The Y chromosome

The Y chromosome (for a review see Ellis, 1991) is an unusual part of the human genome; at about 60 million base pairs, it represents around 2% of the total, yet half of the human population survives quite happily without one. The Y is haploid and paternally inherited. The consequence of this haploidy is that, outside of two small pseudoautosomal regions at the tips of the short and long arms of the chromosome, which can recombine with the X (Rouyer et al., 1986; Freije, Helms, Watson & Donis-Keller, 1992), the Y has no chromosomal partner with which to recombine at meiosis. Thus the Y is passed down virtually intact from father to son. This is in contrast to the X chromosome and autosomes, which are continually being reshuffled by recombination, and means that a comparison of Y chromosomes is a direct comparison of individuals. As Y chromosomes pass down paternal lineages from a common paternal ancestor, they accumulate mutations; if we could look at these mutations, in the form of various kinds of DNA polymorphisms, we could firstly try to determine how modern Ys are related to each other and to their ancestors. Secondly, these polymorphisms define Y chromosome types, and if we could measure the frequencies of these types in different populations, we could also attempt to understand some aspects of the histories of these populations (Spurdle & Jenkins, 1992a).