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Inheritance Patterns of RP (i)

RP is caused by 'faults' in the genes (instructions) in the cells of our body that are required to build and maintain the health of the retina.A fault in a gene is sometimes described as a 'spelling mistake' in the DNA code (or 'mutation'). We inherit our genes in the egg and sperm from our parents. All of our genes come in pairs and when pass our genes on to our children, we pass on just one copy. The child inherits one copy from their mother and one copy from their father.

Genes are carried on structures in the cell called 'chromosomes'. Every cell in our body has 23 pairs of chromosomes. they are numbered by size. Chromosomes one to 22 are called the 'autosomes' and they are same in boys and girls. The 23rd pair are called the 'sex chromosomes' and differ between boys and girls, with girls having two X chromosomes and boys having an X and a Y chromosome.

Different forms of RP can result from a person inheriting one or two abnormal genes, depending on the type of RP.

Genetic conditions can be passed from one generation to the next in a variety of ways. These are called the patterns of inheritance. In the eye clinic, your doctor or genetic counsellor will spend some time asking about other people in your family to try to work out the pattern of inheritance. In some families RP may have been present for several generations but in others there may be no other people known to be affected.

There are three ways in which RP can be inherited or passed on:
  1. Autosomal Dominant
  2. Autosomal Recessive
  3. X-Linked
1. Autosomal Dominant

Autosomal Dominant RP is due to a 'fault' (or mutation) in one gene, which is on one pair of autosomes. In other words, Autosomal dominant RP occurs when a fault in only one copy of a gene is sufficient to cause RP. Genes that can cause Autosomal dominant RP are often important for the structure of the retina. Autosomal Dominant RP affects men and women equally, and can be passed on from an affected parent to a son or a daughter. A person with Autosomal Dominant RP has a one in two risk (50% risk) of passing on the condition to one of their children. This is because he or she will contribute one chromosome from each pair to a child. Depending on whether the chromosome carries the faulty gene or its normal counterpart, the child will be affected or not (Figure 11).

The chance of a child being affected is therefore 50:50 in each pregnancy. Think of this as the tossing of a coin and the chance of the coin coming up heads. Chance actually has no memory and neither does nature. You have the same chance of a coin coming up heads irrespective of the number of times you have tossed it. In other words, if you have Autosomal Dominant RP and you already have a child with the condition, there is still a one in two chance of the next child being affected as well.
Figure 11 - Autosomal Dominant Inheritance

Figure 11 - Autosomal Dominant Inheritance

In some families, Autosomal Dominant RP has been known to exist for several generations while in others it may have been evident for only one or two generations. It does have to start somewhere so that some people who have no affected relatives may have Dominant RP although this in practice is not common. Autosomal Dominant RP usually has a later onset and is less severe than Autosomal Recessive and X-linked RP.

Autosomal Dominant RP can be particularly variable, both between and within families, so some family members can be affected mildly and others more severely, irrespective of their ages. Occasionally the effects of the Autosomonal Dominant RP gene may appear to 'skip a generation' due to the variability in the severity of RP in different individuals or generations.

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