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.
Genetic differences and inheritance patterns 
Autosomal dominant inheritance 
Autosomal traits are associated with a single gene on an autosome (non-sex chromosome)—they are called "dominant" because a single copy—inherited from either parent—is 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 inheritance 
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.
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 form 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, the mating between closely related organisms of traits can clearly be seen on pedigree charts. Pedigree charts of royal families 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 couple 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.
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.
Genomics refers to the field of genetics concerned with structural and functional studies of the genome. 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. 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.
Population 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. 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.
Hardy-Weinberg principle 
The Hardy-Weinberg principle states that when no evolution occurs in a population, the allele and genotype frequencies do not change from one generation to the next. No evolution refers to no mutation, no gene flow, no natural selection, and no genetic drift. To be in equilibrium two more assumptions need to be made that random mating occurs and there are discrete, non-overlapping generations.
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. Mitochondria are inherited from one's mother, and its 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.
Genes and gender 
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 carring two or more copies of the X chromosome, resulting in a potentially toxic dose of X-linked genes.
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.
|X-link Dominant||X-link Recessive||References|
|Alport syndrome||absence of blood in urine|
|Coffin-Lowry syndrome||no cranial malformations|
|colour vision||colour blindness|
|normal clotting factor||haemophilia A & B|
|Strong muscle tissue||Duchenne Muscular Dystrophy|
|fragile X syndrome||normal X chromosome|
|Aicardi syndrome||absence of brain defects|
|absence of autoimmunity||IPEX syndrome|
|Xg Blood type||absence of antigen|
|production of GAGs||Hunter syndrome|
|normal muscle strength||Becker's muscular dystrophy|
|unaffected body||Fabry's disease|
|no progressive blindness||Choroideremia|
|no kidney damage||Dent's disease|
|Rett syndrome||no microcephaly|
|production of HGPRT||Lesch–Nyhan syndrome|
|high levels of copper||Menkes disease|
|normal immune levels||Wiskott–Aldrich syndrome|
|Focal dermal hypoplasia||normal pigmented skin|
|normal pigment in eyes||Ocular albinism|
|vitamin D resistant rickets||absorption of vitamin D|
|Synesthesia||non colour perception|
Human traits with simple inheritance patterns 
|This section's factual accuracy is disputed. (July 2012)|
|Low heart rate||High heart rate|||
|Widow's peak||straight hair line|||
|normal digestive muscle||POLIP syndrome|
|Facial dimples *||No facial dimples|||
|Able to taste PTC||Unable to taste PTC|||
|Unattached (free) earlobe||Attached earlobe|||
|Clockwise hair direction (left to right)||Counter-Clockwise hair direction (right to left)|||
|Cleft chin||smooth chin|||
|straight nose||turned up nose|
|no progressive nerve damage||Friedreich's ataxia|
|Ability to roll tongue (Able to hold tongue in a U shape)||No ability to roll tongue|
|extra finger or toe||Normal five fingers and toes|
|straight pinkies||Crooked pinkies|
|Straight Thumb||Hitchhiker's Thumb|
|Wet-type earwax||Dry-type earwax|||
|Curly hair||Straight hair|
|normal flat palm||Cenani Lenz syndactylism|
|shortness in fingers||Normal finger length|
|A and B blood type||O blood type|||
|Abundant body hair||Little body hair|
|Broad lips||Slender lips|
|Broad nose||narrow nose|
|High blood pressure||Low blood pressure|
|Webbed fingers||Normal separated fingers|
|dominant left thumb||dominant right Thumb|
|Mid digit hair||no mid digit hair|
|Morton's toe||big toe dominance|
|Hair on back of hand||no manus hair|
|Roman nose||no prominent bridge|||
|short stature||tall stature|
|large eyes||Small eyes|
|tone deafness||normal hearing|
|Darker hair||Lighter hair|||
|normal night vision||Night blindness|
|different colour temple highlights||same colour temple highlights|
|Clubbed thumb||regular size|
|oval-shaped face||square shape|||
|separated eyebrows||Joined eyebrow|
|long eyelashes||short eyelashes|
|Marfan's syndrome||normal body proportions|||
|Huntington disease||no nerve damage|||
|normal mucus lining||Cystic fibrosis|||
|Photic sneeze reflex||no ACHOO reflex|||
|forged chin||Receding chin|||
|White Forelock||Dark Forelock|||
|Straight thumb||curved thumb|
|Upington disease||Regular formed hip|
|ability to move ears||can't move ears|
|situs solitus left-sided heart||Situs inversus right-sided heart|
|Ligamentous angustus||Ligamentous Laxity|||
|normal mental abilities||Tay–Sachs disease|
|ability to eat sugar||Galactosemia|||
|Ehlers–Danlos syndrome||strong collagen|
|High cholesterol||Low cholesterol|
|Darwin's tubercle||curved ear helix|
|Almond eye shape||Rounded eye shape|
|two palmar tendons||three palmar tendons|
|no urinal colouration||Beeturia|||
|hemochromatosis Type IV||hemochromatosis Type I, II, III|
|two wrist tendons||three wrist tendon|||
|straight lip line||Cupid's bow|
|Shepherds Crook||strong connective tissue|||
|Wide mouth||narrow mouth|||
|rounded nose shape||pointed nose shape|
|rounded nostrils||flared nostrils|
|hemangiomas||absence of blood birthmarks|||
|no hair on earlobes||hair on earlobes|
|no appearance of body inflammation||Familial Mediterranean fever|
|Boomerang dysplasia||straight formed limbs|
|straight eyes||slanted eyes|
|darker eyebrows than normal hair *||lighter eyebrows than normal hair *|
|Hapsburg lip||no protruding lip or chin|
|long space between commissures||short space between commissures|||
|Straight flush wrist||Madelung's deformity|
|oligodontia||no missing teeth|||
|crooked teeth||straight teeth|||
|Anxiety attacks||Normal brain chemistry|||
|Presence of immune system||Adenosine deaminase deficiency|
|Total leukonychia and Bart pumphrey syndrome||partial leukonychia|||
|Absence of fish-like body odour||Trimethylaminuria|||
|primary hyperhidrosis||little sweating in hands|||
|Mongolian Fold||no retention of fold|||
|Triangular alopecia||hair present on temples|
|Large nose||small nose|||
|Ear pit (congenital appearance of a hole)||absence of ear pit|
|Slower aging||accelerated aging|
|lack of deformities||Carpenter syndrome|
|immunity to poison ivy||susceptibility to poison ivy|
|Normal hemoglobin synthesis||Thalassemia|
|Thick hair(diameter)||Thin hair (diameter)|||
|decreased risk of cancer||Bloom syndrome|
|congenital hearing loss||Pendred syndrome|
|Fatal familial insomnia||lack of fatal insomnia|
|Lactose persistence *||Lactose intolerance *|||
|Prominent chin (V-shaped)||less prominent chin (U-shaped)|||
|Acne prone||Clear complexion|||
|normal height||Cartilage–hair hypoplasia|
- Facial dimples vary if the individual is a homozygote they will have dimples in both cheeks but a heterozygote will have dimples in either the left or right cheek.
- Eyebrow colour depends if they are a heterozygote they will have matching eyebrow and normal hair colour. While a homozygote dominant will have darker eyebrows and a homozygote recessive will have lighter eyebrows 
- Lactose intolerance is inherited as an incomplete dominant trait because individuals express intolerance to an extent.
See also 
- Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. 7th ed. Philadelphia: Saunders, 2007.
- Glossary." Genetics Home Reference. 14 Mar. 2008. U.S. National Library of Medicine. <http://ghr.nlm.nih.gov/>.
- Freeman, Scott, and Jon C. Herron. Evolutionary Analysis. 4th ed. Upper Saddle River: Pearson:Prentice Hall, 2007.
- Campbell, Neil; Jane Reece (2005). Biology. San Francisco: Benjamin Cummings. p. 265. ISBN 0-07-366175-9.
- Online Mendelian Inheritance in Man, ID=194000
- Singapore Science Centre: ScienceNet|Life Sciences|Genetics/ Reproduction
- Online Mendelian Inheritance in Man, ID=126100
- Natural selection at work in genetic variation to taste
- 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.
- Online Mendelian Inheritance in Man, ID=128900
- Online Mendelian Inheritance in Man, ID=119000
- Xue-Jun Zhang et al. "A Gene for Freckles Maps to Chromosome 4q32–q34" Journal of Investigative Dermatology (2004) 122, 286–290 
- Online Mendelian Inheritance in Man, ID=117800