X-linked traits

So far, we have only looked at genetic concepts and terminology using the autosomes. While the sex chromosomes follow the same rules, the difference in distribution puts a new spin on those rules. Females have two X chromosomes, while males only have an X and a Y. Since males only have one X, they only have one allele of each gene on the X. Therefore, the phenotypes for each of the traits coded for by the genes are only influenced by the one allele. This means that dominance and recessiveness are irrelevant in males for all genes on the X, or X-linked genes. Females have two X chromosomes, and so in females, the X-linked genes follow the same rules of dominance and recessivity as do the genes on the autosomes.

An excellent example of an X-linked gene is the one coding for factor VIII. Factor VIII is a protein, that is essential to blood clotting. People who do not have factor VIII have a condition called hemophilia, and they bleed very easily. Most hemophiliacs are males because males only have one copy of the X chromosome, therefore if the X contains a mutant copy of the factor VIII gene that does not produce factor VIII, males automatically have the condition. Since the mutant allele of the gene is recessive, females can get the disease, but they usually act as carriers only. If we use Xh+ and Xh to denote the normal and mutant alleles of the gene respectively, the possible genotypes andphenotypes are as follows: 

Xh+/Xh+:  normal female
Xh+/Xh: normal female, carrier
Xh/Xh: hemophiliac female
Xh+/Y: normal male
Xh/Y: hemophiliac male
Another example of an X-linked gene is Xg, which codes for the Xg blood group. People either have the blood group or they don't. The allele coding for the blood group is dominant, and not having the Xg blood group does not result in a disease. If we use Xg+ and Xg to represent the alleles that code for the Xg group and don't code for it respectively, then the genotypes and phenotypes break down as follows: 
Xg+/Xg+:  female with Xg-positive blood group
Xg+/Xg:  female with Xg-positive blood group
Xg/Xg: female with Xg-negative blood group
Xg+/Y:  male with Xg-positive blood group
Xg/Y:  male with Xg-negative blood group
Finally, it should be noted that since only males have the Y chromosome, only males can pass on the Y to offspring. Therefore, any daughters that a father has will receive his X, while any sons he has will receive the Y.