THE complete sequence of the human X chromosome was published in Nature this week. The work shows that large segments of it match parts of normal chromosomes in birds, confirming the X chromosome’s “non–sex” origins.
Despite the fact that X is much larger than the tiny Y, it seems that both evolved from a pair of conventional chromosomes in early mammals sometime in the past 300 million years – an idea first proposed in 1967. Previously, our main clue that X and Y had a common ancestry was that they swap a few small sections during one kind of…
