"There are a handful of genes known to underlie male infertility but there's still a lot unknown," says Alyssa Kruger, a Ph.D. student within the Department of Human Genetics. Kruger, who works in the lab of principal investigator Jacob Mueller, Ph.D., and their colleagues have been studying the X and Y chromosomes -- which are delivered by sperm to an egg to determine an offspring's sex -- across species and across millions of years of evolution.
Sex chromosomes are unique, Kruger explains, because while they were once an identical pair of chromosomes, they independently evolved distinct sets of genes. Upon examining mice sex chromosomes, they found two recently evolved X-linked gene families that are found only in mice. Interestingly, these genes are also present multiple copies. To figure out what the genes are responsible for, Kruger's team removed them from the genomes of some mice using CRISPR and other technologies.
Copies - Gene - Family - Towards - Male
Removing all copies of one X-linked gene family produced mice whose offspring were biased towards being male by a ratio of 60-40. This discrepancy wasn't a result of more Y-bearing sperm -- the number of X and Y sperm remained the same in the mice. "This suggests to us that this gene is impacting the relative fitness of X versus Y sperm, but we don't know how. Maybe the Y-bearing sperm are swimming faster or in a straighter line, we don't yet know," says Kruger.
Next, the team decided to...
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