We inherit half of our genetic information from each parent and each child will inherit a different set of genes, and only identical twins have exactly the same genes. The genes are carried on the chromosomes and we normally have 22 pairs of matched chromosomes numbered 1 to 22, the Autosomes. We also have two sex-determining chromosomes – females have two x’s and males have an x and a y chromosome.
The genes are sets of instructions for the body to produce the proteins needed for the proper growth and function of the entire human body. We all carry some mistakes in these instructions, but they are largely undetectable and unnoticed.
In Recessive IRD, if a carrier of a gene mutation marries a partner who carries the same gene mutation, then their children may inherit a double copy of the gene mutation and thus have a recessive condition. The chances of a carrier couple having an affected child is one in four or 25%. There is also a 25% chance that the child will inherit a good copy of the gene from both parents and they will not be affected nor will they be carriers. There is a two in four or 50% chance, that the child will inherit a single copy of the mutated gene from either parent and thus once again become a carrier without being affected. This scenario is repeated on each pregnancy. Children of parents who have a recessive IRD are obligatory carriers. Genetic counselling will help parents and patients understand the risks and implications of inherited conditions.
In dominant IRD only one donor parent from either gender, who is affected, may pass the gene mutation on to his/her children with a 50% chance on each pregnancy. As there is a normal copy of the gene present in the cell the IRD is usually less severe, but is usually seen in every generation. Children of an IRD patient who have not inherited the mutated gene cannot pass the trait onto their children.
Recessive X-Linked inheritance
In Recessive X- Linked IRD the gene mutation is carried on the x–Chromosome. Females have two x chromosomes and males have an x and a y.
If the mother carries a gene mutation for IRD on one of her x-chromosomes she has a one in two or 50% chance of passing that gene mutation to her son. As there is no corresponding good copy of the particular gene the son will be affected by the IRD. His daughters will be obligatory carriers of the mutated gene on the x-chromosome.
She has a 50% chance of passing on the gene mutation to her daughter who will then in turn become an unaffected carrier. The mother also has a 50% chance of passing the good copy of the gene to her daughter whose sons will not be at risk for inheriting the IRD. If the precise gene mutation is known in a family carrier testing is available through Retina South Africa at a very reasonable cost. Genetic counseling is a pre-requisite before genetic testing can be done.
This guide does not serve to replace a session with a registered genetic counsellor. Please contact Retina South Africa for details of genetic counsellors in your area.