Retinitis Pigmentosa (RP) is a group eye degeneration disorders that together are the most common cause of heriditary blindness. Retinitis pigmentosa causes abnormalities of the photoreceptors (rods and cones) or the retinal pigment epithelium (RPE) of the retina leading to progressive visual loss. Patients first experience defective dark adaptation or "night blindness," followed by constriction of the peripheral visual field and eventually loss of central vision late in the course of the disease. The name "retinitis pigmentosa" come from the pigments that are deposited on the retinal surface as vision damage accumlates.
The genetics of Retinitis Pigmentosa are variable. At least 34 different genes, or loci, are known to cause RP. Also, RP can be inherited in different ways depending on which gene is involved. It is inherited most frequently inherited (84%) as an autosomal recessive, next as an autosomal dominant (10%), and least frequently (6%) as an X-linked recessive condition.
Cat Eye Syndrome is a rare chromosomal disorder involving colombomas in the eye, renal abnormalities, mental impairemen and imperforate anus. The characteristic look of the eyes ( although not present in all people with this syndrome) give Cat Eye syndrome its name.
Cat Eye syndrome is caused by extra material on chromosome 22. Sometimes this tip of 22 gets cut off and then duplicated leading to a marker extra chromosome called Inverted Duplication (22qter-22q11).
Prader Willi Syndrome (PWS) is a rare genetic disoder involving mental retardation and behaviour problems. The most notable symptom is an eating disorder - they literally never feel full and are constantly ravinous. This leads to gross obesity if the diet is not carefully managed. Prader Willi occurs in about 1 in 12,000 individuals.
The genetics of PWS are very interesting and complicated. Unlike other genetic syndromes, Prader Willi involves imprinting. For most genes, both the one from mom and the one from dad do their job; problems happen if there is a mutation in one or both that stop the genes from working. But with imprinted genes, normally only one gene from a parent works, and the other is disabled, or silenced. In people without Prader Willi, the maternal gene for PWS is silenced and only the paternal copy is active.
Almost always sporadic, and not inherited, PWS is caused by the absence of a working copy of the PraderWilli region on chromosome 15. Most often this is due to an accidental deletion of the paternal Prader Willi gene, or a double dose of genes from mom - in either case there is no functioning Prader Willi gene. Very rarely a defect in the process of imprinting, whether the right genes are switched on or off, can be passed down through a family and cause PWS.