A particularly important category of genetic linkage has to do with the X and Y sex chromosomes. These not only carry the genes that determine male and female traits but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked. Men normally have an X and a Y combination of sex chromosomes, while women have two X's. Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes. Sex cell inheritance patterns
for male and female children
X-linked recessive traits that are not related to feminine body characteristics are primarily expressed in the observable characteristics, or phenotype , of men. This is due to the fact that men only have one X chromosome. Subsequently, genes on that chromosome not coding for gender are usually expressed in the male phenotype even if they are recessive since there are no corresponding genes on the Y chromosome in most cases. In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other. This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes. The "a" recessive allele
will be expressed in his
The "a" recessive allele
will not be expressed in
There are about 1,098 human X-linked genes. Most of them code for something other than female anatomical traits. Many of the non-sex determining X-linked genes are responsible for abnormal conditions such as hemophilia , Duchenne muscular dystrophy , fragile-X syndrome , some high blood pressure, congenital night blindness, G6PD deficiency, and the most common human genetic disorder, red-green color blindness. X-linked genes are also responsible for a common form of baldness referred to as "male pattern...
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