By Allan Tobin and Jennie Dusheck
The X chromosome carries some 10,000 genes-about 10 percent of all human genes. Because the Y chromosome lacks many of these genes, human males have only one copy of many genes. A woman may carry an abnormal recessive allele on one X chromosome without any effect because she usually has a normal allele on the other X chromosome. The normal gene often produces all the protein she needs. Unless she is homozygous, she is fine. In contrast, a man who inherits the same abnormal recessive has no other allele to compensate for the nonfunctioning gene, and he is left missing a crucial protein. Because defective alleles on the X chromosome affect males much more often than they affect females, they are said to be sex linked.
In humans, sex-linked traits include the blood disease hemophilia and color-blindness. About 8 percent of American men are color-blind, because they lack the gene for a pigment responsible for color vision. Hemophilia is caused by a defective allele for a protein that clots blood. In the absence of this protein, the blood will not clot, and, until recently, hemophiliacs who suffered even minor injuries invariably bled to death.
The major treatment for hemophilia in this century has been transfusion, invented in 1908. Transfusion both replaces lost blood and provides the clotting protein that is missing in hemophiliacs. Hemophiliacs who receive frequent transfusions, however, are susceptible to infection by diseases such as hepatitis and AIDS. Today testing programs make it possible to identify and discard most contaminated blood, but by 1995, some 8000 hemophiliacs had AIDS. Today, a safer solution for treating hemophilia is pure clotting protein, or Factor VIII. This protein can be made by isolating the gene for Factor VIII and using recombinant DNA techniques to make the pure protein.
Figure 9.1a: Hemophilia in the family of Queen Victoria. (Click on the figure title to view the image. It will open in a new browser window.) The first person in Queen Victoria's family with hemophilia was her son Leopold. Leopold and two of his sisters, Alice and Beatrice, passed the allele to another generation of royalty. One of Alice's five daughters, Alice Victoria (Alexandra Feodorovna), married the czar of Russia and bore four daughters-Olga, Tatiana, Maria, and Anastasia-and one hemophiliac son, Alexis.
Figure 9.1: The hemophilia allele played an important role in the Russian Revolution of 1917. The Russian Royal Family just before the Revolution. Nikolas II, the last Czar of Russia, married Queen Victoria's granddaughter Alexandra, who carried the allele for hemophilia.