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Human genetics

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File:Human genetics.jpg
For a non-technical introduction to the topic, see Introduction to Genetics.


Genetic Differences and Inheritance Patterns Edit

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 genes. [1]

Autosomal Dominant InheritanceEdit

Autosomal traits are associated with a single 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 InheritanceEdit

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 aspect, 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 and Y-Linked InheritanceEdit

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. Females express X-linked disorders when they are homozygous for the disorder and become carriers when they are heterozygous. An infamous recessive X-linked disorder is Hemophilia A. Hemophilia is a disorder where blood does not clot properly due to a shortage of clotting factor VII. This disorder gained recognition as it traveled through royal families, notably the descendent's of Britain's Queen Victoria. 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.




File:Autosomal Recessive Pedigree Chart .svg

A pedigree is a diagram showing the ancestral relationships and transmission of genetic traits over several generations in a family. 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 been 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 councilors commonly use pedigrees to help couple determine if the parents will be able to produce healthy children.

File:Human male karyotpe.gif




Genomics refers to the field of genetics concerned with structural and functional studies of the genome.[3] 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.[4] In April 2003, the Human Genome Project was able to sequence all the DNA in the human genome, to discover the human genome was composed around 20,000 protein coding genes.

Population GeneticsEdit


Hardy-Weinberg PrincipleEdit

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 Edit

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; an assertion known as the endosymbiotic hypothesis. 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 37 genes.

Genes and human characteristicsEdit

Genes are the fundamental units of inheritance. Genes can be defined as a sequence of DNA in the genome that is required for production of a functional product. Genes have both minor and major effects on human characteristics. Human genes have become prominent in the nature versus nurture debate.

Genes and behaviorEdit

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 traits that were considered undesirable. 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 genderEdit

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.


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

Evolutionary psychologyEdit

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 Edit

Main article: Genetic disorder

Humans have several genetic diseases, often caused by recessive genes. A few examples of human genetic diseases are: Turner Syndrome, Huntington's disease, cancer, autism, and sickle cell anemia. For a more comprehensive list see List of genetic disorders. Genetic disorders happen everywhere and are very common in some places.

  • Cri du Chat syndrome- A disorder caused from a deletion on the short arm of chromosome 5. This deletion results in a phenotype of mental retardation, behavioral problems, and a cat like call. About one in every 50,000 births will have the syndrome.[6]
  • Huntington's disease- A neurological disorder caused by a trinucleotide repeat sequence. Huntingtons is an autosomal dominant trait. Most individuals with the disease will first display the phenotype around 40 years of age. The symptoms are jerky uncontrollable movements, mental retardation, and behavioral problems.[7]
  • Turner syndrome- A condition that effects females caused by a 45, XO genotype instead of the normal XX genotype. These individuals have only one X chromosome. These individuals are phenotypically female, but will be sterile due to undeveloped ovaries.[8]
  • Klinefelter syndrome- A disorder in males caused by the presence of an extra X chromosome. These individuals have a genotype of 47, XXY instead of the normal XY genotype. The symptoms for this syndrome are enlarged breasts, small testes, and sterility.[9]

Human traits with simple inheritance patternsEdit

Main article: List of Mendelian traits in humans
Dominant Recessive References
Widow's peak No Widow's peak [10][11]
Facial Dimples No Facial Dimples [12][13]
Able to taste PTC Unable to taste PTC [14]
Unattached earlobe Attached earlobe [12][15][16]
Cleft chin No Cleft chin [17]
Brunette iris Blue Iris
Color Vision Color Blind
Freckles No Freckles [12][18]
Wet-type earwax Dry-type earwax [15][19]

See alsoEdit

References Edit

  1. Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. 7th ed. Philadelphia: Saunders, 2007.
  4. Glossary." Genetics Home Reference. 14 Mar. 2008. U.S. National Library of Medicine. <>.
  5. Freeman, Scott, and Jon C. Herron. Evolutionary Analysis. 4th ed. Upper Saddle River: Pearson:Prentice Hall, 2007.
  6. Cri-Du-Chat Syndrome." Online Mendelian Inheritance in Man. 2008. Johns Hopkins University. <>
  7. Huntington Disease." Online Mendelian Inheritance in Man. 2008. Johns Hopkins University. <>.
  8. Noonan Syndrome." Online Mendelian Inheritance in Man. 2008. Johns Hopkins University. <>
  9. XX Male Syndrome." Online Mendelian Inheritance in Man. 2008. Johns Hopkins University. <>
  10. Campbell, Neil; Jane Reece (2005). Biology, pp. 265, San Francisco: Benjamin Cummings.
  11. Online Mendelian Inheritance in Man, ID=194000 [1]
  12. 12.0 12.1 12.2 Singapore Science Centre: ScienceNet|Life Sciences|Genetics/ Reproduction
  13. Online Mendelian Inheritance in Man, ID=126100 [2]
  14. Natural selection at work in genetic variation to taste
  15. 15.0 15.1 Cruz-Gonzalez L., Lisker R. (1982). Inheritance of ear wax types, ear lobe attachment and tongue rolling ability. Acta Anthropogenet. 6 (4): 247–54.
  16. Online Mendelian Inheritance in Man, ID=128900 [3]
  17. Online Mendelian Inheritance in Man, ID=119000 [4]
  18. Xue-Jun Zhang et al. "A Gene for Freckles Maps to Chromosome 4q32–q34" Journal of Investigative Dermatology (2004) 122, 286–290 [5]
  19. Online Mendelian Inheritance in Man, ID=117800 [6]

External links Edit


Template:Human genetics

Human chromosomes



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