"It is usually assumed that monozygotic twins always share the same genome and that their hereditary endowments are the same. Indeed, twins have long been used in studies that attempt to separate "nature" from "nurture". Gilbert writes.
[But] take the situation of an egg fertilized by a Y-bearing sperm. It has been found that monozygotic twinning is associated with higher than normal amounts of aneuploidy; so it is possible that if twinning were to occur through the failure of the first two blastomeres to adhere to one another, aneuploidy (abnormal Chromosome numbers) might also occur. In that case, the twins would have different chromosome complements. If the aneuploidy were for the X chromosome, one twin might by male (XY or XYY) while the other would be female (XO).
And that's the problem with assumptions! The first assumption Whitehead makes is that HBS is a behaviour. It isn't. Its a variation in foetal sex formation. The second assumption is the usual chestnut, that if one identical twin is is transsexed (HBS), they both need to be before the cause can be attributed to genes.
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