Genetic basics to help understand retinitis pigmentosa (RP)

 

Retinitis pigmentosa as a genetic disease

The following general discussion of genetics is not intended to replace proper genetic counseling by a medical professional rather it is an overview of the mechanisms involved in autosomal and sex linked diseases, specifically retinitis pigmentosa.

The starting point of any discussion on genetics is the gene. Genes are responsible for determining the form and function of every living organism. It is this inherited unit that is passed from parent to offspring.

Well - this isn't entirely correct. Rather it is chromosomes that are passed from one generation to the next while genes happen to be located on these chromosomes. Genes are composed of DNA (deoxyribonucleic acid) - the basic building block of living things. DNA looks somewhat like a long bead like strand that exists in pairs.

The human genome consists of 23 chromosome pairs. Each chromosome in turn contains unique DNA that when damaged through mutations or errors incorporated into the strand results in the improper functioning of cells. This may lead to the death of tissues, organs or even the body . However, because there are 2 copies of each chromosome and therefore each gene - an altered gene does not necessarily mean that this mutant gene will be the one expressed by the individual as we will see shortly.

What this means to Retinitis pigmentosa

There are four basic types of inheritance in retinitis pigmentosa: autosomal recessive, autosomal dominant, sex linked and digenic. But first an understanding of how chromosomes are passed from parent to offspring.

Back to the 46 chromosomes (23 chromosome pairs) - 23 of which come from dad (sperm) and 23 of which come from mom (egg) during fertilization. 22 of these 23 chromosomes are referred to as autosomal chromosomes and are identical in both males and females. The only differences between the two chromosomes of an autosomal chromosome pair are variances in the DNA that is responsible for character traits such as eye or hair color.

The 23rd chromosome pair differs from the autosomes in that it marks the physical and metabolical variances between males and females. These chromosomes are designated X and Y and determine the sex of a baby, a fact that gives them their name - sex chromosomes.

So, returning to fertilization, when mom provides 22 autosomal chromosomes (not pairs) and an X chromosome and dad provides 22 chromosomes (not pairs) and either an X or Y chromosome, there is a total complement of 23 chromosome pairs. A sex chromosome combination of XX results in a girl while an XY combination results in a boy. This will become important in our discussion of sex linked retinitis pigmentosa.

Every child that is born receives 23 chromosomes (not pairs) from mom (maternal chromosomes) and 23 chromosomes (not pairs) from dad (paternal chromosomes). This means that each chromosome of a chromosome pair has a 50 % chance of being passed onto a child in the next generation. Any genes located on a given chromosome, whether normal or altered, become part of a child's genome. The genes from one parent will interact with the maternal or paternal genes from the child's other parent according to the set of instructions set out by the DNA encoded within them.

With retinitis pigmentosa, which already has more than 7 genes identified with it, scientists have determined four hereditary patterns: autosomal recessive, autosomal dominant, sex linked and digenic degenerative retinal disease.

NEXT PAGE Autosomal recessive/autosomal dominant RP

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