Sex Linkage

Among the 23 pairs of chromosomes in human cells, one pair is the sex chromosomes. (The remaining 22 pairs of chromosomes are referred to as autosomes.) The sex chromosomes determine the sex of humans. There are two types of sex chromosomes: the X chromosome and the Y chromosome. Females have two X chromosomes; males have one X and one Y chromosome. Typically, the female chromosome pattern is designated XX, while the male chromosome pattern is XY. Thus, the genotype of the human male would be 44 XY, while the genotype of the human female would be 44 XX (where 44 represents the autosomes).

In humans, the Y chromosome is much shorter than the X chromosome. Because of this shortened size, a number of sex-linked conditions occur. When a gene occurs on an X chromosome, the other gene of the pair probably occurs on the other X chromosome. Therefore, a female usually has two genes for a characteristic. In contrast, when a gene occurs on an X chromosome in a male, there is usually no other gene present on the short Y chromosome. Therefore, in the male, whatever gene is present on the X chromosome will be expressed.

An example of a sex-linked trait is colorblindness. The gene for colorblindness is found on the X chromosome. A woman is rarely colorblind because she usually has a dominant gene for normal vision on one of her X chromosomes. However, a male has the shortened Y chromosome; therefore, he has no gene to offset a gene for colorblindness on the X chromosome. As a result, the gene for colorblindness expresses itself in the male.

Another example of sex-linked inheritance is the blood disease hemophilia. In hemophilia, the blood does not clot normally because an important blood-clotting protein is missing. The gene for hemophilia occurs on the X chromosome. As females have two X chromosomes, one X chromosome usually has the gene for normal blood clotting. Therefore, the female may be a carrier of hemophilia but normally does not express hemophilia. Males have no offsetting gene on the Y chromosome, so the gene for hemophilia expresses itself in the male. This is why most cases of hemophilia occur in males