The Disappearance of the Y Chromosome (And Other Chromosomes)

Wendy Wu ’22

Sex_Chromosomes
Figure 1: The Y chromosome (below) is noticeably diminished when compared to the X chromosome (above).  

The Y-chromosome is quite unlike its homologue, the X-chromosome. With a relatively lacking number of genes, the Y-chromosome is the only chromosome inessential for life; its major function is to determine the sex of offspring. How did this come to be the case? A part of the reason is that the Y-chromosome does not occur as a pair; it does not have the means to undergo genetic recombination. Harmful mutations, therefore, are not edited out of the gene and as generations pass, the Y-chromosome continues to degenerate. If it continues to do so, scientists argue that it will disappear in five million years. Jason Wilson, a student at the University of Missouri-Kansas City School of Medicine, believes, however, that such degeneration is not limited to the Y-chromosome and that chromosome degeneration is, in fact, an evolutionary phenomenon. 

Wilson made comparisons of mammalian orthologs, or genes in different species that evolved from a common ancestral gene, with the help of genetic databases SPEED and GO. By analyzing 68,006 comparisons across 10,849 genes, he hoped to examine the relationship between gene function and selection that ultimately resulted in the development of orthologs. Of the 6,734 human genes analyzed, Wilson found that genes with diverse functions faced negative selection more often than genes with fewer and more specific functions. Additionally, by comparing the number of GO terms and calculated selective strength, Wilson deduced that genomic isolation lessens the effects of negative selection. These observations fall in line with the nature of Y-linked genes, categorizing the Y-chromosome as a model for chromosome evolution. 

The specialization of the SRY gene and the suppression of recombination of the Y-chromosome, leading to specific function and genomic isolation, has resulted in its rapid divergence from the X-chromosome. Wilson’s research suggests that specialization of function, and therefore selection, are not limited to the sex chromosomes. Overall genetic decay has been occurring and will continue to occur. While it is unlikely our chromosomes will decay to the point of extinction any time soon, prolonged strong selection has the power to do exactly that. Further research into our genetic ancestral lineage can determine the point somatic chromosomes began to experience selection.

 

Works Cited:

[1] J. Wilson, et al., Extinction of chromosomes due to specialization is a universal occurrence. Scientific Reports 10, (2020). doi: https://doi.org/10.1038/s41598-020-58997-2.

[2] Image retrieved from: https://www.sciencesource.com/archive/Male-human-sex-chromosomes–SEM-SS2763357.html

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