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. 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 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:
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
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