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:

50% chance of a having a healthy boy

50% chance of a having a girl without the disease who is a carrier

This means that none of his children would actually show the signs of the disease, but the trait could be passed to his grandsons.

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.