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.
X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Remember, females have two X chromosomes, while males have one X and one Y. Genes on the X chromosome can be recessive or dominant, and their expression in females and males is not the same. Genes on the Y chromosome do not exactly pair up with the genes on the X chromosome. 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 in order 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:
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There is a 50 percent chance that daughters carry the gene and can pass it to the next generation. There is a 50 percent chance that a daughter will not carry the gene and, therefore, cannot pass it on. There is a 50 percent chance that sons do not have the gene and will be healthy. However, there is a 50 percent chance that a son will have inherited the gene and will express the trait or disorder.
Examples of X-linked recessive conditions include red-green color blindness and hemophilia A:
- Red-green color blindness. Red-green color blindness simply means that a person cannot distinguish shades of red and green (usually blue-green). Their visual acuity (ability to see) is normal. There are no serious complications; however, affected individuals 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 4,500 live male births. The occurrence of hemophilia B (Factor IX deficiency) is 1 in 20,000 live male births. 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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