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X-linked Recessive: Red-Green Color Blindness, Hemophilia A

Genes are inherited from our biological parents in specific ways. One of the basic patterns of inheritance of our genes is called X-linked recessive inheritance.

What is X-linked inheritance?

X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females have two X chromosomes; males have one X and one Y. Genes on the X chromosome can be recessive or dominant. Their expression in females and males is not the same. X-linked recessive genes are expressed in females only if there are two copies of the gene (one on each X chromosome). However, for males, there needs to be only one copy of an X-linked recessive gene for the trait or disorder to be expressed. For example, a woman can carry a recessive gene on one of the X chromosomes unknowingly, and pass it on to a son, who will express the trait:

Genetic illustration demonstrating X-linked inheritance
Click Image to Enlarge

There is a 50% chance that daughters carry the gene and can pass it to the next generation. There is a 50% chance that a daughter will not carry the gene and, therefore, cannot pass it on. There is a 50% chance that sons do not have the gene and will be healthy. However, there is a 50% chance that a son will have inherited the gene and will express 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 distinguish shades of red and green (usually blue-green), but their ability to see is normal. There are no serious complications; however, those affected may not be considered for certain occupations involving transportation or the Armed Forces where color recognition is required. Males are affected more often than females, because the gene is located on the X chromosome.

  • Hemophilia A. Hemophilia A is a disorder where the blood cannot clot properly due to a deficiency of a clotting factor called Factor VIII. This results in abnormally heavy bleeding that will not stop, even from a small cut. People with hemophilia A bruise easily and can have internal bleeding into their joints and muscles. The occurrence of hemophilia A (Factor VIII deficiency) is around 1 in 4500 live male births worldwide. The occurrence of hemophilia B (Factor IX deficiency) is one in 20,000 live male births worldwide. Hemophilia A accounts for most cases. Treatment is available by infusion of Factor VIII (blood transfusion). 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. However, not all female carriers present these symptoms. One-third of all cases are thought to be new mutations in the family (not inherited from the mother).



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