The Inheritance of Retinal Degenerations

Page 5

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X-Linked (Sex-linked)

X-linked retinal degeneration (sometimes called sex-linked) is a recessive disorder caused by an altered gene on an X chromosome. With few exceptions, the disease is seen only in males. To understand X-linked inheritance, it is important to remember that females have a pair of similar sex chromosomes called X chromosomes, and males have one X chromosome and a different, smaller Y chromosome. When a male has an altered gene on his only X chromosome, he must develop the disease because the altered gene is not paired with a normal gene and only the altered information of this gene is present. In an affected male, only one copy of the altered gene is needed to cause the disease. About 10 percent of families affected by RP have the X-Iinked form.

A female who has an altered gene on one of her X chromosomes and a normal gene on the other X chromosome is called a carrier. A carrier female usually has no symptoms of retinal degeneration. However, when examined by an ophthalmologist, the eyes of a carrier female may show some mild effects of the altered gene.

When a carrier female and an unaffected male have children, there is I chance in 2, or a 50 percent chance, that each of their sons will have retinal degeneration and also 1 chance in 2 that each of their sons will not have the disease. Remember, for each child conceived, the mother passes on one of her X chromosomes. The sex of the child is determined by the father who passes on either a Y chromosome, for a male, or an X chromosome, for a female. An affected son will receive the X chromosome with the altered gene from his carrier mother and a Y chromosome from his father. A son who receives the X chromosome with the normal gene from his carrier mother will not have retinal degeneration.

There is 1 chance in 2, or a 50 percent chance, that each daughter of a carrier female and an unaffected male will be a carrier and also 1 chance in 2 that each daughter will not be a carrier (a non-carrier). The carrier mother will pass on either the X chromosome with the altered gene, for a carrier daughter, or the X chromosome with the normal gene, for a non-carrier daughter. The unaffected father will always pass on his X chromosome with a normal gene (Figure 7).

Gifure 7, X-LInked
Figure 7. X-linked retinal degeneration with carrier mother and unaffected father
Affected X Chromosome X Chromosome with retinal degeneration gene Unaffected X Chromosome X Chromosome with normal gene Unaffected Y Chromosome Y Chromosome


When a male with an X-linked retinal degeneration and a non-carrier female have children, none of their sons will be affected because, for each son, the male will pass or his Y chromosome, and the non-carrier mother will pass on one of her X chromosomes with a normal gene. Each of this couple's daughters will be a carrier. Although each daughter will receive an X chromosome with a normal gene from her non-carrier mothei she will also receive her father's only X chromosome with the altered gene (Figure 8).

Figure 8, X-Linked
Figure 8. X-linked retinal degeneration with affected father and non-carrier mother.
Affected X Chromosome X Chromosome with retinal degeneration gene Unaffected X Chromosome X Chromosome with normal gene Unaffected Y Chromosome Y Chromosome



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Date last modified January 23, 2000