X-linked Inheritance: Red-Green Color Blindness, Hemophilia
Genes are inherited from our biological parents in specific ways. One of the basic
patterns of inheritance of our genes is called X-linked 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 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
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
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 required. 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.