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atheism | Definition, Philosophy, & Comparison to …

Atheism, in general, the critique and denial of metaphysical beliefs in God or spiritual beings. As such, it is usually distinguished from theism, which affirms the reality of the divine and often seeks to demonstrate its existence. Atheism is also distinguished from agnosticism, which leaves open the question whether there is a god or not, professing to find the questions unanswered or unanswerable.

The dialectic of the argument between forms of belief and unbelief raises questions concerning the most perspicuous delineation, or characterization, of atheism, agnosticism, and theism. It is necessary not only to probe the warrant for atheism but also carefully to consider what is the most adequate definition of atheism. This article will start with what have been some widely accepted, but still in various ways mistaken or misleading, definitions of atheism and move to more adequate formulations that better capture the full range of atheist thought and more clearly separate unbelief from belief and atheism from agnosticism. In the course of this delineation the section also will consider key arguments for and against atheism.

A central, common core of Judaism, Christianity, and Islam is the affirmation of the reality of one, and only one, God. Adherents of these faiths believe that there is a God who created the universe out of nothing and who has absolute sovereignty over all his creation; this includes, of course, human beingswho are not only utterly dependent on this creative power but also sinful and who, or so the faithful must believe, can only make adequate sense of their lives by accepting, without question, Gods ordinances for them. The varieties of atheism are numerous, but all atheists reject such a set of beliefs.

Atheism, however, casts a wider net and rejects all belief in spiritual beings, and to the extent that belief in spiritual beings is definitive of what it means for a system to be religious, atheism rejects religion. So atheism is not only a rejection of the central conceptions of Judaism, Christianity, and Islam; it is, as well, a rejection of the religious beliefs of such African religions as that of the Dinka and the Nuer, of the anthropomorphic gods of classical Greece and Rome, and of the transcendental conceptions of Hinduism and Buddhism. Generally atheism is a denial of God or of the gods, and if religion is defined in terms of belief in spiritual beings, then atheism is the rejection of all religious belief.

It is necessary, however, if a tolerably adequate understanding of atheism is to be achieved, to give a reading to rejection of religious belief and to come to realize how the characterization of atheism as the denial of God or the gods is inadequate.

To say that atheism is the denial of God or the gods and that it is the opposite of theism, a system of belief that affirms the reality of God and seeks to demonstrate his existence, is inadequate in a number of ways. First, not all theologians who regard themselves as defenders of the Christian faith or of Judaism or Islam regard themselves as defenders of theism. The influential 20th-century Protestant theologian Paul Tillich, for example, regards the God of theism as an idol and refuses to construe God as a being, even a supreme being, among beings or as an infinite being above finite beings. God, for him, is being-itself, the ground of being and meaning. The particulars of Tillichs view are in certain ways idiosyncratic, as well as being obscure and problematic, but they have been influential; and his rejection of theism, while retaining a belief in God, is not eccentric in contemporary theology, though it may very well affront the plain believer.

Second, and more important, it is not the case that all theists seek to demonstrate or even in any way rationally to establish the existence of God. Many theists regard such a demonstration as impossible, and fideistic believers (e.g., Johann Hamann and Sren Kierkegaard) regard such a demonstration, even if it were possible, as undesirable, for in their view it would undermine faith. If it could be proved, or known for certain, that God exists, people would not be in a position to accept him as their sovereign Lord humbly on faith with all the risks that entails. There are theologians who have argued that for genuine faith to be possible God must necessarily be a hidden God, the mysterious ultimate reality, whose existence and authority must be accepted simply on faith. This fideistic view has not, of course, gone without challenge from inside the major faiths, but it is of sufficient importance to make the above characterization of atheism inadequate.

Finally, and most important, not all denials of God are denials of his existence. Believers sometimes deny God while not being at all in a state of doubt that God exists. They either willfully reject what they take to be his authority by not acting in accordance with what they take to be his will, or else they simply live their lives as if God did not exist. In this important way they deny him. Such deniers are not atheists (unless we wish, misleadingly, to call them practical atheists). They are not even agnostics. They do not question that God exists; they deny him in other ways. An atheist denies the existence of God. As it is frequently said, atheists believe that it is false that God exists, or that Gods existence is a speculative hypothesis of an extremely low order of probability.

Yet it remains the case that such a characterization of atheism is inadequate in other ways. For one it is too narrow. There are atheists who believe that the very concept of God, at least in developed and less anthropomorphic forms of Judeo-Christianity and Islam, is so incoherent that certain central religious claims, such as God is my creator to whom everything is owed, are not genuine truth-claims; i.e., the claims could not be either true or false. Believers hold that such religious propositions are true, some atheists believe that they are false, and there are agnostics who cannot make up their minds whether to believe that they are true or false. (Agnostics think that the propositions are one or the other but believe that it is not possible to determine which.) But all three are mistaken, some atheists argue, for such putative truth-claims are not sufficiently intelligible to be genuine truth-claims that are either true or false. In reality there is nothing in them to be believed or disbelieved, though there is for the believer the powerful and humanly comforting illusion that there is. Such an atheism, it should be added, rooted for some conceptions of God in considerations about intelligibility and what it makes sense to say, has been strongly resisted by some pragmatists and logical empiricists.

While the above considerations about atheism and intelligibility show the second characterization of atheism to be too narrow, it is also the case that this characterization is in a way too broad. For there are fideistic believers, who quite unequivocally believe that when looked at objectively the proposition that God exists has a very low probability weight. They believe in God not because it is probable that he existsthey think it more probable that he does notbut because belief is thought by them to be necessary to make sense of human life. The second characterization of atheism does not distinguish a fideistic believer (a Blaise Pascal or a Soren Kierkegaard) or an agnostic (a T.H. Huxley or a Sir Leslie Stephen) from an atheist such as Baron dHolbach. All believe that there is a God and God protects humankind, however emotionally important they may be, are speculative hypotheses of an extremely low order of probability. But this, since it does not distinguish believers from nonbelievers and does not distinguish agnostics from atheists, cannot be an adequate characterization of atheism.

It may be retorted that to avoid apriorism and dogmatic atheism the existence of God should be regarded as a hypothesis. There are no ontological (purely a priori) proofs or disproofs of Gods existence. It is not reasonable to rule in advance that it makes no sense to say that God exists. What the atheist can reasonably claim is that there is no evidence that there is a God, and against that background he may very well be justified in asserting that there is no God. It has been argued, however, that it is simply dogmatic for an atheist to assert that no possible evidence could ever give one grounds for believing in God. Instead, atheists should justify their unbelief by showing (if they can) how the assertion is well-taken that there is no evidence that would warrant a belief in God. If atheism is justified, the atheist will have shown that in fact there is no adequate evidence for the belief that God exists, but it should not be part of his task to try to show that there could not be any evidence for the existence of God. If the atheist could somehow survive the death of his present body (assuming that such talk makes sense) and come, much to his surprise, to stand in the presence of God, his answer should be, Oh! Lord, you didnt give me enough evidence! He would have been mistaken, and realize that he had been mistaken, in his judgment that God did not exist. Still, he would not have been unjustified, in the light of the evidence available to him during his earthly life, in believing as he did. Not having any such postmortem experiences of the presence of God (assuming that he could have them), what he should say, as things stand and in the face of the evidence he actually has and is likely to be able to get, is that it is false that God exists. (Every time one legitimately asserts that a proposition is false one need not be certain that it is false. Knowing with certainty is not a pleonasm.) The claim is that this tentative posture is the reasonable position for the atheist to take.

An atheist who argues in this manner may also make a distinctive burden-of-proof argument. Given that God (if there is one) is by definition a very recherch realitya reality that must be (for there to be such a reality) transcendent to the worldthe burden of proof is not on the atheist to give grounds for believing that there is no reality of that order. Rather, the burden of proof is on the believer to give some evidence for Gods existencei.e., that there is such a reality. Given what God must be, if there is a God, the theist needs to present the evidence, for such a very strange reality. He needs to show that there is more in the world than is disclosed by common experience. The empirical method, and the empirical method alone, such an atheist asserts, affords a reliable method for establishing what is in fact the case. To the claim of the theist that there are in addition to varieties of empirical facts spiritual facts or transcendent facts, such as it being the case that there is a supernatural, self-existent, eternal power, the atheist can assert that such facts have not been shown.

It will, however, be argued by such atheists, against what they take to be dogmatic aprioristic atheists, that the atheist should be a fallibilist and remain open-minded about what the future may bring. There may, after all, be such transcendent facts, such metaphysical realities. It is not that such a fallibilistic atheist is really an agnostic who believes that he is not justified in either asserting that God exists or denying that he exists and that what he must reasonably do is suspend belief. On the contrary, such an atheist believes that he has very good grounds indeed, as things stand, for denying the existence of God. But he will, on the second conceptualization of what it is to be an atheist, not deny that things could be otherwise and that, if they were, he would be justified in believing in God or at least would no longer be justified in asserting that it is false that there is a God. Using reliable empirical techniques, proven methods for establishing matters of fact, the fallibilistic atheist has found nothing in the universe to make a belief that God exists justifiable or even, everything considered, the most rational option of the various options. He therefore draws the atheistical conclusion (also keeping in mind his burden-of-proof argument) that God does not exist. But he does not dogmatically in a priori fashion deny the existence of God. He remains a thorough and consistent fallibilist.

Such a form of atheism (the atheism of those pragmatists who are also naturalistic humanists), though less inadequate than the first formation of atheism, is still inadequate. God in developed forms of Judaism, Christianity, and Islam is not, like Zeus or Odin, construed in a relatively plain anthropomorphic way. Nothing that could count as God in such religions could possibly be observed, literally encountered, or detected in the universe. God, in such a conception, is utterly transcendent to the world; he is conceived of as pure spirit, an infinite individual who created the universe out of nothing and who is distinct from the universe. Such a realitya reality that is taken to be an ultimate mysterycould not be identified as objects or processes in the universe can be identified. There can be no pointing at or to God, no ostensive teaching of God, to show what is meant. The word God can only be taught intralinguistically. God is taught to someone who does not understand what the word means by the use of descriptions such as the maker of the universe, the eternal, utterly independent being upon whom all other beings depend, the first cause, the sole ultimate reality, or a self-caused being. For someone who does not understand such descriptions, there can be no understanding of the concept of God. But the key terms of such descriptions are themselves no more capable of ostensive definition (of having their referents pointed out) than is God, where that term is not, like Zeus, construed anthropomorphically. (That does not mean that anyone has actually pointed to Zeus or observed Zeus but that one knows what it would be like to do so.)

In coming to understand what is meant by God in such discourses, it must be understood that God, whatever else he is, is a being that could not possibly be seen or be in any way else observed. He could not be anything material or empirical, and he is said by believers to be an intractable mystery. A nonmysterious God would not be the God of Judaism, Christianity, and Islam.

This, in effect, makes it a mistake to claim that the existence of God can rightly be treated as a hypothesis and makes it a mistake to claim that, by the use of the experimental method or some other determinate empirical method, the existence of God can be confirmed or disconfirmed as can the existence of an empirical reality. The retort made by some atheists, who also like pragmatists remain thoroughgoing fallibilists, is that such a proposed way of coming to know, or failing to come to know, God makes no sense for anyone who understands what kind of reality God is supposed to be. Anything whose existence could be so verified would not be the God of Judeo-Christianity. God could not be a reality whose presence is even faintly adumbrated in experience, for anything that could even count as the God of Judeo-Christianity must be transcendent to the world. Anything that could actually be encountered or experienced could not be God.

At the very heart of a religion such as Christianity there stands a metaphysical belief in a reality that is alleged to transcend the empirical world. It is the metaphysical belief that there is an eternal, ever-present creative source and sustainer of the universe. The problem is how it is possible to know or reasonably believe that such a reality exists or even to understand what such talk is about.

It is not that God is like a theoretical entity in physics such as a proton or a neutrino. They are, where they are construed as realities rather than as heuristically useful conceptual fictions, thought to be part of the actual furniture of the universe. They are not said to be transcendent to the universe, but rather are invisible entities in the universe logically on a par with specks of dust and grains of sand, only much, much smaller. They are on the same continuum; they are not a different kind of reality. It is only the case that they, as a matter of fact, cannot be seen. Indeed no one has an understanding of what it would be like to see a proton or a neutrinoin that way they are like Godand no provision is made in physical theory for seeing them. Still, there is no logical ban on seeing them as there is on seeing God. They are among the things in the universe, and thus, though they are invisible, they can be postulated as causes of things that are seen. Since this is so it becomes at least logically possible indirectly to verify by empirical methods the existence of such realities. It is also the case that there is no logical ban on establishing what is necessary to establish a causal connection, namely a constant conjunction of two discrete empirical realities. But no such constant conjunction can be established or even intelligibly asserted between God and the universe, and thus the existence of God is not even indirectly verifiable. God is not a discrete empirical thing or being, and the universe is not a gigantic thing or process over and above the things and processes in the universe of which it makes sense to say that the universe has or had a cause. But then there is no way, directly or indirectly, that even the probability that there is a God could be empirically established.

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atheism | Definition, Philosophy, & Comparison to …

Atheism | CARM.org

Atheism is a lack of belief in any God and deities as well as a total denial of the existence of any god. It is a growing movement that is becoming more aggressive, more demanding, and less tolerant of anything other than itself – as is exemplified by its adherents. Is atheism a sound philosophical system as a worldview or is it ultimately self-defeating? Is the requirement of empirical evidence for God a mistake in logic or is it a fair demand? Can we prove that God exists or is that impossible? Find out more about atheism, its arguments, and its problems here at CARM. Learn how to deal with the arguments raised against the existence of God that seek to replace Him with naturalism, materialism, and moral relativism.

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Atheism | CARM.org

Atheism – Simple English Wikipedia, the free encyclopedia

Atheism is rejecting the belief in a god or gods. It is the opposite of theism, which is the belief that at least one god exists.A person who rejects belief in gods is called an atheist.Theism is the belief in one or more gods. Adding an a, meaning “without”, before the word theism results in atheism, or literally, “without theism”.. Atheism is not the same as agnosticism: agnostics say that …

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Atheism – Simple English Wikipedia, the free encyclopedia

atheism r/atheism – reddit: the front page of the internet

This happened around last year when they just found out that i was an atheist. My parents sat down with me (and for some reason they roped my brother in too) to kinda talk it out with them, the why and how and all that.

So my father was talking about how god had blessed him and his family with a luxurious and comfortable life. I, thinking that my parents would hear me out since they got out of their own way just to talk about religion with us, told them that i believed that they worked hard and earned the money themselves.

Surprisingly enough, my father immediately blew his top off and yelled at me, insisting that it was by god’s grace that we are now able to live such a good life. He then, for some reason told me that my ability to draw was a god-given talent. Naturally, i was pissed. After all, i went to years and years of art class just to be able to draw like i do now, though it only looks nice in my family’s standards since i’m the only one in my family that can draw. But i didn’t say anything back since i don’t want to start another war with m parents.

Seriously, if it really was just god’s grace that allowed my family to live comfortably, why have i never seen god just bestow upon my father a paycheck? Why is it that he’s so happy about having all his hard work credited to an invisible sky daddy? Call me greedy or selfish, but if someone took all the credit to my hard work i’d be bloody pissed. But hey, thanks for reading this.

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atheism r/atheism – reddit: the front page of the internet

Cryptocurrency News: This Week on Bitfinex, Tether, Coinbase, & More

Cryptocurrency News
On the whole, cryptocurrency prices are down from our previous report on cryptos, with the market slipping on news of an exchange being hacked and a report about Bitcoin manipulation.

However, there have been two bright spots: 1) an official from the U.S. Securities and Exchange Commission (SEC) said that Ethereum is not a security, and 2) Coinbase is expanding its selection of tokens.

Let’s start with the good news.
SEC Says ETH Is Not a Security
Investors have some reason to cheer this week. A high-ranking SEC official told attendees of the Yahoo! All Markets Summit: Crypto that Ethereum and Bitcoin are not.

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Cryptocurrency News: This Week on Bitfinex, Tether, Coinbase, & More

Ripple Price Forecast: XRP vs SWIFT, SEC Updates, and More

Ripple vs SWIFT: The War Begins
While most criticisms of XRP do nothing to curb my bullish Ripple price forecast, there is one obstacle that nags at my conscience. Its name is SWIFT.

The Society for Worldwide Interbank Financial Telecommunication (SWIFT) is the king of international payments.

It coordinates wire transfers across 11,000 banks in more than 200 countries and territories, meaning that in order for XRP prices to ascend to $10.00, Ripple needs to launch a successful coup. That is, and always has been, an unwritten part of Ripple’s story.

We’ve seen a lot of progress on that score. In the last three years, Ripple wooed more than 100 financial firms onto its.

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Ripple Price Forecast: XRP vs SWIFT, SEC Updates, and More

Cryptocurrency News: Bitcoin ETFs, Andreessen Horowitz, and Contradictions in Crypto

Cryptocurrency News
This was a bloody week for cryptocurrencies. Everything was covered in red, from Ethereum (ETH) on down to the Basic Attention Token (BAT).

Some investors claim it was inevitable. Others say that price manipulation is to blame.

We think the answers are more complicated than either side has to offer, because our research reveals deep contradictions between the price of cryptos and the underlying development of blockchain projects.

For instance, a leading venture capital (VC) firm launched a $300.0-million crypto investment fund, yet liquidity continues to dry up in crypto markets.

Another example is the U.S. Securities and Exchange Commission’s.

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Cryptocurrency News: Bitcoin ETFs, Andreessen Horowitz, and Contradictions in Crypto

Cryptocurrency News: Looking Past the Bithumb Crypto Hack

Another Crypto Hack Derails Recovery
Since our last report, hackers broke into yet another cryptocurrency exchange. This time the target was Bithumb, a Korean exchange known for high-flying prices and ultra-active traders.

While the hackers made off with approximately $31.5 million in funds, the exchange is working with relevant authorities to return the stolen tokens to their respective owners. In the event that some is still missing, the exchange will cover the losses. (Source: “Bithumb Working With Other Crypto Exchanges to Recover Hacked Funds,”.

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Cryptocurrency News: Looking Past the Bithumb Crypto Hack

Cryptocurrency News: XRP Validators, Malta, and Practical Tokens

Cryptocurrency News & Market Summary
Investors finally saw some light at the end of the tunnel last week, with cryptos soaring across the board. No one quite knows what kicked off the rally—as it could have been any of the stories we discuss below—but the net result was positive.

Of course, prices won’t stay on this rocket ride forever. I expect to see a resurgence of volatility in short order, because the market is moving as a single unit. Everything is rising in tandem.

This tells me that investors are simply “buying the dip” rather than identifying which cryptos have enough real-world value to outlive the crash.

So if you want to know when.

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Cryptocurrency News: Bitcoin ETF Rejection, AMD Microchip Sales, and Hedge Funds

Cryptocurrency News
Although cryptocurrency prices were heating up last week (Bitcoin, especially), regulators poured cold water on the rally by rejecting calls for a Bitcoin exchange-traded fund (ETF). This is the second time that the proposal fell on deaf ears. (More on that below.)

Crypto mining ran into similar trouble, as you can see from Advanced Micro Devices, Inc.‘s (NASDAQ:AMD) most recent quarterly earnings. However, it wasn’t all bad news. Investors should, for instance, be cheering the fact that hedge funds are ramping up their involvement in cryptocurrency markets.

Without further ado, here are those stories in greater detail.
ETF Rejection.

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Cryptocurrency News: What You Need to Know This Week

Cryptocurrency News
Cryptocurrencies traded sideways since our last report on cryptos. However, I noticed something interesting when playing around with Yahoo! Finance’s cryptocurrency screener: There are profitable pockets in this market.

Incidentally, Yahoo’s screener is far superior to the one on CoinMarketCap, so if you’re looking to compare digital assets, I highly recommend it.

But let’s get back to my epiphany.

In the last month, at one point or another, most crypto assets on our favorites list saw double-digit increases. It’s true that each upswing was followed by a hard crash, but investors who rode the trend would have made a.

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Cryptocurrency News: What You Need to Know This Week

Cryptocurrency News: New Exchanges Could Boost Crypto Liquidity

Cryptocurrency News
Even though the cryptocurrency news was upbeat in recent days, the market tumbled after the U.S. Securities and Exchange Commission (SEC) rejected calls for a Bitcoin (BTC) exchange-traded fund (ETF).

That news came as a blow to investors, many of whom believe the ETF would open the cryptocurrency industry up to pension funds and other institutional investors. This would create a massive tailwind for cryptos, they say.

So it only follows that a rejection of the Bitcoin ETF should send cryptos tumbling, correct? Well, maybe you can follow that logic. To me, it seems like a dramatic overreaction.

I understand that legitimizing cryptos is important. But.

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Cryptocurrency News: New Exchanges Could Boost Crypto Liquidity

Bitcoin Rise: Is the Recent Bitcoin Price Surge a Sign of Things to Come or Another Misdirection?

What You Need to Know About the Bitcoin Price Rise
It wasn’t that long ago that Bitcoin (BTC) dominated headlines for its massive growth, with many cryptocurrency millionaires being made. The Bitcoin price surged ever upward and many people thought the gravy train would never stop running—until it did.

Prices crashed, investors abandoned the space, and lots of people lost money. Cut to today and we’re seeing another big Bitcoin price surge; is this time any different?

I’m of a mind that investors ought to think twice before jumping back in on Bitcoin.

Bitcoin made waves when it once again crested above $5,000. Considering that it started 2019 around $3,700,.

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Bitcoin Rise: Is the Recent Bitcoin Price Surge a Sign of Things to Come or Another Misdirection?

Cryptocurrency News: Vitalik Buterin Doesn’t Care About Bitcoin ETFs

Cryptocurrency News
While headline numbers look devastating this week, investors might take some solace in knowing that cryptocurrencies found their bottom at roughly $189.8 billion in market cap—that was the low point. Since then, investors put more than $20.0 billion back into the market.

During the rout, Ethereum broke below $300.00 and XRP fell below $0.30, marking yearly lows for both tokens. The same was true down the list of the top 100 biggest cryptos.

Altcoins took the brunt of the hit. BTC Dominance, which reveals how tightly investment is concentrated in Bitcoin, rose from 42.62% to 53.27% in just one month, showing that investors either fled altcoins at higher.

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Cryptocurrency News: Vitalik Buterin Doesn’t Care About Bitcoin ETFs

Human genetics – Wikipedia

Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Genes can be the common factor of the qualities of most human-inherited traits. Study of human genetics can be useful as it can answer questions about human nature, understand the diseases and development of effective disease treatment, and understand genetics of human life. This article describes only basic features of human genetics; for the genetics of disorders please see: medical genetics.

Inheritance of traits for humans are based upon Gregor Mendel’s model of inheritance. Mendel deduced that inheritance depends upon discrete units of inheritance, called factors or genes.[1]

Autosomal traits are associated with a single gene on an autosome (non-sex chromosome)they are called “dominant” because a single copyinherited from either parentis enough to cause this trait to appear. This often means that one of the parents must also have the same trait, unless it has arisen due to an unlikely new mutation. Examples of autosomal dominant traits and disorders are Huntington’s disease and achondroplasia.

Autosomal recessive traits is one pattern of inheritance for a trait, disease, or disorder to be passed on through families. For a recessive trait or disease to be displayed two copies of the trait or disorder needs to be presented. The trait or gene will be located on a non-sex chromosome. Because it takes two copies of a trait to display a trait, many people can unknowingly be carriers of a disease. From an evolutionary perspective, a recessive disease or trait can remain hidden for several generations before displaying the phenotype. Examples of autosomal recessive disorders are albinism, cystic fibrosis.

X-linked genes are found on the sex X chromosome. X-linked genes just like autosomal genes have both dominant and recessive types. Recessive X-linked disorders are rarely seen in females and usually only affect males. This is because males inherit their X chromosome and all X-linked genes will be inherited from the maternal side. Fathers only pass on their Y chromosome to their sons, so no X-linked traits will be inherited from father to son. Men cannot be carriers for recessive X linked traits, as they only have one X chromosome, so any X linked trait inherited from the mother will show up.

Females express X-linked disorders when they are homozygous for the disorder and become carriers when they are heterozygous. X-linked dominant inheritance will show the same phenotype as a heterozygote and homozygote. Just like X-linked inheritance, there will be a lack of male-to-male inheritance, which makes it distinguishable from autosomal traits. One example of an X-linked trait is CoffinLowry syndrome, which is caused by a mutation in ribosomal protein gene. This mutation results in skeletal, craniofacial abnormalities, mental retardation, and short stature.

X chromosomes in females undergo a process known as X inactivation. X inactivation is when one of the two X chromosomes in females is almost completely inactivated. It is important that this process occurs otherwise a woman would produce twice the amount of normal X chromosome proteins. The mechanism for X inactivation will occur during the embryonic stage. For people with disorders like trisomy X, where the genotype has three X chromosomes, X-inactivation will inactivate all X chromosomes until there is only one X chromosome active. Males with Klinefelter syndrome, who have an extra X chromosome, will also undergo X inactivation to have only one completely active X chromosome.

Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. Since Y chromosomes can only be found in males, Y linked traits are only passed on from father to son. The testis determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics.

A pedigree is a diagram showing the ancestral relationships and transmission of genetic traits over several generations in a family. Square symbols are almost always used to represent males, whilst circles are used for females. Pedigrees are used to help detect many different genetic diseases. A pedigree can also be used to help determine the chances for a parent to produce an offspring with a specific trait.

Four different traits can be identified by pedigree chart analysis: autosomal dominant, autosomal recessive, x-linked, or y-linked. Partial penetrance can be shown and calculated from pedigrees. Penetrance is the percentage expressed frequency with which individuals of a given genotype manifest at least some degree of a specific mutant phenotype associated with a trait.

Inbreeding, or mating between closely related organisms, can clearly be seen on pedigree charts. Pedigree charts of royal families often have a high degree of inbreeding, because it was customary and preferable for royalty to marry another member of royalty. Genetic counselors commonly use pedigrees to help couples determine if the parents will be able to produce healthy children.

A karyotype is a very useful tool in cytogenetics. A karyotype is picture of all the chromosomes in the metaphase stage arranged according to length and centromere position. A karyotype can also be useful in clinical genetics, due to its ability to diagnose genetic disorders. On a normal karyotype, aneuploidy can be detected by clearly being able to observe any missing or extra chromosomes.[1]

Giemsa banding, g-banding, of the karyotype can be used to detect deletions, insertions, duplications, inversions, and translocations. G-banding will stain the chromosomes with light and dark bands unique to each chromosome. A FISH, fluorescent in situ hybridization, can be used to observe deletions, insertions, and translocations. FISH uses fluorescent probes to bind to specific sequences of the chromosomes that will cause the chromosomes to fluoresce a unique color.[1]

Genomics refers to the field of genetics concerned with structural and functional studies of the genome.[1] A genome is all the DNA contained within an organism or a cell including nuclear and mitochondrial DNA. The human genome is the total collection of genes in a human being contained in the human chromosome, composed of over three billion nucleotides.[2] In April 2003, the Human Genome Project was able to sequence all the DNA in the human genome, and to discover that the human genome was composed of around 20,000 protein coding genes.

Medical genetics is the branch of medicine that involves the diagnosis and management of hereditary disorders. Medical genetics is the application of genetics to medical care. It overlaps human genetics, for example, research on the causes and inheritance of genetic disorders would be considered within both human genetics and medical genetics, while the diagnosis, management, and counseling of individuals with genetic disorders would be considered part of medical genetics.

Population genetics is the branch of evolutionary biology responsible for investigating processes that cause changes in allele and genotype frequencies in populations based upon Mendelian inheritance.[3] Four different forces can influence the frequencies: natural selection, mutation, gene flow (migration), and genetic drift. A population can be defined as a group of interbreeding individuals and their offspring. For human genetics the populations will consist only of the human species. The Hardy-Weinberg principle is a widely used principle to determine allelic and genotype frequencies.

In addition to nuclear DNA, humans (like almost all eukaryotes) have mitochondrial DNA. Mitochondria, the “power houses” of a cell, have their own DNA. Mitochondria are inherited from one’s mother, and their DNA is frequently used to trace maternal lines of descent (see mitochondrial Eve). Mitochondrial DNA is only 16kb in length and encodes for 62 genes.

The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila), and some plants (Ginkgo). In this system, the sex of an individual is determined by a pair of sex chromosomes (gonosomes). Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two distinct sex chromosomes (XY), and are called the heterogametic sex.

Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.However, females carry two or more copies of the X chromosome, resulting in a potentially toxic dose of X-linked genes.[4]

To correct this imbalance, mammalian females have evolved a unique mechanism of dosage compensation. In particular, by way of the process called X-chromosome inactivation (XCI), female mammals transcriptionally silence one of their two Xs in a complex and highly coordinated manner.[4]

GeneticChromosomal

[35]

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Human genetics – Wikipedia

Human genetics – Wikipedia

Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Genes can be the common factor of the qualities of most human-inherited traits. Study of human genetics can be useful as it can answer questions about human nature, understand the diseases and development of effective disease treatment, and understand genetics of human life. This article describes only basic features of human genetics; for the genetics of disorders please see: medical genetics.

Inheritance of traits for humans are based upon Gregor Mendel’s model of inheritance. Mendel deduced that inheritance depends upon discrete units of inheritance, called factors or genes.[1]

Autosomal traits are associated with a single gene on an autosome (non-sex chromosome)they are called “dominant” because a single copyinherited from either parentis enough to cause this trait to appear. This often means that one of the parents must also have the same trait, unless it has arisen due to an unlikely new mutation. Examples of autosomal dominant traits and disorders are Huntington’s disease and achondroplasia.

Autosomal recessive traits is one pattern of inheritance for a trait, disease, or disorder to be passed on through families. For a recessive trait or disease to be displayed two copies of the trait or disorder needs to be presented. The trait or gene will be located on a non-sex chromosome. Because it takes two copies of a trait to display a trait, many people can unknowingly be carriers of a disease. From an evolutionary perspective, a recessive disease or trait can remain hidden for several generations before displaying the phenotype. Examples of autosomal recessive disorders are albinism, cystic fibrosis.

X-linked genes are found on the sex X chromosome. X-linked genes just like autosomal genes have both dominant and recessive types. Recessive X-linked disorders are rarely seen in females and usually only affect males. This is because males inherit their X chromosome and all X-linked genes will be inherited from the maternal side. Fathers only pass on their Y chromosome to their sons, so no X-linked traits will be inherited from father to son. Men cannot be carriers for recessive X linked traits, as they only have one X chromosome, so any X linked trait inherited from the mother will show up.

Females express X-linked disorders when they are homozygous for the disorder and become carriers when they are heterozygous. X-linked dominant inheritance will show the same phenotype as a heterozygote and homozygote. Just like X-linked inheritance, there will be a lack of male-to-male inheritance, which makes it distinguishable from autosomal traits. One example of an X-linked trait is CoffinLowry syndrome, which is caused by a mutation in ribosomal protein gene. This mutation results in skeletal, craniofacial abnormalities, mental retardation, and short stature.

X chromosomes in females undergo a process known as X inactivation. X inactivation is when one of the two X chromosomes in females is almost completely inactivated. It is important that this process occurs otherwise a woman would produce twice the amount of normal X chromosome proteins. The mechanism for X inactivation will occur during the embryonic stage. For people with disorders like trisomy X, where the genotype has three X chromosomes, X-inactivation will inactivate all X chromosomes until there is only one X chromosome active. Males with Klinefelter syndrome, who have an extra X chromosome, will also undergo X inactivation to have only one completely active X chromosome.

Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. Since Y chromosomes can only be found in males, Y linked traits are only passed on from father to son. The testis determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics.

A pedigree is a diagram showing the ancestral relationships and transmission of genetic traits over several generations in a family. Square symbols are almost always used to represent males, whilst circles are used for females. Pedigrees are used to help detect many different genetic diseases. A pedigree can also be used to help determine the chances for a parent to produce an offspring with a specific trait.

Four different traits can be identified by pedigree chart analysis: autosomal dominant, autosomal recessive, x-linked, or y-linked. Partial penetrance can be shown and calculated from pedigrees. Penetrance is the percentage expressed frequency with which individuals of a given genotype manifest at least some degree of a specific mutant phenotype associated with a trait.

Inbreeding, or mating between closely related organisms, can clearly be seen on pedigree charts. Pedigree charts of royal families often have a high degree of inbreeding, because it was customary and preferable for royalty to marry another member of royalty. Genetic counselors commonly use pedigrees to help couples determine if the parents will be able to produce healthy children.

A karyotype is a very useful tool in cytogenetics. A karyotype is picture of all the chromosomes in the metaphase stage arranged according to length and centromere position. A karyotype can also be useful in clinical genetics, due to its ability to diagnose genetic disorders. On a normal karyotype, aneuploidy can be detected by clearly being able to observe any missing or extra chromosomes.[1]

Giemsa banding, g-banding, of the karyotype can be used to detect deletions, insertions, duplications, inversions, and translocations. G-banding will stain the chromosomes with light and dark bands unique to each chromosome. A FISH, fluorescent in situ hybridization, can be used to observe deletions, insertions, and translocations. FISH uses fluorescent probes to bind to specific sequences of the chromosomes that will cause the chromosomes to fluoresce a unique color.[1]

Genomics refers to the field of genetics concerned with structural and functional studies of the genome.[1] A genome is all the DNA contained within an organism or a cell including nuclear and mitochondrial DNA. The human genome is the total collection of genes in a human being contained in the human chromosome, composed of over three billion nucleotides.[2] In April 2003, the Human Genome Project was able to sequence all the DNA in the human genome, and to discover that the human genome was composed of around 20,000 protein coding genes.

Medical genetics is the branch of medicine that involves the diagnosis and management of hereditary disorders. Medical genetics is the application of genetics to medical care. It overlaps human genetics, for example, research on the causes and inheritance of genetic disorders would be considered within both human genetics and medical genetics, while the diagnosis, management, and counseling of individuals with genetic disorders would be considered part of medical genetics.

Population genetics is the branch of evolutionary biology responsible for investigating processes that cause changes in allele and genotype frequencies in populations based upon Mendelian inheritance.[3] Four different forces can influence the frequencies: natural selection, mutation, gene flow (migration), and genetic drift. A population can be defined as a group of interbreeding individuals and their offspring. For human genetics the populations will consist only of the human species. The Hardy-Weinberg principle is a widely used principle to determine allelic and genotype frequencies.

In addition to nuclear DNA, humans (like almost all eukaryotes) have mitochondrial DNA. Mitochondria, the “power houses” of a cell, have their own DNA. Mitochondria are inherited from one’s mother, and their DNA is frequently used to trace maternal lines of descent (see mitochondrial Eve). Mitochondrial DNA is only 16kb in length and encodes for 62 genes.

The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila), and some plants (Ginkgo). In this system, the sex of an individual is determined by a pair of sex chromosomes (gonosomes). Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two distinct sex chromosomes (XY), and are called the heterogametic sex.

Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.However, females carry two or more copies of the X chromosome, resulting in a potentially toxic dose of X-linked genes.[4]

To correct this imbalance, mammalian females have evolved a unique mechanism of dosage compensation. In particular, by way of the process called X-chromosome inactivation (XCI), female mammals transcriptionally silence one of their two Xs in a complex and highly coordinated manner.[4]

GeneticChromosomal

[35]

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Human genetics – Wikipedia

Human Genetics – medschool.ucla.edu

A hub of deep expertise, the Department of Human Genetics helps partners across UCLA interpret data and leverage genomic technology to improve study design and solve medical problems.

We demystify genetic complexities to provide vital insights for a range of clinical and research applications. We strive to improve the care of as many patients as possible by pushing our capabilities, developing novel ways to address unanswered questions.

Your next collaboration is right down the street.

Our enviable proximity to the worlds brightest scientific minds enables both thriving scheduled events and impromptu sidewalk powwows. A casual conversation during your coffee run could lead to your next big publication.

Come find out why innovation lives here.

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Jonathan Flint, MDPsychiatrist Dr. Jonathan Flint has been named a fellow of the Royal Society, an academy that includes some of the worlds most eminent scientists.Learn More

Bogdan Pasaniuc, PhDA team of researchers from UCLA, Cedars-Sinai Cancer and Dana-Farber Cancer Institute has identified 34 genes that are associated with an increased risk for developing the earliest stages of ovarian cancer.Learn More

Deborah Krakow, MDNamed to Los Angeles Magazines 2019 list of Top Doctors in Los Angeles specializing in Genetics. The list identifies doctors considered to be at the top of their fields.Learn More

Julian Martinez, MD, PhD Named to Los Angeles Magazines 2019 list of Top Doctors in Los Angeles specializing in Genetics. The list identifies doctors considered to be at the top of their fields.Learn More

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Human Genetics – medschool.ucla.edu

Department of Human Genetics | The University of Chicago

The Department of Human Genetics is the home within the Division of Biological Sciences for the study of basic principles of genetics and genomics as applied to human disease. We provide broad training in experimental genetics and genomics, statistical and population genetics, bioinformatics, and clinical genetics. A common theme throughout our research is the application of basic genetic principles and strategies to the study of disease mechanism, disease susceptibility, and the genetic architecture of complex traits. Our faculty bridge between basic and clinical research and train students for careers in academia, industry, and medicine.

The Department of Human Genetics has an unwavering commitment to diversity, inclusion, free expression, and open discourse.These values are at the core of our roles as scientists, as teachers, and as citizens of a free society.

Science, including genetics, plays a central role in many crucial issues of our time. We are committed to generating rigorous scientific knowledge, training future scientists, and preparing our students to be well-informed citizens in a democratic society.

Excerpt from:

Department of Human Genetics | The University of Chicago

Human genetics – Wikipedia

Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Genes can be the common factor of the qualities of most human-inherited traits. Study of human genetics can be useful as it can answer questions about human nature, understand the diseases and development of effective disease treatment, and understand genetics of human life. This article describes only basic features of human genetics; for the genetics of disorders please see: medical genetics.

Inheritance of traits for humans are based upon Gregor Mendel’s model of inheritance. Mendel deduced that inheritance depends upon discrete units of inheritance, called factors or genes.[1]

Autosomal traits are associated with a single gene on an autosome (non-sex chromosome)they are called “dominant” because a single copyinherited from either parentis enough to cause this trait to appear. This often means that one of the parents must also have the same trait, unless it has arisen due to an unlikely new mutation. Examples of autosomal dominant traits and disorders are Huntington’s disease and achondroplasia.

Autosomal recessive traits is one pattern of inheritance for a trait, disease, or disorder to be passed on through families. For a recessive trait or disease to be displayed two copies of the trait or disorder needs to be presented. The trait or gene will be located on a non-sex chromosome. Because it takes two copies of a trait to display a trait, many people can unknowingly be carriers of a disease. From an evolutionary perspective, a recessive disease or trait can remain hidden for several generations before displaying the phenotype. Examples of autosomal recessive disorders are albinism, cystic fibrosis.

X-linked genes are found on the sex X chromosome. X-linked genes just like autosomal genes have both dominant and recessive types. Recessive X-linked disorders are rarely seen in females and usually only affect males. This is because males inherit their X chromosome and all X-linked genes will be inherited from the maternal side. Fathers only pass on their Y chromosome to their sons, so no X-linked traits will be inherited from father to son. Men cannot be carriers for recessive X linked traits, as they only have one X chromosome, so any X linked trait inherited from the mother will show up.

Females express X-linked disorders when they are homozygous for the disorder and become carriers when they are heterozygous. X-linked dominant inheritance will show the same phenotype as a heterozygote and homozygote. Just like X-linked inheritance, there will be a lack of male-to-male inheritance, which makes it distinguishable from autosomal traits. One example of an X-linked trait is CoffinLowry syndrome, which is caused by a mutation in ribosomal protein gene. This mutation results in skeletal, craniofacial abnormalities, mental retardation, and short stature.

X chromosomes in females undergo a process known as X inactivation. X inactivation is when one of the two X chromosomes in females is almost completely inactivated. It is important that this process occurs otherwise a woman would produce twice the amount of normal X chromosome proteins. The mechanism for X inactivation will occur during the embryonic stage. For people with disorders like trisomy X, where the genotype has three X chromosomes, X-inactivation will inactivate all X chromosomes until there is only one X chromosome active. Males with Klinefelter syndrome, who have an extra X chromosome, will also undergo X inactivation to have only one completely active X chromosome.

Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. Since Y chromosomes can only be found in males, Y linked traits are only passed on from father to son. The testis determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics.

A pedigree is a diagram showing the ancestral relationships and transmission of genetic traits over several generations in a family. Square symbols are almost always used to represent males, whilst circles are used for females. Pedigrees are used to help detect many different genetic diseases. A pedigree can also be used to help determine the chances for a parent to produce an offspring with a specific trait.

Four different traits can be identified by pedigree chart analysis: autosomal dominant, autosomal recessive, x-linked, or y-linked. Partial penetrance can be shown and calculated from pedigrees. Penetrance is the percentage expressed frequency with which individuals of a given genotype manifest at least some degree of a specific mutant phenotype associated with a trait.

Inbreeding, or mating between closely related organisms, can clearly be seen on pedigree charts. Pedigree charts of royal families often have a high degree of inbreeding, because it was customary and preferable for royalty to marry another member of royalty. Genetic counselors commonly use pedigrees to help couples determine if the parents will be able to produce healthy children.

A karyotype is a very useful tool in cytogenetics. A karyotype is picture of all the chromosomes in the metaphase stage arranged according to length and centromere position. A karyotype can also be useful in clinical genetics, due to its ability to diagnose genetic disorders. On a normal karyotype, aneuploidy can be detected by clearly being able to observe any missing or extra chromosomes.[1]

Giemsa banding, g-banding, of the karyotype can be used to detect deletions, insertions, duplications, inversions, and translocations. G-banding will stain the chromosomes with light and dark bands unique to each chromosome. A FISH, fluorescent in situ hybridization, can be used to observe deletions, insertions, and translocations. FISH uses fluorescent probes to bind to specific sequences of the chromosomes that will cause the chromosomes to fluoresce a unique color.[1]

Genomics refers to the field of genetics concerned with structural and functional studies of the genome.[1] A genome is all the DNA contained within an organism or a cell including nuclear and mitochondrial DNA. The human genome is the total collection of genes in a human being contained in the human chromosome, composed of over three billion nucleotides.[2] In April 2003, the Human Genome Project was able to sequence all the DNA in the human genome, and to discover that the human genome was composed of around 20,000 protein coding genes.

Medical genetics is the branch of medicine that involves the diagnosis and management of hereditary disorders. Medical genetics is the application of genetics to medical care. It overlaps human genetics, for example, research on the causes and inheritance of genetic disorders would be considered within both human genetics and medical genetics, while the diagnosis, management, and counseling of individuals with genetic disorders would be considered part of medical genetics.

Population genetics is the branch of evolutionary biology responsible for investigating processes that cause changes in allele and genotype frequencies in populations based upon Mendelian inheritance.[3] Four different forces can influence the frequencies: natural selection, mutation, gene flow (migration), and genetic drift. A population can be defined as a group of interbreeding individuals and their offspring. For human genetics the populations will consist only of the human species. The Hardy-Weinberg principle is a widely used principle to determine allelic and genotype frequencies.

In addition to nuclear DNA, humans (like almost all eukaryotes) have mitochondrial DNA. Mitochondria, the “power houses” of a cell, have their own DNA. Mitochondria are inherited from one’s mother, and their DNA is frequently used to trace maternal lines of descent (see mitochondrial Eve). Mitochondrial DNA is only 16kb in length and encodes for 62 genes.

The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila), and some plants (Ginkgo). In this system, the sex of an individual is determined by a pair of sex chromosomes (gonosomes). Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two distinct sex chromosomes (XY), and are called the heterogametic sex.

Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.However, females carry two or more copies of the X chromosome, resulting in a potentially toxic dose of X-linked genes.[4]

To correct this imbalance, mammalian females have evolved a unique mechanism of dosage compensation. In particular, by way of the process called X-chromosome inactivation (XCI), female mammals transcriptionally silence one of their two Xs in a complex and highly coordinated manner.[4]

GeneticChromosomal

[35]

Original post:

Human genetics – Wikipedia

Human genetics | biology | Britannica.com

Human genetics, study of the inheritance of characteristics by children from parents. Inheritance in humans does not differ in any fundamental way from that in other organisms.

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genetics: Human genetics

Some geneticists specialize in the hereditary processes of human genetics. Most of the emphasis is on understanding and treating genetic

The study of human heredity occupies a central position in genetics. Much of this interest stems from a basic desire to know who humans are and why they are as they are. At a more practical level, an understanding of human heredity is of critical importance in the prediction, diagnosis, and treatment of diseases that have a genetic component. The quest to determine the genetic basis of human health has given rise to the field of medical genetics. In general, medicine has given focus and purpose to human genetics, so the terms medical genetics and human genetics are often considered synonymous.

A new era in cytogenetics, the field of investigation concerned with studies of the chromosomes, began in 1956 with the discovery by Jo Hin Tjio and Albert Levan that human somatic cells contain 23 pairs of chromosomes. Since that time the field has advanced with amazing rapidity and has demonstrated that human chromosome aberrations rank as major causes of fetal death and of tragic human diseases, many of which are accompanied by mental retardation. Since the chromosomes can be delineated only during mitosis, it is necessary to examine material in which there are many dividing cells. This can usually be accomplished by culturing cells from the blood or skin, since only the bone marrow cells (not readily sampled except during serious bone marrow disease such as leukemia) have sufficient mitoses in the absence of artificial culture. After growth, the cells are fixed on slides and then stained with a variety of DNA-specific stains that permit the delineation and identification of the chromosomes. The Denver system of chromosome classification, established in 1959, identified the chromosomes by their length and the position of the centromeres. Since then the method has been improved by the use of special staining techniques that impart unique light and dark bands to each chromosome. These bands permit the identification of chromosomal regions that are duplicated, missing, or transposed to other chromosomes.

Micrographs showing the karyotypes (i.e., the physical appearance of the chromosome) of a male and a female have been produced. In a typical micrograph the 46 human chromosomes (the diploid number) are arranged in homologous pairs, each consisting of one maternally derived and one paternally derived member. The chromosomes are all numbered except for the X and the Y chromosomes, which are the sex chromosomes. In humans, as in all mammals, the normal female has two X chromosomes and the normal male has one X chromosome and one Y chromosome. The female is thus the homogametic sex, as all her gametes normally have one X chromosome. The male is heterogametic, as he produces two types of gametesone type containing an X chromosome and the other containing a Y chromosome. There is good evidence that the Y chromosome in humans, unlike that in Drosophila, is necessary (but not sufficient) for maleness.

A human individual arises through the union of two cells, an egg from the mother and a sperm from the father. Human egg cells are barely visible to the naked eye. They are shed, usually one at a time, from the ovary into the oviducts (fallopian tubes), through which they pass into the uterus. Fertilization, the penetration of an egg by a sperm, occurs in the oviducts. This is the main event of sexual reproduction and determines the genetic constitution of the new individual.

Human sex determination is a genetic process that depends basically on the presence of the Y chromosome in the fertilized egg. This chromosome stimulates a change in the undifferentiated gonad into that of the male (a testicle). The gonadal action of the Y chromosome is mediated by a gene located near the centromere; this gene codes for the production of a cell surface molecule called the H-Y antigen. Further development of the anatomic structures, both internal and external, that are associated with maleness is controlled by hormones produced by the testicle. The sex of an individual can be thought of in three different contexts: chromosomal sex, gonadal sex, and anatomic sex. Discrepancies between these, especially the latter two, result in the development of individuals with ambiguous sex, often called hermaphrodites. The phenomenon of homosexuality is of uncertain cause and is unrelated to the above sex-determining factors. It is of interest that in the absence of a male gonad (testicle) the internal and external sex anatomy is always female, even in the absence of a female ovary. A female without ovaries will, of course, be infertile and will not experience any of the female developmental changes normally associated with puberty. Such a female will often have Turners syndrome.

If X-containing and Y-containing sperm are produced in equal numbers, then according to simple chance one would expect the sex ratio at conception (fertilization) to be half boys and half girls, or 1 : 1. Direct observation of sex ratios among newly fertilized human eggs is not yet feasible, and sex-ratio data are usually collected at the time of birth. In almost all human populations of newborns, there is a slight excess of males; about 106 boys are born for every100 girls. Throughout life, however, there is a slightly greater mortality of males; this slowly alters the sex ratio until, beyond the age of about 50 years, there is an excess of females. Studies indicate that male embryos suffer a relatively greater degree of prenatal mortality, so the sex ratio at conception might be expected to favour males even more than the 106 : 100 ratio observed at birth would suggest. Firm explanations for the apparent excess of male conceptions have not been established; it is possible that Y-containing sperm survive better within the female reproductive tract, or they may be a little more successful in reaching the egg in order to fertilize it. In any case, the sex differences are small, the statistical expectation for a boy (or girl) at any single birth still being close to one out of two.

During gestationthe period of nine months between fertilization and the birth of the infanta remarkable series of developmental changes occur. Through the process of mitosis, the total number of cells changes from 1 (the fertilized egg) to about 2 1011. In addition, these cells differentiate into hundreds of different types with specific functions (liver cells, nerve cells, muscle cells, etc.). A multitude of regulatory processes, both genetically and environmentally controlled, accomplish this differentiation. Elucidation of the exquisite timing of these processes remains one of the great challenges of human biology.

Immunity is the ability of an individual to recognize the self molecules that make up ones own body and to distinguish them from such nonself molecules as those found in infectious microorganisms and toxins. This process has a prominent genetic component. Knowledge of the genetic and molecular basis of the mammalian immune system has increased in parallel with the explosive advances made in somatic cell and molecular genetics.

There are two major components of the immune system, both originating from the same precursor stem cells. The bursa component provides B lymphocytes, a class of white blood cells that, when appropriately stimulated, differentiate into plasma cells. These latter cells produce circulating soluble proteins called antibodies or immunoglobulins. Antibodies are produced in response to substances called antigens, most of which are foreign proteins or polysaccharides. An antibody molecule can recognize a specific antigen, combine with it, and initiate its destruction. This so-called humoral immunity is accomplished through a complicated series of interactions with other molecules and cells; some of these interactions are mediated by another group of lymphocytes, the T lymphocytes, which are derived from the thymus gland. Once a B lymphocyte has been exposed to a specific antigen, it remembers the contact so that future exposure will cause an accelerated and magnified immune reaction. This is a manifestation of what has been called immunological memory.

The thymus component of the immune system centres on the thymus-derived T lymphocytes. In addition to regulating the B cells in producing humoral immunity, the T cells also directly attack cells that display foreign antigens. This process, called cellular immunity, is of great importance in protecting the body against a variety of viruses as well as cancer cells. Cellular immunity is also the chief cause of the rejection of organ transplants. The T lymphocytes provide a complex network consisting of a series of helper cells (which are antigen-specific), amplifier cells, suppressor cells, and cytotoxic (killer) cells, all of which are important in immune regulation.

One of the central problems in understanding the genetics of the immune system has been in explaining the genetic regulation of antibody production. Immunobiologists have demonstrated that the system can produce well over one million specific antibodies, each corresponding to a particular antigen. It would be difficult to envisage that each antibody is encoded by a separate gene; such an arrangement would require a disproportionate share of the entire human genome. Recombinant DNA analysis has illuminated the mechanisms by which a limited number of immunoglobulin genes can encode this vast number of antibodies.

Each antibody molecule consists of several different polypeptide chainsthe light chains (L) and the longer heavy chains (H). The latter determine to which of five different classes (IgM, IgG, IgA, IgD, or IgE) an immunoglobulin belongs. Both the L and H chains are unique among proteins in that they contain constant and variable parts. The constant parts have relatively identical amino acid sequences in any given antibody. The variable parts, on the other hand, have different amino acid sequences in each antibody molecule. It is the variable parts, then, that determine the specificity of the antibody.

Recombinant DNA studies of immunoglobulin genes in mice have revealed that the light-chain genes are encoded in four separate parts in germ-line DNA: a leader segment (L), a variable segment (V), a joining segment (J), and a constant segment (C). These segments are widely separated in the DNA of an embryonic cell, but in a mature B lymphocyte they are found in relative proximity (albeit separated by introns). The mouse has more than 200 light-chain variable region genes, only one of which will be incorporated into the proximal sequence that codes for the antibody production in a given B lymphocyte. Antibody diversity is greatly enhanced by this system, as the V and J segments rearrange and assort randomly in each B-lymphocyte precursor cell. The mechanisms by which this DNA rearrangement takes place are not clear, but transposons are undoubtedly involved. Similar combinatorial processes take place in the genes that code for the heavy chains; furthermore, both the light-chain and heavy-chain genes can undergo somatic mutations to create new antibody-coding sequences. The net effect of these combinatorial and mutational processes enables the coding of millions of specific antibody molecules from a limited number of genes. It should be stressed, however, that each B lymphocyte can produce only one antibody. It is the B lymphocyte population as a whole that produces the tremendous variety of antibodies in humans and other mammals.

Plasma cell tumours (myelomas) have made it possible to study individual antibodies, since these tumours, which are descendants of a single plasma cell, produce one antibody in abundance. Another method of obtaining large amounts of a specific antibody is by fusing a B lymphocyte with a rapidly growing cancer cell. The resultant hybrid cell, known as a hybridoma, multiplies rapidly in culture. Since the antibodies obtained from hybridomas are produced by clones derived from a single lymphocyte, they are called monoclonal antibodies.

As has been stated, cellular immunity is mediated by T lymphocytes that can recognize infected body cells, cancer cells, and the cells of a foreign transplant. The control of cellular immune reactions is provided by a linked group of genes, known as the major histocompatibility complex (MHC). These genes code for the major histocompatibility antigens, which are found on the surface of almost all nucleated somatic cells. The major histocompatibility antigens were first discovered on the leukocytes (white blood cells) and are therefore usually referred to as the HLA (human leukocyte group A) antigens.

The advent of the transplantation of human organs in the 1950s made the question of tissue compatibility between donor and recipient of vital importance, and it was in this context that the HLA antigens and the MHC were elucidated. Investigators found that the MHC resides on the short arm of chromosome 6, on four closely associated sites designated HLA-A, HLA-B, HLA-C, and HLA-D. Each locus is highly polymorphic; i.e., each is represented by a great many alleles within the human gene pool. These alleles, like those of the ABO blood group system, are expressed in codominant fashion. Because of the large number of alleles at each HLA locus, there is an extremely low probability of any two individuals (other than siblings) having identical HLA genotypes. (Since a person inherits one chromosome 6 from each parent, siblings have a 25 percent probability of having received the same paternal and maternal chromosomes 6 and thus of being HLA matched.)

Although HLA antigens are largely responsible for the rejection of organ transplants, it is obvious that the MHC did not evolve to prevent the transfer of organs from one person to another. Indeed, information obtained from the histocompatibility complex in the mouse (which is very similar in its genetic organization to that of the human) suggests that a primary function of the HLA antigens is to regulate the number of specific cytotoxic T killer cells, which have the ability to destroy virus-infected cells and cancer cells.

More is known about the genetics of the blood than about any other human tissue. One reason for this is that blood samples can be easily secured and subjected to biochemical analysis without harm or major discomfort to the person being tested. Perhaps a more cogent reason is that many chemical properties of human blood display relatively simple patterns of inheritance.

Certain chemical substances within the red blood cells (such as the ABO and MN substances noted above) may serve as antigens. When cells that contain specific antigens are introduced into the body of an experimental animal such as a rabbit, the animal responds by producing antibodies in its own blood.

In addition to the ABO and MN systems, geneticists have identified about 14 blood-type gene systems associated with other chromosomal locations. The best known of these is the Rh system. The Rh antigens are of particular importance in human medicine. Curiously, however, their existence was discovered in monkeys. When blood from the rhesus monkey (hence the designation Rh) is injected into rabbits, the rabbits produce so-called Rh antibodies that will agglutinate not only the red blood cells of the monkey but the cells of a large proportion of human beings as well. Some people (Rh-negative individuals), however, lack the Rh antigen; the proportion of such persons varies from one human population to another. Akin to data concerning the ABO system, the evidence for Rh genes indicates that only a single chromosome locus (called r) is involved and is located on chromosome 1. At least 35 Rh alleles are known for the r location; basically the Rh-negative condition is recessive.

A medical problem may arise when a woman who is Rh-negative carries a fetus that is Rh-positive. The first such child may have no difficulty, but later similar pregnancies may produce severely anemic newborn infants. Exposure to the red blood cells of the first Rh-positive fetus appears to immunize the Rh-negative mother, that is, she develops antibodies that may produce permanent (sometimes fatal) brain damage in any subsequent Rh-positive fetus. Damage arises from the scarcity of oxygen reaching the fetal brain because of the severe destruction of red blood cells. Measures are available for avoiding the severe effects of Rh incompatibility by transfusions to the fetus within the uterus; however, genetic counselling before conception is helpful so that the mother can receive Rh immunoglobulin immediately after her first and any subsequent pregnancies involving an Rh-positive fetus. This immunoglobulin effectively destroys the fetal red blood cells before the mothers immune system is stimulated. The mother thus avoids becoming actively immunized against the Rh antigen and will not produce antibodies that could attack the red blood cells of a future Rh-positive fetus.

Human serum, the fluid portion of the blood that remains after clotting, contains various proteins that have been shown to be under genetic control. Study of genetic influences has flourished since the development of precise methods for separating and identifying serum proteins. These move at different rates under the impetus of an electrical field (electrophoresis), as do proteins from many other sources (e.g., muscle or nerve). Since the composition of a protein is specified by the structure of its corresponding gene, biochemical studies based on electrophoresis permit direct study of tissue substances that are only a metabolic step or two away from the genes themselves.

Electrophoretic studies have revealed that at least one-third of the human serum proteins occur in variant forms. Many of the serum proteins are polymorphic, occurring as two or more variants with a frequency of not less than 1 percent each in a population. Patterns of polymorphic serum protein variants have been used to determine whether twins are identical (as in assessing compatibility for organ transplants) or whether two individuals are related (as in resolving paternity suits). Whether the different forms have a selective advantage is not generally known.

Much attention in the genetics of substances in the blood has been centred on serum proteins called haptoglobins, transferrins (which transport iron), and gamma globulins (a number of which are known to immunize against infectious diseases). Haptoglobins appear to relate to two common alleles at a single chromosome locus; the mode of inheritance of the other two seems more complicated, about 18 kinds of transferrins having been described. Like blood-cell antigen genes, serum-protein genes are distributed worldwide in the human population in a way that permits their use in tracing the origin and migration of different groups of people.

Hundreds of variants of hemoglobin have been identified by electrophoresis, but relatively few are frequent enough to be called polymorphisms. Of the polymorphisms, the alleles for sickle-cell and thalassemia hemoglobins produce serious disease in homozygotes, whereas others (hemoglobins C, D, and E) do not. The sickle-cell polymorphism confers a selective advantage on the heterozygote living in a malarial environment; the thalassemia polymorphism provides a similar advantage.

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Human genetics | biology | Britannica.com


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