Science Fair Project Encyclopedia
Genetics of humans
Humans (Homo sapiens) are a eukaryotic species. A human has 46 chromosomes (22 autosomes and 2 sex chromosomes, with an XY sex determination system. At present estimate, humans have approximately 20,000-25,000 genes . Humans share 98.5% of their DNA with their closest living evolutionary relative, the bonobos.
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Chromosomes
Humans have 46 chromosomes, arranged in pairs (i.e. they are diploid). 22 Of these are autosomes, and 2 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.
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 have mitochondrial DNA
Genetic disorders
Humans have several genetic diseases, often caused by recessive genes.
External links
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