What are the three main inheritance patterns of sight loss?
Autosomal dominant inheritance (AD)
Autosomal dominant inheritance means that the faulty gene is on an autosome (any chromosome other than the sex chromosomes) and that only one faulty copy is needed to cause sight loss.
The majority of families affected by autosomal dominant conditions will know them well, as any person who is affected will very probably also have an affected parent, and may have affected siblings or aunts, uncles or cousins. When family members come to decide about having children, they are usually aware that there may be a risk of passing on the faulty gene.
Autosomal recessive inheritance (AR)
Autosomal recessive inheritance means that the faulty gene is on an autosome (any chromosome other than the sex chromosomes) and that both copies need to be faulty to cause sight loss. Autosomal recessive mutations often result in the retinal cell missing an important genetic instruction, leaving the cell struggling to do its job.
Autosomal recessive conditions happen when both of the affected person’s parents each carry only one faulty copy of the gene. They experience no symptoms as their healthy copy of the gene can compensate. However, the affected child ends up inheriting two faulty copies, one from each parent.
x-linked inheritance (XL)
X-linked inheritance means that the faulty gene is located on the X chromosome, the larger of the two sex chromosomes. X-linked inheritance occurs fairly frequently in retinitis pigmentosa, and in these cases the condition is sometimes referred to as X-linked RP, or XLRP. Choroideremia is also an X-linked condition.
X-linked conditions affect males far more frequently than females. Females have two X chromosomes, so even if one of them contains a faulty gene, its healthy counterpart on the other X chromosome can often compensate and sight remains unaffected. Males have only one X chromosome, and one Y chromosome that contains very little genetic information. If their X chromosome contains a faulty gene, there is no second copy to compensate, and they will develop sight loss. If there are a number of men and boys in an extended family who are living with sight loss, but no affected females, then X-linked inheritance is highly likely.
A genetic counsellor will be able to tell you more about your condition and how it may effect your family. For further information on genetic counselling see, How can a genetic counsellor help with inherited sight loss?
Further information on genes and genetic testing can be found on the Retina UK website.
