Inheritance Patterns
Eye Conditions & Care | Inheritance Patterns
Inheritance Patterns for Retinitis Pigmentosa
RP results from a large number of gene defects; so far around a hundred gene defects have been found to cause the disease. All the gene defects for RP and other inherited retinal diseases have yet to be identified.
The information below is aimed at helping understand the ways that the disease can be inherited. To correctly identify the cause of RP you should consult your Ophthalmologist who may refer you to a Geneticist and a Genetic Counsellor. It is important to identify the form of RP in order to be able to make decisions based on the correct information and to be able to benefit from cures in the future if and when they are found.
Different forms of RP can result from a person inheriting one or two abnormal genes, depending on the type of RP. There are three ways in which RP can be inherited or passed on:
Autosomal Dominant Inheritance
This is the pattern of inheritance where RP is known to exist in a family, affecting both males and females. The probability of RP being passed from an affected parent to a child is exactly 50 per cent.
The diagram indicates the four possible combinations of genetic information that may be passed on by the parents. The mutant dominant gene is represented by "D" and the normal gene by "d".
Autosomal Recessive Inheritance
This form means each parent is a carrier (have the gene present) but is not affected by the disease themselves. There will usually be no known history of RP in the family. If two carriers who show no obvious symptoms have a child, there is a 25 per cent chance that he or she will have RP.
An affected individual is unlikely to pass on the condition as long as their partner is not a carrier. There is about one in 200 chance of their partner being a carrier depending on the frequency of carriers in the general population. The risk of finding a partner who is a carrier among a relative is however much higher (over one in 100) and this is why cousin marriage carries a higher risk of affected children.
The diagram indicates how the recessive genes are passed from the two carrier parents to their children. The mutant RP gene is represented by "r" and the normal gene by "R".
X-linked Recessive Inheritance
This is a pattern of inheritance where only males develop the disease, but female members of a family are carriers. Some carriers can develop a mild form of RP. For example, if a man has X-linked RP, his sons will not develop RP, but all of his daughters will become carriers. These daughters will each have a 50 per cent chance of producing an affected son and a 50 per cent chance of daughters who will be carriers. This inheritance pattern is sometimes difficult to identify in a family where there have been no sons for several generations, as the faulty gene could have passed down a line of female carriers and then suddenly affect a male child.
This diagram depicts the four possible combinations of genetic input from a couple, each contributing a copy of each of their genes. The woman is a 'carrier' of X-linked RP; the mutant gene on the X chromosome is represented by "r". The man can only convey normal genes.
If you would like more information about inheritance patterns and the availability of genetic testing in the U.A.E. please contact the Foresight office.