Friday, December 17, 2010

Life Lines 81


Males have an X and a Y chromosome plus 23 pairs of chromosomes that are found in both sexes. Females have two X chromosomes. The X and Y chromosomes are called sex chromosomes. The Y chromosome is not essential for life (any female is living testimony to that inference). But it sure is needed to make a male. The Y chromosome has a gene that converts a neutral embryonic future sex organ (the gonads) into testes. The testes then make hormones that produce the male sexual apparatus. The absence of that gene on the Y normally occurs when the fertilized egg has two X chromosomes and the gonads become ovaries and the absence of the male hormones lead to female sex apparatus formation. Sometimes gene mutations can mess things up leading to XY females or apparent XX males.

The Y is found in half of a male’s sperm. Those are the sperm that produce sons. The X is found in the other half of the male’s sperm and they produce daughters. Biologists know that it is the male who determines the sex of his children but for generations many women were punished for not giving husbands sons!

The genes of the Y stay together but the genes of all other chromosomes have an opportunity to switch bits of their chromosomes when sperm or eggs are made. Since fathers pass their Y chromosomes only to their sons and these genes always stay together, the mutations that arise in the Y chromosome will stay there unless they lead to infertility. That is not at all unusual because many genes for making sperm are found on the Y chromosome.

The sequence of most of the genes for our Y chromosome is known because it is a small chromosome. Samples from people around the world in different continents have been studies and there is no variation except in Africa! Perhaps larger samples will reveal some differences, but the rate of change of normal to mutant is relatively low compared to some other genetic systems in our cells. This implies that our common Y ancestor or Ur-grandfather (whom we call Y chromosome Adam) left his African homeland with his mitochondrial Eve partner about 250,000 years ago and none of his European, Asian, or New World descendants have established Y chromosome variations that survived.

Just as mitochondrial Eve, our common ancestor of humanity, had kin in the thousands, so too Y chromosome Adam had lots of male kin. They just didn’t luck out to get their Y chromosome to us. When we study pedigrees, we often note people who keep following their family name back. They are following their father’s Y chromosome as far as it goes. Thus J. Pierpont Morgan, the financier and T. H. Morgan the Nobel Prize geneticist, share both a common last name, a relative who stepped off the Mayflower, and a common Y chromosome. They share very few of their other genes from that Mayflower ancestor.

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