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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.
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.
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.
Human traits with simple inheritance patterns
|Widow's Peak||No Widow's Peak|||
|Facial Dimples||No Facial Dimples|||
|Able to taste PTC||Unable to taste PTC|||
|Unattached earlobe||Attached earlobe|||
|Cleft chin||No Cleft chin|||
|Wet-type earwax||Dry-type earwax|||
- Campbell, Neil (2005). Biology. San Francisco: Benjamin Cummings. pp. pp. 265. Unknown parameter
- Online Mendelian Inheritance in Man, ID=194000 
- Online Mendelian Inheritance in Man, ID=126100 
- 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