The timing of genetic degeneration of sex chromosomes

Research output: Contribution to journalReview articlepeer-review


Genetic degeneration is an extraordinary feature of sex chromosomes, with loss of functions of Y-linked genes in species with XY systems, and W-linked genes in ZW systems, eventually affecting almost all genes. Although degeneration is familiar to most biologists, important aspects are not yet well understood, including how quickly a Y or W chromosome can become completely degenerated. I review current understanding of the time-course of degeneration. Degeneration starts after crossing over between the sex chromosome pair stops, and theoretical models predict an initially fast degeneration rate and a later much slower one. It has become possible to estimate the two quantities that the models suggest are the most important in determining degeneration rates — the size of the sex-linked region, and the time when recombination became suppressed (which can be estimated using Y-X or W-Z sequence divergence). However, quantifying degeneration is still difficult. I review evidence on gene losses (based on coverage analysis), or loss of function (by classifying coding sequences into functional alleles and pseudogenes). I also review evidence about whether small genome regions degenerate, or only large ones, whether selective constraints on the genes in a sex-linked region also strongly affect degeneration rates, and about how long it takes before all (or almost all) genes are lost.
Original languageEnglish
Article number20200093
Number of pages11
JournalPhilosophical Transactions of the Royal Society B: Biological Sciences
Issue number1832
Early online date12 Jul 2021
Publication statusPublished - 30 Aug 2021


  • evolutionary strata
  • chromosome fusion
  • dosage compensation


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