Originally posted by heyllo22
Autosomes are those chromosomes within the cell nucleus including the sex-chromosomes x and y. These nuclear chromosomes derive 50-50 from one's parents. Normally, males have one x and one y and females have two x chromosomes. Females inherit one x from their mother and one from their father. Because females have two x's, those x's can recombine, but one x is inactivated so that female offspring receive an x that is a blend of paternal and maternal x. Because x cannot combine with y, the male y constitutes a record of direct descent on the paternal line. The mitochondrial DNA is outside the cell nucleus and normally is not subject to recombination, it is passed from a mother to her offspring but only passed on by her female offspring to their children, hence it constitutes a record of direct descent on the maternal line.
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