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Among humans, some men have two Xs and a Y ("XXY", see Klinefelter syndrome), or one X and two Ys (see XYY syndrome), and some women have three Xs or a single X instead of a double X ("X0", see Turner syndrome).
There are other exceptions in which SRY is damaged (leading to an XY female), or copied to the X (leading to an XX male).
Recombination between the X and Y chromosomes proved harmful—it resulted in males without necessary genes formerly found on the Y chromosome, and females with unnecessary or even harmful genes previously only found on the Y chromosome.
In mammals, the Y chromosome contains the gene SRY, which triggers testis development.
Over time, genes that were beneficial for males and harmful to (or had no effect on) females either developed on the Y chromosome or were acquired through the process of translocation.
This re-estimation of the age of the therian XY system is based on the finding that sequences that are on the X chromosomes of marsupials and eutherian mammals are present on the autosomes of platypus and birds.
Edmund Beecher Wilson independently discovered the same mechanisms the same year.
Stevens proposed that chromosomes always existed in pairs and that the Y chromosome was the pair of the X chromosome discovered in 1890 by Hermann Henking.