The Inheritance of Retinal Degenerations

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Digenic

Recently, scientists identified a fourth inheritance pattern through which a retinal degeneration can be passed from one generation to the next, called digenic. So far, only a few families affected by RP have been identified as digenic. Remember that genes come in pairs. Although each pair of genes specifies one trait, each gene of the pair may carry different instructions for that trait and each will influence how the trait is actually produced in the body. In a digenic retinal degenerative disease, an affected person has disease-causing mutations in two different gene pairs. Each of the gene pairs has one normal gene and one altered gene. The two altered genes act together to cause retinal degeneration, even though they lie on separate autosomes.

Carriers, individuals who have a mutation in one of the gene pairs but not the other, will not be affected. When an unaffected carrier with a mutation in one gene has children with an unaffected carrier with a mutation in the other gene the chances for having children who are affected, unaffected or carriers resembles autosomal recessive inheritance for the first two generations. There is 1 chance in 4, or 25 percent chance, that both parents will pass on their altered gene copies, and therefore the disease, to each child. There is 1 chance in 2, or a 50 percent chance, that each child will be a carrier of one of the altered genes. If both parents pass on normal genes, the child wil not be affected and will not be a carrier. There is 1 chance in 4, or a 25 percent chance that each child will neither have retinal degeneration nor be a carrier (Figure 9).

Figure 9, Digenic
Figure 9. Digenic retinal degeneratino with unaffected carrier parents.
Affected Chromosome Affected Chromosome Chromosome with altered genes Unaffected Chromosome Unaffected Chromosome Chromosomes with normal genes


From the second to third generations of digenic inheritance, there can be direct parent- to-child transmission, similar to autosomal dominant inheritance. When a person (whether male or female) with retinal degeneration due to these two altered genes ha children with an unaffected non-carrier partner, there is 1 chance in 4, or 25 percent chance, that the affected parent will pass on both altered gene copies, and therefore the disease, to each child. There is 1 chance in 2, or a 50 percent chance, that each child will be a carrier of one of the altered genes. And if both parents pass on normal genes, the child will not be affected and will not be a carrier. There is 1 chance in 4, or a 25 percent chance, that each child will neither have retinal degeneration nor be a carrier (Figure 10).

Figure 10, Digenic
Figure 10. Digenic retinal degeneration with affected parent and affected parent.
Affected Chromosome Affected Chromosome Chromosome with altered genes Unaffected Chromosome Unaffected Chromosome Chromosomes with normal genes


Other combinations are more complicated to describe and very complete genetic counseling will be needed.


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Date last modified December 4, 1998