What is X-linked inheritance?

X-linked inheritance means that the gene causing the trait or disorder is located on the X chromosome. Females have two X chromosomes. Males have one X chromosome and one Y chromosome. Genes on the X chromosome can be recessive or dominant. Their effect in males and females is not the same. For males, there needs to be only one copy of a nonworking gene (on their 1 copy of the X chromosome) for the trait or disorder to appear. Females are generally not affected unless both copies of the gene (one on each X chromosome) are not working. Females who carry only 1 copy of an affected gene may not have any symptoms of the condition or may show mild symptoms compared with males.

There is a 1 in 2 chance that daughters carry the gene and can pass it to the next generation. There is a 1 in 2 chance that a daughter will not carry the gene and so cannot pass it on. There is a 1 in 2 chance that sons don't have the gene and will be healthy. But there is a 1 in 2 chance that a son will have inherited the gene and will have the trait or disorder.

What are some of the different types of X-linked recessive conditions?

Examples of X-linked recessive conditions include red-green color blindness and hemophilia A:

• Red-green color blindness. Red-green color blindness means that a person cannot see shades of red and green (usually blue-green). But they can otherwise see normally. This condition has no serious complications. But people affected may not be able to work in certain jobs, such as transportation or the Armed Forces, where seeing color is needed. Males are affected more often than females because the gene is located on the X chromosome.

• Hemophilia. Hemophilia is a disorder in which the blood cannot clot correctly because of a lack of a clotting factor called factor VIII. This results in heavy bleeding that will not stop, even from a small cut. People with hemophilia bruise easily and can have internal bleeding into their joints and muscles. The occurrence of hemophilia A (factor VIII deficiency) is around 1 in 4,500 live male births worldwide. The occurrence of hemophilia B (factor IX deficiency) is 1 in 20,000 live male births worldwide. Hemophilia A accounts for most cases. Treatment for hemophilia A is available by infusion of factor VIII. Female carriers of the gene may show some mild signs of factor VIII deficiency, such as bruising easily or taking longer than usual to stop bleeding when cut. But not all female carriers have these symptoms. One-third of all cases are thought to be new mutations in the family (not inherited from the mother). Treatment for hemophilia B includes replacement of factor IX.