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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 a 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, Tay-Sachs disease.

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 a X-linked trait is Coffin-Lowry 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.

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Human genetics - Wikipedia, the free encyclopedia

Dr. Karin Blakemore
Dr. Blakemore is the Director of Maternal Fetal Medicine and Prenatal Genetics whose clinical practice focuses on caring for expectant mothers and their deve...

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Human evolutionary genetics studies how one human genome differs from the other, the evolutionary past that gave rise to it, and its current effects. Differences between genomes have anthropological, medical and forensic implications and applications. Genetic data can provide important insight into human evolution.

Biologists classify humans, along with only a few other species, as great apes (species in the family Hominidae). The Hominidae include two distinct species of chimpanzee (the bonobo, Pan paniscus, and the common chimpanzee, Pan troglodytes), two species of gorilla (the western gorilla, Gorilla gorilla, and the eastern gorilla, Gorilla graueri), and two species of orangutan (the Bornean orangutan, Pongo pygmaeus, and the Sumatran orangutan, Pongo abelii).

Apes, in turn, belong to the primates order (>400 species). Data from both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) indicate that primates belong to the group of Euarchontoglires, together with Rodentia, Lagomorpha, Dermoptera, and Scandentia.[1] This is further supported by Alu-like short interspersed nuclear elements (SINEs) which have been found only in members of the Euarchontoglires.[2]

A phylogenetic tree like the one shown above is usually derived from DNA or protein sequences from populations. Often mitochondrial DNA or Y chromosome sequences are used to study ancient human demographics. These single-locus sources of DNA do not recombine and are almost always inherited from a single parent, with only one known exception in mtDNA.[3] Individuals from the various continental groups tend to be more similar to one another than to people from other continents. The tree is rooted in the common ancestor of chimpanzees and humans, which is believed to have originated in Africa. Horizontal distance in the diagram corresponds to two things:

Chimpanzees and humans belong to different genera, indicated in red. Formation of species and subspecies is also indicated, and the formation of races is indicated in the green rectangle to the right (note that only a very rough representation of human phylogeny is given). Note that vertical distances are not meaningful in this representation.

The separation of humans from their closest relatives, the African apes (chimpanzees and gorillas), has been studied extensively for more than a century. Five major questions have been addressed:

As discussed before, different parts of the genome show different sequence divergence between different hominoids. It has also been shown that the sequence divergence between DNA from humans and chimpanzees varies greatly. For example the sequence divergence varies between 0% to 2.66% between non-coding, non-repetitive genomic regions of humans and chimpanzees.[5] Additionally gene trees, generated by comparative analysis of DNA segments, do not always fit the species tree. Summing up:

The divergence time of humans from other apes is of great interest. One of the first molecular studies, published in 1967 measured immunological distances (IDs) between different primates.[7] Basically the study measured the strength of immunological response that an antigen from one species (human albumin) induces in the immune system of another species (human, chimpanzee, gorilla and Old World monkeys). Closely related species should have similar antigens and therefore weaker immunological response to each other's antigens. The immunological response of a species to its own antigens (e.g. human to human) was set to be 1.

The ID between humans and gorillas was determined to be 1.09, that between humans and chimpanzees was determined as 1.14. However the distance to six different Old World monkeys was on average 2.46, indicating that the African apes are more closely related to humans than to monkeys. The authors consider the divergence time between Old World monkeys and hominoids to be 30 million years ago (MYA), based on fossil data, and the immunological distance was considered to grow at a constant rate. They concluded that divergence time of humans and the African apes to be roughly ~5 MYA. That was a surprising result. Most scientists at that time thought that humans and great apes diverged much earlier (>15 MYA).

The gorilla was, in ID terms, closer to human than to chimpanzees; however, the difference was so slight that the trichotomy could not be resolved with certainty. Later studies based on molecular genetics were able to resolve the trichotomy: chimpanzees are phylogenetically closer to humans than to gorillas. However, the divergence times estimated later (using much more sophisticated methods in molecular genetics) do not substantially differ from the very first estimate in 1967.

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Human evolutionary genetics - Wikipedia, the free encyclopedia

CHICAGO (AP) Dr. Janet Rowley, a pioneer in cancer genetics research, has died at age 88.

Rowley spent most of her career at the University of Chicago, where she also obtained her medical degree. She died Tuesday of ovarian cancer complications at her home nearby, the university said in a statement.

Rowley conducted landmark research with leukemia in the 1970s, linking cancer with genetic abnormalities work that led to targeted drug treatment for leukemia. She identified a genetic process called translocation, now widely accepted. By 1990, more than 70 translocations had been identified in various cancers, according to her biography on the National Library of Medicine's website.

She is a recipient of the National Medal of Science, the nation's highest scientific honor and the Presidential Medal of Freedom, the nation's highest civilian honor.

"Janet Rowley's work established that cancer is a genetic disease," Mary-Claire King, president of the American Society of Human Genetics, said recently. "We are still working from her paradigm."

Rowley, known among colleagues for her intelligence and humility, called receiving the presidential award, in 2009, "quite remarkable."

"I've never regretted being in science and being in research," Rowley said at the time. "The exhilaration that one gets in making new discoveries is beyond description."

With her silvery hair and twinkling eyes, Rowley was a recognizable figure at the University of Chicago, often seen riding her bike around the South Side campus, even up until a few months ago despite her disease. She remained active in research until close to her death and hoped that her own cancer could contribute to understanding of the disease.

Just last month, she was well enough to attend a celebration of the 50th anniversary of the presidential medal in Washington alongside other previous recipients and this year's winners, who include several scientists, former President Bill Clinton, Oprah Winfrey, baseball's Ernie Banks and Loretta Lynn.

Rowley was born in New York City in 1925 and at age 15 won a scholarship to an advanced academic program at the University of Chicago. She went to medical school there when the quota was just three women in a class of 65, the university said.

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Janet Rowley, cancer genetics pioneer, dies at 88

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Newswise Singapore, 18 December 2013 A recent study led by scientists from the National University of Singapore (NUS) opens a possible new route for treatment of Spinal Muscular Atrophy (SMA), a devastating disease that is the most common genetic cause of infant death and also affects young adults. As there is currently no known cure for SMA, the new discovery gives a strong boost to the fight against SMA.

SMA is caused by deficiencies in the Survival Motor Neuron (SMN) gene. This gene controls the activity of various target genes. It has long been speculated that deregulation of some of these targets contributes to SMA, yet their identity remained unknown.

Using global genome analysis, the research team, led by Associate Professor Christoph Winkler of the Department of Biological Sciences at the NUS Faculty of Science and Dr Kelvin See, a former A*STAR graduate scholar in NUS who is currently a Research Fellow at the Genome Institute of Singapore (GIS), found that deficiency in the SMN gene impairs the function of the Neurexin2 gene. This in turn limits the neurotransmitter release required for the normal function of nerve cells. The degeneration of motor neurons in the spinal cord causes SMA. This is the first time that scientists establish an association between Neurexin2 and SMA.

Preliminary experimental data also showed that a restoration of Neurexin2 activity can partially recover neuron function in SMN deficient zebrafish. This indicates a possible new direction for therapy of neurodegeneration.

Collaborating with Assoc Prof Winkler and the NUS researchers are Dr S. Mathavan and his team at GIS, as well as researchers from the University of Wuerzburg in Germany. The breakthrough discovery was first published in scientific journal Human Molecular Genetics last month.

Small zebrafish provides insights into human neurodegenerative disease

SMA is a genetic disease that attacks a distinct type of nerve cells called motor neurons in the spinal cord. The disease has been found to be caused by a defect in the SMN gene, a widely used gene that is responsible for normal motor functions in the body.

To study how defects in SMN cause neuron degeneration, the scientists utilised a zebrafish model, as the small fish has a relatively simple nervous system that allows detailed imaging of neuron behaviour.

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Study Provides New Insights Into Cause of Human Neurodegenerative Disease

A team of geneticists has identified a possible link between mutations that cause early cognitive impairments, such as dyslexia, and schizophrenia and autism.

The study was led by members of an Icelandic biopharmaceutical company specialising in the human genome, called deCODE genetics, and was based on work done by those before them into possible links between copy number variant (CNV) mutations and schizophrenia and autism.

CNVs occur when parts of the genome have an abnormal number of copies -- this could be represented as a deletion or duplication of a section of a particular chromosome. A number of these CNVs have in the past been identified in those suffering from psychiatric disorders, and the deCODE genetics team sought to track down how these markers alter the brain over time by comparing the genetics of sufferers of psychiatric disorders against healthy volunteers that carry those same mutations.

"In a small fraction of patients with schizophrenia or autism, alleles of CNVs in their genomes are probably the strongest factors contributing to the pathogenesis of the disease," write the authors in the paper, published in Nature. "These CNVs may provide an entry point for investigations into the mechanisms of brain function and dysfunction alike."

Working alongside the Central Institute of Mental Health in Mannheim, Germany, the team used a genealogical database of more than 100,000 Icelanders to track down carriers of the mutations. They found 26 CNV alleles, already identified as being markers for an increased predisposition of the disorders, in just 1.16 percent of candidates -- those 1,178 people carried one or more of the mutations each. According to a report by medwireNews, of these 167 carried specific neuropsychiatric-related CNVs but had not been diagnosed with any such condition.

The team then went about administering a series of neuropsychiatric and cognitive tests to those 167 individuals, along with a healthy control group, schizophrenia sufferers and carriers of other unrelated CNVs.

What they found, was a distinct link between mild cognitive impairments and CNVs linked to neuropsychiatric disorders, which makes sense, considering autism and schizophrenia are cognitive impairments. The carriers of neuropsychiatric-linked CNVs performed significantly worse in cognitive tasks than those with unrelated CNVs, and were more likely to have a history of learning disabilities such as dyslexia. They did, however, perform far better than patients with schizophrenia.

Digging further, the team broke down the specific CNVs. They found that those that performed poorly in the cognitive tasks and also had a history of dyslexia and dyscalculia carried the same CNV -- a deletion in chromosome 15, known as 15q11.2. Carrying out MRI scans of these volunteers' brains, they found the structure had altered in the same regions that are altered in patients with early signs of schizophrenia and in those with dyslexia

"This study provides one of the first footholds into biochemical understanding of humans' unique cognitive abilities," lead author on the study and deCODE genetics CEO Kari Stefansson said in a statement. "The findings also provide insight into which cognitive abilities put individuals at risk of developing schizophrenia and demonstrate that control carriers provide an opportunity to study cognitive abnormalities without the confounding effects of psychosis or medication."

This is not the first time a significant genetic link has been made between different cognitive impairments. Earlier this year a paper published in Nature Communications revealed the results of a novel study that involved the descendants of those living in isolated small towns in northern Finland, where cases of neuropsychiatric disorders are unusually common.

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Genetic markers for schizophrenia linked to unrelated cognitive impairments

Data obtained from a Neanderthal woman's toe bone points to incest and inbreeding among early humans, an international genetics team reported on Wednesday.

The fossil's genetic map, or genome, reported from Denisova cave in Siberia's Altai Mountains dates to more than 50,000 years ago. The cave was home at separate times to both Neanderthals and the so-called Denisovans, two sister families of now-extinct early humans. (See also "New Type of Ancient Human Found.")

Adding to increasing evidence of a tangled human family tree, the new Neanderthal genome study released by the journal Nature also suggests that another previously unknown archaic human species shared its genes with some of our ancestors. The study authors suggest that it was Homo erectus, one of the earliest human species, which first arose around 1.8 million years ago. (See also "Why Am I a Neanderthal?")

The report, led by Germany's Kay Prfer of the Max Planck Institute for Evolutionary Anthropology in Leipzig, builds on recent prehistoric genetics results that argue against theories that modern humans arose completely from one "out of Africa" migration more than 60,000 years ago that spread worldwide without mating with other early humans.

Instead, it looks like early modern humans sometimes mated with archaic human cousins they met along the way. People of non-African origin broadly have genes that are 1.5 percent to 2.1 percent Neanderthal, according to the study, with proportions higher among Asians and Native Americans. Similarly, 5 percent of the genome of people of Australian and Papua New Guinea descent looks Denisovan, as does 0.2 percent of the genes of people from Asia.

"We don't have one ancestral group, but proportions of ancestral groups," says computational biologist Rasmus Nielsen of the University of California, Berkeley, who was not part of the study team. "I think they make a convincing argument."

"In my view, this paper heralds the completion of the Neanderthal genome project in terms of mapping an entire genome," says paleontologist and human origins expert Richard Potts of the Smithsonian's National Museum of Natural History in Washington, D.C. "That's pretty cool science."

Kissing Cousins

In 2010, the study's toe bone first turned up at Denisova Cave, where excellent fossil preservation conditions had allowed for the genetic mapping of the then-surprising Denisovan finger bone found in 2008. Gene tests showed the toe belonged to a Neanderthal, and Prfer and colleagues began calculating its full genetic map.

Photograph by B. Viola, MPI f. Evolutionary Anthropology

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Ancient Incest Uncovered in Neanderthal Genome