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Inheritance Patterns of RP (ii)

2. Autosomal Recessive

Autosomal Recessive RP, like Autosomal Dominant RP, affects men and women equally because the faulty genes are carried on the autosomes, but that is where the resemblance ends. The mechanism is different because in order to have recessive RP it is necessary to have a 'fault' on both copies of the relevant gene. That is both chromosomes of a pair carry this faulty gene. An affected person inherits one faulty gene from each parent who is called a carrier. Each parent will have one normal and one faulty copy of the gene and they do not have any symptoms of RP. In Autosomal Recessive RP, one normal copy is enough to keep the retina healthy, so carriers have normal vision.

We all have several faulty recessive genes and they only may cause problems if we have a child with a person who carries a copy of one of the same genes.

Having introduced the idea of tossing a coin in the case of illustrating how Autosomal Dominant RP is passed on, think of both parents, each tossing a coin. Their risk of having a child with Autosomal Recessive RP is the same as both coins coming up heads; this comes to a one in four chance (25% risk of having an affected child). To describe how this comes about, there are four possible outcomes if you toss two coins, those are: both coins heads, both coins tails, coin one heads and coin two tails, and, coin one tails and coin two heads.

Only the first outcome of both coins heads corresponds to two faulty genes, one on each chromosome and this is the one in four of the possible outcomes. That is where we get the risk of an affected child being one in four. And the other three outcomes correspond to an unaffected child.

Let us look at the other three outcomes more closely, bearing in mind that they correspond to an unaffected child. Outcome two, both coins tails, both genes are normal, so the child is normal and is not a carrier. Both outcome three and four have one coin heads and one coin tails, this is the same as one gene of the pair normal and the other faulty, exactly like the parents who are carriers. So these three outcomes all produce a normal child but two in three of these will be a carrier (Figure 12).

To summarise this, parents who are carriers have a one in four chance of having an affected child (25%) and a three in four chance of having an unaffected child (75%). There is a 50% chance of the child being a carrier (two in four chance).
Figure 12 - Autosomal Recessive Inheritance

Figure 12 - Autosomal Recessive Inheritance

Finally, anyone who is affected with Autosomal Recessive RP has to contribute one of the faulty genes of the pair to their children, which means that all of their children are carriers. An affected individual is unlikely to pass on the condition as long as their partner is not a carrier. There is of course a small risk of the partner being a carrier, perhaps one in two hundred or less depending on the frequency of carriers of the particular faulty gene in the general population. Couples who marry within the family have a higher chance of a partner being a carrier depending on how they are related; and this is why having a partner who is a relative carries a higher risk of affected children.

There are many forms of Autosomal Recessive RP with Syndromic RP (RP Syndromes) commonly being inherited in an autosomal recessive manner. It is known for example that the various forms of Usher syndrome and Bardet-Biedl syndrome show recessive inheritance. Infantile onset RP termed Leber congenital amaurosis is also inherited in this way. Autosomal Recessive RP usually has an earlier onset and is more severe than Autosomal Dominant RP.

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