Re: Which is the origin of the names of sexual chromosomes X and Y ?

A good question. Most people assume that both the X and Y chromosomes are so called because of their shape. You can see chromosomes under a microscope when they are in the process of dividing--mitosis or meiosis. The chromosomes look like Xs because they have been duplicated and are attached by a centromere. During division the centromere split and the two sister chromosomes each go into sister cells.

When scientists first looked at the division process, they noticed that in some cells, there was an "extra" chromosome. H. Henking in 1891 first noticed this and called the extra chromosome "X" for extra. (Other scientists at the time were calling the extra chromosome the "accessory".) In 1901, C. McClung made the connection that this extra chromosome had something to do with sex since only males have the extra. E. B. Wilson and Nettie Stevens in 1905 did a more detailed study and figured out that X was seen as extra only because it wasn't paired. Females have 2X chromosomes so you don't see any extra chromosome in the cells. Wilson and Stevens were looking at insects where males are XO (O as in zero); so they saw an extra X.

In mammals, males are XY. The Y chromosome is smaller and shaped differently from its partner. The name Y was a natural extension of its shape and the fact that it "follows" X.

So a rather long answer (as usual) for an interesting question.