More Than Just a Shrunken X

	The small size of the Y lead to its reputation as a just a "rotten X."

In addition to these critical "housekeeping" genes, the Y chromosome contains the genetic information necessary to the formation of sperm. How do we know?

Early chromosome mapping research by the Human Genome Project led to an important discovery: about one in every 4,000 boys is born with a piece of his Y chromosome missing. When these boys grow up, they produce few or no sperm. In almost every case, the fathers of these boys (from whom they inherited their Y chromosomes) have intact Y's, so the genetic deletion is not inherited, but is rather a mutation that occurs either in the process of sperm production in the father or in the earliest stages of an embryo's life.

	The sex of a baby alligator depends on how warm it was inside the egg.

It turns out that missing any one of several regions of the Y causes problems with sperm production. Among these regions, one is missing in 12% of men who produce absolutely no sperm, called the Azoospermia Factor C Region. Azoospermia, (pronounced AY-zoh-SPER-mia,) means "absence of sperm."

Reading the Book of Y

The Y chromosome can be thought of as a book containing 24 million letters. In 1992, David C. Page at the Whitehead Institute began a search for the chapters missing in these infertile males. But finding the missing chapters didn't automatically reveal what genes - i.e., sentences - were in the missing chapters. Page realized that sequencing - figuring out all the letters in all the words in each sentence - would be the fastest way to reveal all the genes in this Azoospermia Factor C Region and to shed light on why it's so often deleted.

A gene on the X causes hemophilia, a famous example of sex-linked inheritance.

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Photos: Genetics & IVF Institute; Janet Stein Carter, University of Cincinnati-Claremont College