X-linked recessive diseases most often occur in males. Males have only one X chromosome. A single recessive gene on that X chromosome will cause the disease.
The Y chromosome is the other half of the XY gene pair in the male. However, the Y chromosome doesn't contain most of the genes of the X chromosome. Because of that, it doesn't protect the male. Diseases such as hemophilia and Duchenne muscular dystrophy occur from a recessive gene on the X chromosome.
TYPICAL SCENARIOS
In each pregnancy, if the mother is a carrier of a certain disease (she has only one abnormal X chromosome) and the father is not a carrier for the disease, the expected outcome is:
- 25% chance of a healthy boy
- 25% chance of a boy with disease
- 25% chance of a healthy girl
- 25% chance of a carrier girl without disease
If the father has the disease and the mother is not a carrier, the expected outcomes are:
- 100% chance of a healthy boy
- 100% chance of a carrier girl without disease
X-LINKED RECESSIVE DISORDERS IN FEMALES
Females can get an X-linked recessive disorder, but this is very rare. An abnormal gene on the X chromosome from each parent would be required, since a female has two X chromosomes. This could occur in the two scenarios below.
In each pregnancy, if the mother is a carrier and the father has the disease, the expected outcomes are:
- 25% chance of a healthy boy
- 25% chance of a boy with the disease
- 25% chance of a carrier girl
- 25% chance of a girl with the disease
If both the mother and the father have the disease, the expected outcomes are:
- 100% chance of the child having the disease, whether boy or girl
The odds of either of these two scenarios are so low that X-linked recessive diseases are sometimes referred to as male only diseases. However, this is not technically correct.
Female carriers can have a normal X chromosome that is abnormally inactivated. This is called "skewed X-inactivation." These females may have symptoms similar to those of males, or they may have only mild symptoms.