IRDs can be caused by one of many possible genetic changes, but they all result in a similar set of symptoms related to vision. Researchers have identified hundreds of different genetic changes that can each cause IRDs, although most IRD cases are caused by changes to one of a small number of commonly affected genes.
Genes are instructions for cells to make proteins in the body. Everyone carries two copies of each gene, one inherited from each parent. IRDs are the result of a change (mutation) in the relevant gene.
IRDs usually run in families, but the way it is passed from parents to their children varies depending on the specific genetic changes responsible for a person’s IRD.
Autosomal recessive IRDs occur when there are two faulty copies of the involved gene, one from each parent. Most IRD cases are inherited in an autosomal recessive pattern.
Autosomal recessive IRDs tend to produce signs and symptoms between 30 and 40 years of age and usually cause more severe sight loss.
Autosomal dominant IRDs occur when there is one faulty copy of the involved gene.
Autosomal dominant IRDs are generally less severe than other forms of IRD and usually result in symptoms from around 30 years of age.
In diseases with X-linked inheritance, the affected gene is located on the X chromosome. Typically, females have two X chromosomes, whilst males have one X and one Y chromosome.
Females with one affected copy and one normal copy are known as carriers.
As males only have one copy of the X chromosome gene, males with one affected copy do not have a second working copy and are therefore affected with the genetic disorder.
X-linked IRDs can result in severe vision loss, often with blindness or near-blindness by the age of 40.
Many cases of IRDs occur in people without any known family history. Parents may have passed the genetic changes onto their offspring but did not develop symptoms themselves. In these cases, it may not be possible to determine how the IRD is inherited.
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