Autosomal recessive inheritance is the most common type of inheritance for retinal dystrophies. When this type of condition is present in a family, it is often seen only in one child or in siblings, not in the parents or other relatives.
As shown in the figure, to have symptoms of Bietti's Crystalline Dystrophy (BCD), an individual must have two copies of the same disease gene. We all have two copies of each gene, since we inherit one from our father and one from our mother.
In the figure above, the N stands for the normal copy of the BCD gene and the B stands for the abnormal copy of the BCD gene. Both parents have an N gene and a B gene and are therefore called “carriers.” In this scenario, there is a 75 percent chance that a child will not have BCD. In each child there is a 50 percent chance to inherit the N gene and a 50 percent chance to inherit the B gene from each parent. There is:
- a 25 percent chance that a child will inherit both N genes and be unaffected
- a 50 percent chance that a child will inherit one N gene and one B gene and be unaffected, but be a carrier of the mutant gene
- a 25 percent chance for a child to inherit both B genes and be affected
These recessive genes can be passed on for generations without causing disease, since two copies are needed to cause symptoms.