Molecular Biology of Mammalian Sex Chromosomes

Here I describe how molecular techniques have been applied to mammalian sex chromosomes, and the remarkable insights they have provided into the structure, sequence and gene content of the X and Y in humans and other mammals. Mammal species share an XY sex chromosome pair, with a medium-sized X, and a much smaller Y bearing few genes and a lot of repetitive DNA. To zero in on sex chromosomes, molecular scale maps of the X and Y chromosomes were constructed for humans and other mammals, which built on the genetic and physical maps described in Chapter 2. This paved the way to applying the rapidly developing DNA sequencing methods to reveal the most intimate molecular details of human sex chromosomes as part of the Human Genome Project. This revealed not only the gene content of the X and Y, but peculiarities of their structure. The tips of the X and Y were homologous over a pseudoautosomal region, which paired and recombined at male meiosis. The Y, in particular, bore genes that had become specialized and often greatly amplified within ‘palindrome’ loops.