Why is the Y Chromosome Useful?
Posted February 1, 1998
Human females have 23 pairs of homologous chromosomes, but human males have only 22. In males, the 23rd pair of chromosomes consists of an X and a Y chromosome, so
named because of their shapes. The Y chromosome is known to determine maleness, but until recently, little more was known about this chromosome. Some scientists even hypothesized that
the Y chromosome was becoming vestigial, well on its way to becoming extinct.
Two years ago, biologist David Page began to think about how the Y chromosome was different from other chromosomes. He began looking at couples who had infertility
problems. In one-fifth of these couples, a defect in sperm production is the cause of the infertility. Page identified a gene he named DAZ, for "deleted azoospermia," on the Y chromosome.
In fertile men, DAZ is switched on in the sperm-producing cells in the testes. In men without this gene, the testes make no sperm at all, resulting in infertility. This indicates that
DAZ is necessary for normal sperm production. When Page looked more closely at the Y chromosome, he found that DAZ appeared in multiple copies. The more copies of DAZ there are on the
Y chromosome, the more sperm produced. But the Y chromosome mutates easily and often, which may result in loss of copies of DAZ. Unlike other chromosomes, the Y chromosome cannot cross
over during meiosis because it has no homologue. This means that a normal, fertile male could have a son with no DAZ genes, who would then be infertile. In fact, one in every 8000 newborn
boys is missing this part of the Y chromosome.
Page's team has also identified a gene he called DAZL, for "DAZ-like," on chromosome 3. This gene is expressed in the testes, like DAZ, but it is also expressed in
the ovaries of females. Scientists hypothesize that DAZL is in fact the ancestor of DAZ. That is, all mammals inherited DAZL on chromosome 3, but over time copies of DAZL found their
way onto the 23rd pair of chromosomes during crossing over. Once on the Y chromosome, DAZL became DAZ, the gene that governed sperm production in the testes.
The discovery of DAZ and DAZL is just one of the surprises resulting from the Human Genome Project. Once all the human genes have been mapped, it may be possible
to cure many human problems, including infertility in men. What new information will be found? We will just have to wait and see.
References
Radetsky, Peter. "Y?" Discover, November, 1997, Vol. 18, No. 11, pp. 88-93.
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