Scientists have long suspected that the sex chromosome that only males carry is deteriorating and could disappear entirely within a few million years, but until now, no one has understood the evolutionary processes that control this chromosome's demise. Now, a pair of Penn State scientists has discovered that this sex chromosome, the Y chromosome, has evolved at a much more rapid pace than its partner chromosome, the X chromosome, which both males and females carry. This rapid evolution of the Y chromosome has led to a dramatic loss of genes on the Y chromosome at a rate that, if maintained, eventually could lead to the Y chromosome's complete disappearance. The research team, which includes Associate Professor of Biology Kateryna Makova, the team's leader, and National Science Foundation Graduate Research Fellow Melissa Wilson, will publish its results in the 17 July 2009 issue of the journal PLoS Genetics. "There are three classes of mammals," said Makova, "egg-laying mammals, like the platypus and the echidna; marsupials, like the opossum and the wallaby; and all other mammals -- called eutherians -- which include humans, dogs, mice, and giraffes. The X and Y chromosomes of marsupials and eutherians evolved from a pair of non-sex chromosomes to become sex chromosomes."

Scientists have a better understanding of what causes an abnormal number of chromosomes in offspring, a condition called aneuploidy that encompasses the most common genetic disorders in humans, such as Down syndrome, and is a leading cause of pregnancy loss. To pinpoint what goes awry in these cases, researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of Tennessee, Knoxville studied mice. They found that if a mother's egg cell has a mutation in just one copy of a gene, called Bub1, then she is more likely to have fewer offspring that survive to birth.