Biology 11th Edition by Cecie Starr ,Ralph Taggart

تمرين 20

About 300 million years ago, before mammals began their great adaptive radiation, their X and Y chromosomes were about the same in size. When paired, the two sex chromosomes typically synapsed and exchanged alleles along their length. The X chromosome now carries 1,141 genes. Over time, however, the Y chromosome lost most of itself and now contains only 255 genes. Its big claim to fame is ownership of the SRY gene, the master of sex determination.
Think about the dosage compensation theory, sketched out in Section 15.2. According to this theory, X chromosome inactivation is natures way of compensating for a double dose of X-linked genes in XX embryos, because there are not enough genes left on the puny Y chromosome to balance out their expression. And yet, about 15 percent of the genes on an inactivated X chromosome escape being painted to varying degrees-which means women make more copies of certain proteins than men do.
Besides this, another 10 percent of the X-linked genes might or might not get painted in individual embryos-which means women differ significantly from one another in which X-linked genes are active.
Now consider this: Human and chimpanzee genomes differ by 1.5 percent. Women differ from men by 1 percent! Go ahead and let your brain chew on that one.
You may wish to start with this recent article: L. Carel and H. Willard, "X-Inactivation Profile Reveals Extensive Variability in X-Linked Gene Expression in Females," Nature 2005; 434(7031):400-404.