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 variant 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. A variant 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. Females with Turner syndrome (who have only 1 X chromosome) may also have X-linked recessive conditions if they carry the recessive trait on the one X chromosome they have.

Inheritance - sex-linked recessive; Genetics - sex-linked recessive; X-linked recessive

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Updated by: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.