Review: Sex Chromosome Evolution and the Expression of Sex-Specific Genes in the Placenta

Evolution of the X and Y chromosome from an original autosome. The X chromosome has stayed more-or-less intact, while the Y has degenerated progressively.

Comparative mapping of genes on the human X in distantly related mammals, and other vertebrates. Blue denotes the X conserved region shared by the X chromosomes of marsupials and placental mammals, and green the recently added region that is autosomal in marsupials. The blue and green regions represent two evolutionary blocks of the genome that are highly conserved in all vertebrates. However, they are both autosomal in the chicken, and also the platypus, dating the evolution of our X and Y at 166 million year ago. The human Y chromosome, too, is made up from the same two evolutionary blocks, but there is almost nothing left of the original (blue) Y chromosome.

The expression of male-derived genes (blue) from the sex chromosomes in the placenta in mouse (top) and human (bottom). Placentas are represented as small ovals within pregnant mouse and human females (pink). Genes from the Y chromosome (left) are represented by blue patterns that are different in the two species (representing the products of different HY antigens in mouse and human). The paternally derived X chromosome (right) is inactivated in the extraembryonic membranes of the mouse, so the placenta expresses only the maternal X (pink), but in human the maternally and paternally derived X chromosomes are randomly inactivated or co-expressed in the placenta (pink and blue pattern).