By tracing the pattern of affected and unaffected family members, it is often possible to
determine if the retinal degeneration in a family is dominant or recessive and whether
the mutated gene lies on an autosome or on a sex chromosome. An inheritance
pattern can be determined only when more than one member of a family has a retinal
degeneration. Once a pattern has been identified, genetic counseling should be
provided to any family member who is at risk for having the disease or who is at risk for
having children with the disease. In most cases, RP and other retinal degenerative
disease are inherited in either an autosomal dominant, an autosomal recessive or an X
linked pattern. Another unusual inheritance pattern, known as digenic, has been seen
in a small number of families affected by RP.
Autosomal Dominant
In an autosomal dominant retinal degenerative disease, an affected person has one
altered gene (the mutated gene) paired with one normal gene. These genes lie on one
of the 22 pairs of autosomal chromosomes. About 20 percent of all families with RP
have autosomal dominant RP.
When a person with an autosomal dominant retinal degeneration (whether male or
female) and an unaffected partner have children there is 1 chance in 2, or a 50 percen
chance, that the affected parent will pass on the altered gene, and therefore the
disease, to each child. There is also chance in 2 that he (or she) will pass on the
normal gene to each child. The unaffected partner will pass on one of his or her norm~
genes (Figure 5). A child who does not have an altered gene does not have the
disease and cannot pass the gene (and the disease) on to his or her future children.
Figure 5. Autosomal dominant retinal degeneration
Chromosome with
retinal degeneration gene
Chromosome with normal gene
Most often, families with an autosomal dominant retinal degeneration can trace the
disease back through several generations. Rarely, in some of these families, the
disease seems to have skipped one or more generations. In some families, it is
possible for members to inherit a gene for autosomal dominant retinal degeneration but
not develop the disease. This gene is said to have reduced penetrance, which means
that the gene's effect is somehow modified, or reduced, and sometimes does not cause
retinal degeneration even when present. However, in other families, the disease in
affected members may be so mild that there is no, or very few, symptoms. This gene is
said to have variable expressivity, which means that the severity of the effect of the
altered gene can vary from one affected family member to another.
The age at which family members first show symptoms of retinal degeneration may also
vary. Some family members can experience symptoms of the condition early in life,
possibly in childhood, whereas other affected members in the same family may not have
symptoms until they are well into middle age. When a family member dies before
symptoms of retinal degeneration are discovered, but has affected children, it often
seems that the disease has skipped a generation. Any person at risk for a retinal
degeneration should have a thorough examination by an ophthalmologist to detect mild
or late onset cases of the disease.
Chromosome with
retinal degeneration gene
Chromosome with normal gene
