X-Linked sight loss is caused by a faulty gene on an X chromosome and it affects mainly males, although females can be affected mildly and occasionally severely. As mentioned earlier, the sex chromosomes in women are an identical pair of X chromosomes while men have one X and one Y chromosome. A man with X-Linked sight loss will have a faulty gene on his X chromosome and therefore he will pass on this chromosome to all his daughters, but to none of his sons who receive his Y chromosome (the Y chromosome does not have a copy of the gene). None of his sons will be affected as they receive his Y chromosome whereas all daughters of an affected man are carriers of the gene.

Female carriers have an X chromosome with the faulty gene and also an X chromosome with the normal copy. This normal copy of the gene usually ‘protects’ them from developing the full features of X-linked sight loss. However, some carriers of X-linked sight loss may have some signs of mild sight loss. There may be changes that can be seen in their retina either directly or with specialised retinal imaging, or abnormal function of the retina that can be detected by measuring electrical activity of the retina – a common test in retina clinic. The changes, although mild, may vary a great deal among individuals.

A carrier woman will pass on either one or the other of her X chromosomes to her children and it is a one in two chance whether she passes on the X chromosome with the faulty gene. If that chromosome goes to a son, he will have X-Linked sight loss. If it goes to a daughter, she will be a carrier like her mother.

To summarise this, an affected male cannot pass on X-Linked sight loss to his sons, but all his daughters will be carriers; and a carrier female has a 50 per cent risk or one in two chance of having an affected son, and the same risk of having a daughter who is a carrier like herself.