Inheritance Patterns

The two sex chromosomes differ in the genes they carry. The Y chromosome is very small, and appears to carry very few genes other than the SRY gene that determines male sex. Many genes are carried on the X chromosome, however, and these are as essential for males as they are for females. Genes carried on the X chromosome are said to be X-linked.

X-linked dominant alleles affect both males and females, although males may be more severely affected since they inherit only a single X chromosome and thus lack a compensating normal allele. An example of a disorder caused by an X-linked dominant allele is congenital generalized hypertrichosis, which causes dense hair growth on the face and other regions in both sexes. X-linked dominant alleles can be inherited by both males and females, but fathers cannot pass them on to sons.

X-linked recessive alleles affect males more often and more severely than females. A male inherits his single X chromosome from his mother. Because a male has only one X chromosome, he expresses every allele on it, including harmful recessive ones. Examples of conditions due to recessive X-linked alleles include Duchenne muscular dystrophy, one form of hemophilia, and red-green colorblindness. These conditions are much more common in males than in females. Female carriers have a 50 percent chance of giving birth to a male child affected by the recessive allele. The genetic status of the father with respect to X-linked conditions is not relevant in this case, because he donates a Y chromosome to his male children.

Since females have two X chromosomes, they are less likely than males to express harmful recessive X-linked traits. Female children have only a 25 percent chance of inheriting two recessive alleles from a carrier mother and an affected father.