Human genetics

A discipline concerned with genetically determined resemblances and differences among human beings. Technological advances in the visualization of human chromosomes have shown that abnormalities of chromosome number or structure are surprisingly common and of many different kinds, and that they account for birth defects or mental impairment in many individuals as well as for numerous early spontaneous abortions. Progress in molecular biology has clarified the molecular structure of chromosomes and their constituent genes and the ways in which change in the molecular structure of a gene can lead to a disease. Concern about possible genetic damage through environmental agents and the possible harmful effects of hazardous substances in the environment on prenatal development has also stimulated research in human genetics. The medical aspects of human genetics have become prominent as nonhereditary causes of ill health or early death, such as infectious disease or nutritional deficiency, have declined, at least in developed countries.

In normal humans, the nucleus of each normal cell contains 46 chromosomes, which comprise 23 different pairs. Of each chromosome pair, one is paternal and the other maternal in origin. In turn, only one member of each pair is handed on through the reproductive cell (egg or sperm) to each child. Thus, each egg or sperm has only 23 chromosomes, the haploid number; fusion of egg and sperm at fertilization will restore the double, or diploid, chromosome number of 46. See also Chromosome.

The segregation of chromosome pairs during meiosis allows for a large amount of “shuffling” of genetic material as it is passed down the generation. Two parents can provide 2²³ × 2²³ different chromosome combinations. This enormous source of variation is multiplied still further by the mechanism of crossing over, in which homologous chromosomes exchange segments during meiosis. See also Crossing-over (genetics); Meiosis.

Twenty-two of the 23 chromosome pairs, the autosomes, are alike in both sexes; the other pair comprises the sex chromosomes. A female has a pair of X chromosomes; a male has a single X, paired with a Y chromosome which he has inherited from his father and will transmit to each of his sons. Sex is determined at fertilization, and depends on whether the egg (which has a single X chromosome) is fertilized by an X-bearing or a Y-bearing sperm. See also Sex determination.

Any gene occupies a specific chromosomal position, or locus. The alternative genes at a particular locus are said to be alleles. If a pair of alleles are identical, the individual is homozygous; if they are different, the individual is heterozygous. See also Allele.

Genetic variation has its origin in mutation. The term is usually applied to stable changes in DNA that alter the genetic code and thus lead to synthesis of an altered protein. The genetically significant mutations occur in reproductive cells and can therefore be transmitted to future generations. Natural selection acts upon the genetic diversity generated by mutation to preserve beneficial mutations and eliminate deleterious ones.

A very large amount of genetic variation exists in the human population. Everyone carries many mutations, some newly acquired but others inherited through innumerable generations. Though the exact number is unknown, it is likely that everyone is heterozygous at numerous loci, perhaps as many as 20%. See also Mutation.

The patterns of inheritance of characteristics determined by single genes or gene pairs depend on two conditions: (1) whether the gene concerned is on an autosome (autosomal) or on the X chromosome (X-linked); (2) whether the gene is dominant, that is, expressed in heterozygotes (when it is present on only one member of a chromosomal pair and has a normal allele) or is recessive (expressed only in homozygotes, when it is present at both chromosomes). See also Dominance.

A quantitative trait is one that is under the control of many factors, both genetic and environmental, each of which contributes only a small amount to the total variability of the trait. The phenotype may show continuous variation (for example, height and skin color), quasicontinuous variation (taking only integer values—such as the number of ridges in a fingerprint), or it may be discontinuous (a presence/absence trait, such as diabetes or mental retardation). With discontinuous traits, it is assumed that there exists an underlying continuous variable and that individuals having a value of this variable above (or below) a threshold possess the trait.

A trait that “runs in families” is said to be familial. However, not all familial traits are hereditary because relatives tend to share common environments as well as common genes.

The variability of almost any trait is partly genetic and partly environmental. A rough measure of the relative importance of heredity and environment is an index called heritability. For example, in humans, the heritability of height is about 0.75. That is, about 75% of the total variance in height is due to variability in genes that affect height and 25% is due to exposure to different environments.

Hereditary diseases

Medical genetics has become an integral part of preventive medicine (that is, genetic counseling, including prenatal diagnostics). Hereditary diseases may be subdivided into three classes: chromosomal diseases; hereditary diseases with simple, mendelian modes of inheritance; and multifactorial diseases.

One out of 200 newborns suffers from an abnormality that is caused by a microscopically visible deviation in the number or structure of chromosomes. The most important clinical abnormality is Down syndrome—a condition due to trisomy of chromosome 21, one of the smallest human chromosomes. This chromosome is present not twice but three times; the entire chromosome complement therefore comprises 47, not 46, chromosomes. Down syndrome occurs one to two times in every 1000 births; its pattern of abnormalities derives from an imbalance of gene action during embryonic development. Down syndrome is a good example of a characteristic pattern of abnormalities that is produced by a single genetic defect. See also Down syndrome.

Other autosomal aberrations observed in living newborns that lead to characteristic syndromes include trisomies 13 and 18 (both very rare), and a variety of structural aberrations such as translocations (exchanges of chromosomal segments between different chromosomes) and deletions (losses of chromosome segments). Translocations normally have no influence on the health status of the individual if there is no gain or loss of chromosomal material (these are called balanced translocations). However, carriers of balanced translocations usually run a high risk of having children in whom the same translocation causes gain or loss of genetic material, and who suffer from a characteristic malformation syndrome.

Clinical syndromes caused by specific aberrations vary, but certain clinical signs are common: low birth weights (small for date); a peculiar face; delayed general, and especially mental, development, often leading to severe mental deficiency; and multiple malformations, including abnormal development of limbs, heart, and kidneys. See also Congenital anomalies.

Less severe signs than those caused by autosomal aberrations are found in individuals with abnormalities in number (and, sometimes, structure) of sex chromosomes. This is because in individuals having more than one X chromosome, the additional X chromosomes are inactivated early in pregnancy. For example, in women, one of the two X chromosomes is always inactivated. Inactivation occurs at random so that every normal woman is a mosaic of cells in which either one or the other X chromosome is active. Additional X chromosomes that an individual may have received will also be inactivated; in trisomies, genetic imbalance is thus avoided to a certain degree. However, inactivation is not complete; therefore, individuals with trisomies—for example, XXY (Klinefelter syndrome), XXX (triple-X syndrome), or XYY—or monosomies (XO; Turner syndrome) often show abnormal sexual development, intelligence, or behavior.

In contrast to chromosomal aberrations, the genetic defects in hereditary diseases with simple, mendelian modes of inheritance cannot be recognized by microscopic examination; as a rule, they must be inferred more indirectly from the phenotype and the pattern of inheritance in pedigrees. The defects are found in the molecular structure of the DNA. Often, one base pair only is altered, although sometimes more complex molecular changes, such as deletions of some bases or abnormal recombination, are involved. Approximately 1% of all newborns have, or will develop during their lives, a hereditary disease showing a simple mendelian mode of inheritance.

In medical genetics, a condition is called dominant if the heterozygotes deviate in a clearly recognizable way from the normal homozygotes, in most cases by showing an abnormality. Since such dominant mutations are usually rare, almost no homozygotes are observed.

In some dominant conditions, the harmful phenotype may not be expressed in a gene carrier (this is called incomplete penetrance), or clinical signs may vary in severeness between carriers (called variable expressivity). Penetrance and expressivity may be influenced by other genetic factors; sometimes, for example, by the sex of the affected person, whereas in other instances, the constitution of the “normal” allele has been implicated. Environmental conditions may occasionally be important. In most cases, however, the reasons are unknown.

X-linked modes of inheritance occur when the mutant allele is located on the X chromosome. The most important X-linked mode of inheritance is the recessive one. Here, the males (referred to as hemizygotes since they have only one allele) are affected, since they have no normal allele. The female heterozygotes, on the other hand, will be unaffected, since the one normal allele is sufficient for maintaining function. A classical example is hemophilia A, in which one of the serum factors necessary for normal blood clotting is inactive or lacking. (The disease can now be controlled by repeated substitution of the deficient blood factor—a good example for phenotypic therapy of a hereditary disease by substitution of a deficient gene product.) Male family members are affected whereas their sisters and daughters, while being unaffected themselves, transmit the mutant gene to half their sons. Only in very rare instances, when a hemophilic patient marries a heterozygous carrier, are homozygous females observed. See also Sex-linked inheritance.

There are thousands of hereditary diseases with simple mendelian modes of inheritance, but most common anomalies and diseases are influenced by genetic variability at more than one gene locus. Most congenital malformations, such as congenital heart disease, cleft lip and palate, neural tube defects and many others, fall into this category, as do the constitutional diseases, such as diabetes mellitus, coronary heart disease, anomalies of the immune response and many mental diseases, such as schizophrenia or affective disorders. All of these conditions are common and often increase in frequency with advanced age.

Biochemical genetics

Biochemical genetics began with the study of inborn errors of metabolism. These are diseases of the body chemistry in which a small molecule such as a sugar or amino acid accumulates in body fluids because an enzyme responsible for its metabolic breakdown is deficient. This molecular defect is the result of mutation in the gene coding for the enzyme protein. The accumulated molecule, dependent on its nature, is responsible for the causation of a highly specific pattern of disease.

The field of biochemical genetics expanded with the recognition that similar heritable defective enzymes interfere with the breakdown of very large molecules, such as mucopolysaccharides and the complex lipids that are such prominent components of brain substance. The resultant storage disorders present with extreme alterations in morphology and bony structure and with neurodegenerative disease.

The majority of hereditary disorders of metabolism are inherited in an autosomal recessive fashion. In these families, each parent carries a single mutant gene on one chromosome and a normal gene on the other. Most of these mutations are rare. In populations with genetic diversity, most affected individuals carry two different mutations in the same gene. Some metabolic diseases are coded for by genes on the X chromosome. Most of these disorders are fully recessive, and so affected individuals are all males, while females carrying the gene are clinically normal. The disorders that result from mutations in the mitochondrial genome are inherited in nonmendelian fashion because mitochondrial DNA is inherited only from the mother. Those that carry a mutation are heteroplasmic; that is, each carries a mixed population of mitochondria, some with the mutation and some without.

Phenylketonuria (PKU) is a prototypic biochemical genetic disorder. It is an autosomally recessive disorder in which mutations demonstrated in a sizable number of families lead, when present in the genes on both chromosomes, to defective activity of the enzyme that catalyzes the first step in the metabolism of phenylalanine. This results in accumulation of phenylalanine and a recognizable clinical disease whose most prominent feature is severe retardation of mental development. See also Phenylketonuria.

The diseases that result from mutation in mitochondrial DNA have been recognized as such only since the 1990s. They result from point mutations, deletions, and other rearrangements. A majority of these disorders express themselves chemically in elevated concentrations of lactic acid in the blood or cerebrospinal fluid. Many of the disorders are known as mitochondrial myopathies (diseases of muscles) because skeletal myopathy or cardiomyopathy are characteristic features.

Surely, people must always have been curious about how physical traits and aspects of character are passed down from generation to generation. But, until the middle of the nineteenth century, thoughts about heredity were extremely confused. Indeed, throughout the medieval period in Europe, all organisms were thought to grow from replicas created by God at the beginning of the universe. In these terms, Eve literally contained within her ovaries the whole of the rest of the human race, generation after generation of miniatures packed one inside the other, each waiting for an act of fertilization. Even after the discovery of sperm this ‘pre-formation hypothesis’ was not discarded, and some held that an individual was pre-formed in the sperm, and simply nurtured by the mother. Towards the end of the nineteenth century, anatomists found that they could see thread-like objects, which could be stained with coloured dyes, inside dividing cells. They called these structures chromosomes, and later suggested that they might carry heritable information from cell to cell. At about the same time, based on a series of breeding experiments on ornamental plants, Gregor Mendel formulated the concept that later became known as the gene — a unit of heredity, passed from generation to generation in a way that follows simple mathematical laws. These disparate observations were finally unified during the middle of the twentieth century with the discovery that genetic information carried by chromosomes resides in the double helix of deoxyribonucleic acid (DNA), first described by Francis Crick and James Watson. DNA has the remarkable property of self-replication — it copies itself faithfully each time a cell divides. It was soon clear that genes consist of lengths of DNA, and that their major function is to direct the synthesis of proteins, ensuring that the structure of proteins is always the same and that they are made in the right amount in the right place at the right time.

Proteins are made up of 20 different amino acids, and the extensive differences in their structures — from the fluid haemoglobin of our blood to the tough keratin of our skin — reflect differences in the number and order of amino acids in their constituent peptide chains. The information responsible for selecting and arranging the amino acids in these peptide chains is encoded by the order of the four bases that make up each strand of the DNA helix — adenine (A), guanine (G), cytosine (C), and thymine (T). Each individual amino acid is represented by a unique three-letter ‘word’, spelled out by a particular sequence of three bases in the gene. Even more remarkable, the code is the same throughout almost all of the animal kingdom.

In recent years it has been feasible to isolate human DNA, cut it into pieces, and insert these into bacterial cells, where they grow and multiply. In this way, it has been possible to prepare ‘libraries’ containing most of a person's genes (the ‘genome’), to isolate individual genes, and to examine their structure and function.

The human genome

Normal human cells (except eggs and sperm) have 46 chromosomes, arranged in pairs, one member of each pair coming from each parent. Twenty-two pairs are called autosomes and the other pair are the sex chromosomes, designated X and Y. Females have two X chromosomes (one from the mother, the other from the father) while males have one maternal X and one paternal Y sex chromosome. ‘Germ cells’ (sperm and eggs) are unusual in having only 23 unpaired chromosomes, each of which is created by a process in which maternal and paternal chromosome pairs become closely wound round each other in the germinal cells that give rise to the eggs and sperm. The closer together a pair of genes are on the same chromosome the less chance they will have to cross over from one to another. Hence the number of crossovers is a measure of the distance between genes. Indeed, many years ago it was realized that if two different genes are on the same chromosome, and particularly if they are close together, they will tend to be inherited together. They are then said to be linked.

Through studies of individual families, and later by generating genetic markers by analysing DNA itself, geneticists have been able to obtain a ‘linkage map’ of the human chromosomes, assigning many different genes to particular chromosomes. In essence, this looks like a road map in which the towns (genes) are clearly marked, although it tells us nothing about the state of the roads (DNA) in between. Early in the new millennium, as part of the Human Genome Project, this work will be extended, and the complete sequence of all the bases that make up the 30-40 000 genes of the human genome will be worked out.

Mutations and human diversity

Occasionally, during cell division, when new strands of DNA are synthesized, a different base is inserted, by mistake, into a gene. This is called a mutation. Without this slight imperfection in the mechanism of DNA replication we would still be swimming round in the primeval soup. For, while many mutations, or polymorphisms, are neutral (that is, the slight changes in the proteins produced have no effect on the function of the organism), others may be beneficial. Although it is still a topic of some controversy, there is general agreement that this is the way in which Darwinian evolution has occurred; the occurrence of mutations occasionally leads to changes in organisms that enable them to adapt better to their current environments, or to new ones. While this may not be the only mechanisms for the gradual emergence of different species there is compelling evidence that it has been a major force behind human evolution. The existence of polymorphisms, and our new-found ability to analyse DNA with ease, are providing major insights into the origins of human beings, the ways in which different races have evolved, and, indeed, the whole basis of human diversity. As well as the DNA that resides in the nuclei of our cells, our mitochondria, the chemical dynamos that energize the cell, have their own DNA. Since mitochondrial DNA is all derived from our mothers, it is of particular value in tracing our evolutionary past in a direct line.

Harmful mutations

Unfortunately, not all mutations are simply neutral or even beneficial; occasionally they cause disease, because they affect some vital process in the body. Inherited diseases due to single defective genes usually follow Mendelian patterns of inheritance. Some are said to be ‘dominant’ because they occur with the inheritance of a single defective gene on only one of the pair of chromosomes. Others are ‘recessive’ — it is necessary to inherit the mutant gene from both parents to have the disease. ‘Carriers’, or ‘heterozygotes’, are individuals who have a recessive disease gene on only one of the pair of chromosomes, and who therefore are not affected by the disease but can pass it on their children if they mate with a partner who is also a carrier. Yet other diseases are sex-linked; that is, they are carried on the X chromosome and, if recessive, are not expressed in females (who have two X chromosomes) but may be transmitted to a son to whom the mother passes down an X chromosome bearing the mutant gene.

Although there are over 4000 single-gene (monogenic) disorders, most are very rare because they produce such severe disorders that they usually prevent reproduction, and therefore cannot disseminate within a population. However, a few monogenic diseases — the inherited anaemias, sickle cell anaemia and thalassaemia, for example — are extremely common. This is because carriers are made more resistant to malaria by the presence of the mutation in their cells. Therefore they tend to survive slightly longer than non-affected individuals in malarious countries, and hence have more children. In this way, the frequency of the disease increases until it reaches an equilibrium, at which it is balanced by the loss from the community of severely affected homozygotes (those who have received the defective gene from both parents). This kind of interaction may explain why cystic fibrosis, a disease that affects the lung and bowel, is so common in European populations: the mutation may have made carriers resistant to one or more of the severe infections that swept through Europe in the past, possibly cholera.

Many of the common diseases of Western society — heart attacks, stroke, diabetes, and dementia, for example — are the result of complex interactions between our genetic make-up and the environment. These diseases do not run predictably through families like single-gene disorders, but there may be a familial tendency, which probably reflects the action of several different genes combined with the effects of lifestyle. Other common diseases, cancer in particular, also reflect interactions between our genes and environments.

Many cancers result from the acquisition of mutations in a family of genes called oncogenes, which normally serve important housekeeping functions for our cells. They tell the cells how and when to divide, identify those with damaged DNA and either ensure that it is repaired or programme the potentially harmful cells to die, and regulate how they interact with their fellow cells. Cancer appears to be due to the acquisition of mutations in these genes; the frequency with which this occurs may be related both to exposure to environmental mutagens such as tobacco smoke, and to chemical agents that are by-products of our metabolism.

Other genetic defects are responsible for congenital malformation and for mental retardation. Sometimes these conditions result from major chromosomal abnormalities, involving either their numbers or structure. However, many malformations of this type can be traced to the action of single mutant genes — an observation that is providing clues about the mechanisms that control human development.

Manipulating the human genome

Our new-found ability to analyse human DNA, which has led to the discovery of the mutations that cause many monogenic diseases, means that they can be identified in carriers and appropriate counselling and advice can be given. It is also now possible to diagnose most of these diseases prenatally by chorionic villus sampling — removing and genetically analysing a tiny amount of tissue from round the fetus between 9 and 13 weeks of pregnancy. This allows prenatal diagnosis, and gives the mother the option of termination of a pregnancy if the fetus has a particularly severe condition. Soon it may be feasible to correct genetic disease or alter the genetic machinery of cells in a way that may be used to treat cancer or other acquired diseases. The treatment of single-gene disorders can, in principle, be approached by either germ-cell gene therapy (in which a ‘good’ gene is injected into a fertilized egg and therefore distributes among all the cells of the body, including future germ cells) or somatic-cell therapy (in which the gene is inserted into a particular cell population in the body of one individual — the stem cells of the bone marrow for example). Although germ cell gene therapy offers the prospect of eliminating disease from a family for all future generations, it is currently not permitted because of the possible risk of making damaging errors to the genes. On the other hand, somatic-cell gene therapy poses no major ethical problems beyond those of any form of tissue or organ transplantation. Human genes can also be inserted into cultured cells or whole animals (a process called transgenesis) in order for the recipient cells or animals to produce molecules of therapeutic value — insulin to treat diabetes, or clotting factors to treat haemophilia, for example.

Biological determinism and the future

The remarkable developments in human genetics over the last half of the twentieth century led to the notion that most of people's mental achievements, personality traits, and behaviour can be explained by their genetic make-up, shaped in the past by Darwinian evolution. Much of this thinking ignores the important role of the environment in making us what we are. But because it carries such conviction, and because it is assumed that we will gradually learn how to manipulate our genetic make-up, this view is causing considerable concern about the possible resurgence of the eugenics movement — a philosophy for the ‘betterment’ of mankind by selective breeding, which stimulated the study of human genetics at the end of the nineteenth century. Although it will be a long time before we know the relative role of nature and nurture in shaping human beings, there is little doubt that our ability to modify the human genome will increase dramatically during the new millennium, and society will be faced with the dilemma of deciding how far it wishes to shape its destiny in this way.

— D. J. Weatherall

Bibliography

• Bodmer, W. and McKie, R. (1994). The book of man. Little, Brown and Co, London.
• Bowdler, P. J. (1989). The Mendelian revolution. Johns Hopkins University Press, Baltimore.
• Jones, S. (1993). The language of the genes. Harper Collins, London.
• Lewin, B. (1997). Genes VI. Oxford University Press, Oxford.
• Raskó, I. and Downes, C. S. (1995). Genes in medicine. Chapman and Hall, London

See also evolution, human; gene therapy; heredity.

The study of the genetic aspects of humans as a species.

A karyotype of a human male, showing 46 chromosomes including XY sex chromosomes.

Human genetics describes the study of inheritance as it occurs in human beings. This article describes only basic features of human genetics; for the genetics of disorders please see: Medical genetics.

Chromosomes

Humans have 46 chromosomes, arranged in 23 pairs (i.e. they are diploid). 44 (22 pairs) of these chromosomes are autosomes, and 2 (1 pair) are sex chromosomes. Humans have an XY sex determination system, so that females have the sex chromosomes XX and the males XY. The Y chromosome is shorter than the X chromosome, so that males are hemizygous over this region. X-linked recessive genes are thus expressed more often in males. A Humans' gender is determined by the x and the y chromosomes.

Number of genes

Estimates of the number of genes humans have has been possible since DNA sequencing was first introduced. Estimates however have varied wildly, though the present best guess is 20,000-25,000, estimates of up to 40,000 have been in the past.

Mitochondrial DNA

In addition to nuclear DNA, humans (like almost all eukaryotes) have mitochondrial DNA. Mitochondria, the "power houses" of a cell, have their own DNA because they are descended from a proteobacterium that merged with eukaryotic cells over 2 billion years ago. Mitochondria are inherited from one's mother, and its DNA is frequently used to trace maternal lines of descent (see mitochondrial Eve).

Genes and human characteristics

Genes have both minor and major effects on human characteristics. Human genes have become prominent in the nature versus nurture debate.

Genes and behavior

Genes have a strong influence on human behavior. IQ is largely heritable. However, this has been questioned. The stance that humans inherit substantial behavioral characteristics is called psychological nativism, compared to the stance that human behavior and culture are virtually entirely constructed (tabula rasa).

In the early 20th century, eugenics was policy in parts of the United States and Europe. The goal was to reduce or eliminate people whose genes were considered inferior. One form of eugenics was compulsory sterilization of people deemed mentally unfit. Hitler's eugenics programs turned social consciousness against the practice, and psychological nativism became associated with racism and sexism.

Genes and gender

The biggest genetic difference among healthy humans is in gender. Scientists debate the extent to which genes and culture affect gender roles. The case of David Reimer was once a case in point for the tabula rasa camp, though recently that same case has become evidence for a strong genetic component to gender identity.

Genes and race

Most genetic diversity occurs within races rather than between them. Common concepts of racial categories do not accurately match genetic characteristics.

Evolutionary psychology

Evolutionary psychology explains many human behaviors as more or less moderated by genes that evolved in the hunter-gatherer stage of human cultural development. See for example Stockholm syndrome.

Genetic disorders

Main article: Genetic disorder

Humans have several genetic diseases, often caused by recessive genes. See List of genetic disorders. Genetic disorders happen everywhere and are very common in some places.

Human traits with simple inheritance patterns

Dominant	Recessive	References
Widow's Peak	No Widow's Peak	[1][2]
Facial Dimples	No Facial Dimples	[3][4]
Able to taste PTC	Unable to taste PTC	[5]
Unattached earlobe	Attached earlobe	[3][6][7]
Cleft chin	No Cleft chin	[8]
Freckles	No Freckles	[3][9]
Wet-type earwax	Dry-type earwax	[6][10]

See also

• Human evolutionary genetics
• Human genome
• Genetic genealogy
• Genealogical DNA test

Human mitochondrial DNA (mtDNA) haplogroups

most recent common mt-ancestor

L0

L1

L2

L3

L4

L5

L6

L7

M

N

CZ

D

E

G

Q

A

I

O

R

S

W

X

Y

C

Z

B

F

pre-HV

pre-JT

P

UK

HV

JT

U

K

H

V

J

T

Human Y-chromosome DNA (Y-DNA) haplogroups

Y-most recent common ancestor

|

A

BR

B

CR

C

DE

F

D

E

G

H

IJ

K

I

J

L

M

NO

P

N

O

Q

R

References

1. ^ Campbell, Neil; Jane Reece (2005). Biology. San Francisco: Benjamin Cummings, pp. 265. 
2. ^ Online Mendelian Inheritance in Man, ID=194000 [1]
3. ^ ^{a b c} http://www.science.edu.sg/ssc/detailed.jsp?artid=4862&type=6&root=4&parent=4&cat=40
4. ^ Online Mendelian Inheritance in Man, ID=126100 [2]
5. ^ http://www.medicalnewstoday.com/articles/10009.php
6. ^ ^{a b} Cruz-Gonzalez L., Lisker R. (1982). "Inheritance of ear wax types, ear lobe attachment and tongue rolling ability.". Acta Anthropogenet. 6 (4): 247-54. PMID 7187238. 
7. ^ Online Mendelian Inheritance in Man, ID=128900 [3]
8. ^ Online Mendelian Inheritance in Man, ID=119000 [4]
9. ^ Xue-Jun Zhang et al. "A Gene for Freckles Maps to Chromosome 4q32–q34" Journal of Investigative Dermatology (2004) 122, 286–290 [5]
10. ^ Online Mendelian Inheritance in Man, ID=117800 [6]

External links

• Human Genome Project
• How many Genes do humans have?

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Human genetics
Human evolutionary genetics · Human genetic engineering · Human genetic history · Human genetic variation · Human genome · Human evolution

Human chromosomes
{1} {2} {3} {4} {5} {6} {7} {8} {9} {10} {11} {12} {13} {14} {15} {16} {17} {18} {19} {20} {21} {22} {X} {Y}

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