appear as an amorphous blob under the microscope and only take on a well-defined shape during mitosis. This shape is vaguely X-shaped for all chromosomes. It is entirely coincidental that the Y chromosome, during mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape. Variations Most therian mammals have only one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes. In mammals, the Y chromosome contains a gene, SRY, which triggers embryonic development as a male. The Y chromosomes of humans and other mammals also contain other genes needed for normal sperm production. There are exceptions, however. 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). Origins and evolution Before Y chromosome Many ectothermic vertebrates have no sex chromosomes. If they have different sexes, sex is determined environmentally rather than genetically. For some of them, especially reptiles, sex depends on the incubation temperature. Some vertebrates are hermaphrodites, although other than a very few ray-finned fish, they are sequential (the same organism produces male or female gametes, but never both, at different points in its life), rather than simultaneous (the same organism producing both male and female gametes at the same time). Origin The X and Y chromosomes are thought to have evolved from a pair of identical chromosomes, termed autosomes, when an ancestral animal developed an allelic variation, a so-called "sex locus" – simply possessing this allele caused the organism to be male. The chromosome with this allele became the Y chromosome, while the other member of the pair became the X chromosome. 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. Until recently, the X and Y chromosomes were thought to have diverged around 300 million years ago. However, research published in 2010, and particularly research published in 2008 documenting the sequencing of the platypus genome,