Question

In mammals, sex is determined by the X and Y chromo- somes. Females are XX; males are XY. The Y chromo- some contains a gene that causes development of testes, which then causes the embryo to become male. The Y Redrawn from Kohn et al. (2000). chromosome does not undergo crossing over with the X during spermatogenesis in males, but the two X's cross over with each other during oogenesis in females.

a. The Y chromosome is thought to have once been the same size as the large, fully functional X chromo- some. But during the evolution of the mammals, the Y chromosome seems to have accumulated an enor- mous number of deleterious mutations. It has also lost almost all of its genes and has shrunk to a rudimentary chromosome containing just the testis-determining gene, a few other genes, and some nonfunctional remnants of other genes. Why has this occurred?
b. Birds use a reverse system, in which females have two different chromosomes (called WZ in birds) and males have two of the same kind of chromosome (ZZ). In birds, sex is determined by a gene on the W chromosome that causes ovary formation, which then causes the bird embryo to become female. Would you predict one of these chromosomes might have accumulated mutations in the same way that the Y chromosome has? If so, which one?
c. Some plants also have genetically determined sex but are polyploid. Should their sex chromosomes show accumulation of mutations?

Answer 1

The Y chromosome has shrunk and lost genes compared to the X chromosome due to its lack of recombination, which may lead to accumulated mutations. The avian W chromosome may experience similar evolutionary pressures. In polyploid plants, mutations might accumulate in sex chromosomes without recombination, but polyploidy could mitigate this effect.

Step-by-step explanation:

The Y chromosome in mammals has undergone significant evolutionary changes. It has lost a substantial number of its original genes besides accumulating many deleterious mutations. This reduction in size and gene content may have occurred because, unlike other chromosomes which undergo genetic recombination at each generation, the Y chromosome does not undergo crossing over with its homolog, the X chromosome, except for a small region at the Y chromosome tips. This lack of recombination may have led to a failure to repair DNA damage, resulting in the loss or degradation of genes.

In birds, sex determination follows a ZW system, where the Z chromosome is present in both sexes (ZZ in males and ZW in females), but the W chromosome determines female development. The W chromosome could be subject to similar evolutionary pressures as the mammalian Y chromosome, potentially leading to a reduction in size and gene content

In the case of polyploid plants with sex chromosomes, the same accumulation of mutations might happen if they have a similar system where one sex chromosome does not undergo recombination, but the presence of multiple sets of chromosomes (polyploidy) can buffer against this effect.