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Sex linked and digenic cases in retinitis pigmentosa (RP)
Sex linked retinitis pigmentosa
The human genome consists of 46 chromosomes. These chromosomes are grouped into pairs. 22 of the pairs are identical and are called autosomal chromosomes while the 23rd pair is the sex chromosomes. Besides varying in length - the shorter of the two being termed Y and the longer termed X - it is these two sex chromosomes that as their name implies determine the sex of a child. Males have a XY chromosome pair configuration and females an XX chromosome pair configuration.
With X linked recessive retinal pigmentosa, the mutation has occurred on the X chromosome - making the presence of a normal X chromosome all-important.
As you will probably see, because males have only one X chromosome, the presence of a mutant X chromosome for retinitis pigmentosa assures that they will have RP. In contrast, a female has two X chromosomes. Since the trait is recessive, this means that the combination of an altered X chromosome for RP and a normal X chromosome assures that a female will be only a carrier of the disease. Generally, her symptoms will be mild if at all. Only an ophthalmologist is usually able to see the affects of the sex-linked retinitis pigmentosa in her eyes.
Digenic
Digenic inheritance is rarer than the other three forms. It involves 2 different genes on different chromosome pairs. Remember that each gene pair causes the expression of a particular trait in an individual. There are numerous types of retinitis pigmentosa and already some 7 genes associate with them.
In the digenic RP situation, when certain altered genes for retinitis pigmentosa located on different chromosomes are present in the same individual, they interact with each other to cause retinitis pigmentosa. There is a 25 % chance of a person with digenic RP passing on the degenerative disease to the next generation.
An example
For arguments sake, we will designate chromosomes 2 and 3 as the chromosomes that carry the mutant genes for digenic RP. The symbol (ii) symbolizes the altered state of the gene and (i) the normal gene. There are four chromosome 2 and 3 combinations that may be passed by an affected individual (one who has digenic RP) to next generation.
The genome of a digenic RP parent (using the description above) is 2i2ii and 3i3ii. The affected person's egg or sperm will have chromosome 2 and 3 combinations of 2i3i, 2i3ii, 2ii3i or 2ii3ii. The probability of passing 2ii3ii (digenic RP) onto the next generation is therefore 25 %.The table below show the four ways that chromosomes 2 and 3 can combine to be passed on to the next generation. (ii) symbolizes the altered gene while (i) symbolizes the normal gene.
In the case of a carrier (ie. full complement of chromosome 2 and 3 chromosomes represented by 2i2i3i3ii or 2i2ii3i3i)) - only one of the two gene pairs has an altered gene. Because either chromosome 2 or 3 has a normal chromosome pair, the altered gene has no other mutant gene to interact with to cause the expression of RP.
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Contact: retinitisp@yahoo.ca
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