Y chromosome: Encyclopedia II - Y chromosome - Origins

Y chromosome - Origins

Many cold-blooded 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, others are hermaphroditic.

X and Y chromosome diverged about 350 million years ago, when some reptile developed a gene which makes all its owners to be males. The chromosome with this gene became Y chromosome, and similar chromosome without it became X chromosome. So initially, X and Y chromosomes were almost the same. Genes which were beneficial for males and harmful for females (male genes) either moved into Y chromosome or developed in it. This was beneficial for both sexes.

However, recombination between X and Y chromosomes was harmful because it provided males without some male genes or females with some male genes. As a result, male genes assembled around the sex determining gene in order to make this less probable. Later, Y chromosome changed in such a way that the areas around the sex determining gene completely lost their ability to recombine with X chromosome.

With time, the larger and larger areas lost ability to recombine with the X chromosome. However, without recombination it is hard to get rid of harmful mutations. Therefore, harmful mutations increasingly damaged male genes until some stopped functioning and became genetic junk. The useless genes were then removed from Y chromosome.

As a result of this process, for humans 95% of Y chromosome is unable to recombine, and Y chromosome contains only 70 to 300 working genes versus more than 1000 for X chromosome. For some other animals, the degradation of Y chromosome is even more severe. For example, the Y chromosome in kangaroos contains only the SRY gene.

For humans and some other primates, the Y chromosome is able to "recombine" with itself (see below). This process, called gene conversion, may slow down the process of degradation.

After only an SRY (or other sex-determining) gene remains from the whole Y chromosome, there are following possibilities:

1. The gene is connected to X chromosome or some autosome, making it the new Y chromosome. The whole process starts again. This happened with two species of rodents (Ellobius tancrei and E. lutescens). In one species, both sexes have unpaired X chromosomes; in the other, both females and males have XX.
2. Part of some autosome is connected to both X and Y chromosome. This happened with one species of drosophila.

Contrary to popular belief, the SRY gene cannot be destroyed in the way to extinct all males or all species. This is because any mutation which prevents SRY from functioning would remove the species from genetic pool of males. Indeed, kangaroos who diverged from humans 130 million years ago, still have almost the same SRY as humans, even if all other genes of kangaroo's Y-chromosome have been destroyed.

Adapted from the Wikipedia article "Origins", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki