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24 Aug 2014

The X chromosome and its family history surprises

I learn recently that the X chromosome contains no material from a man’s father and none from your father’s father if you are a woman.  This leads to the curiously imbalanced chart below, which you will spot contains a Fibonacci pattern.

Table 1.  This table shows the contribution ancestors make to the total (pair) of X sex-chromosomes for a female: I have italicised the female’s father’s contribution.

In my family, the female (my mother)’s F-M-F-M-F-M (father’s mother’s father’s mother’s father’s mother)and F-M-M-M-F-M, are by complete coincidence, the same person, Elizabeth Cock born 1770 in Gwithian Sands, Cornwall.  Elizabeth accounts for three-eighths of the material on one of my mother’s X chromosomes, and thus three-sixteenths of my X chromosome.  This is either 12 or 24 times what she ‘ought’ to contribute being only 1 out of my 128 5xgreat-grandparents.

My X-chromosome is made up of my mother’s two X-chromosomes combined.  My Y-chromosome has been passed down from father to son, down to myself, so my father did not pass on to me an X chromosome.

My sisters have a second X-chromosome, from their father, which is made up of their paternal grandmother’s two X-chromosomes combined.  A stonking quarter of this comes from one lady, Ann Charlton, born 1785 in Whittonstall, south Northumberland, our 4xgreat-grandmother.  She is their F-M-F-M-F-M.  As there are 64 people in this generation, Ann is oversubscribed by a factor of 16, or of 8 – if you treat this X-chromosome as being strictly on the paternal side.  Were Ann to have other son’s daughter’s son’s daughter’s son’s daughters (which she does, the Embletons), we could in theory identify genes on the chromosome for which she was responsible.
As the pair of sex chromosomes are only 2 out of 46, the fact that some grandparents did or didn’t contribute makes very little difference overall.

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