Thursday, April 11, 2013

The X Chromosome

The previous post described the inheritance of mitochondrial DNA, Y-DNA, and autosomal DNA (Chromosomes 1 through 22), but left out X inheritance, an interesting special case.

You probably know that men carry an X chromosome and a Y chromosome, and women carry two X's.   These are the chromosomes that determine whether you're a man or a woman.    

Women get an X from each parent, recombine them, and pass them on to sons and daughters.   Men get only one, from their mothers, and pass that one chromosome on intact to their daughters.   That causes an interesting pattern of inheritance, shown in charts here from Blaine T. Blettinger's blog, The Genetic Genealogist .

The chart below shows which ancestors can contribute to a male's X chromosome.

This chart shows which ancestors can contribute to a female's X chromosome.

X ancestors include the ancestors who contributed your mitochondrial line, but they also include many more, in the case of females even including some ancestors from the father's side.   One way of thinking of X inheritance is that it can't involve two males in a row in the inheritance line, since X's no man gets an X from his father - he gets a Y.

Because of the special case of fathers passing on the X they inherited from their mothers intact, females have one X that came directly from their paternal grandmothers.    So, in contrast to autosomal DNA, which is divided and recombined every generation, X DNA is divided and recombined only when inherited from a female.   This makes it hard to estimate how many generations back a given size match on the X is.

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