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A proposed path for the evolution of the human sex chromosomes. Lahn and Page [6] postulate four inversions on the human Y chromosome, which suppressed recombination between the 'proto' sex chromosomes. Each inversion (designated 1-4) is thought to have reduced the size of the pseudoautosomal region (white) and enlarged the non-recombining portions of the X (yellow) and Y (blue) chromosomes. Time points at which the human X and Y may have diverged from the sex chromosomes of other mammals are indicated.
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Figure 1: A proposed path for the evolution of the human sex chromosomes. Lahn and Page [6] postulate four inversions on the human Y chromosome, which suppressed recombination between the 'proto' sex chromosomes. Each inversion (designated 1-4) is thought to have reduced the size of the pseudoautosomal region (white) and enlarged the non-recombining portions of the X (yellow) and Y (blue) chromosomes. Time points at which the human X and Y may have diverged from the sex chromosomes of other mammals are indicated.

Mentions: In mammals, Lahn and Page [6] have found evidence that recombination ceased along the sex chromosomes in an unexpected, stepwise fashion. First, a block of DNA surrounding the SRY gene stopped recombining, and then discrete non-recombining blocks evolved along most of the chromosome length (Figure 1). By comparing X-Y nucleotide divergence at 19 homologous genes located in the non-recombining region of the X and Y chromosomes, they identified four 'evolutionary strata' along the human X chromosome. Within each stratum, the X and Y copies of genes differ by about the same amount, indicating that recombination ceased at the same time for all the genes in question. But the different groups clearly originated at different time points [6]. Large inversions, which are known to suppress recombination, could account for this stepwise pattern. Interestingly, one such potentially important polymorphic inversion (3.5 megabases) has been identified on the human Y chromosome [2].

Towards a complete sequence of the human Y chromosome

Bachtrog D, Charlesworth B - Genome Biol. (2001)

Bottom Line: A few dozen genes are known on the human Y chromosome.The completion of the human genome sequence will allow identification of the remaining loci, which should shed further light on the function and evolution of this peculiar chromosome.

Affiliation: Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK. Doris.Bachtrog@ed.ac.uk

ABSTRACT
A few dozen genes are known on the human Y chromosome. The completion of the human genome sequence will allow identification of the remaining loci, which should shed further light on the function and evolution of this peculiar chromosome.

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