PRA is a disease which causes problems with the retina in the eye. The retina contains special cells called 'photoreceptors' which are importnat in vision. In a healthy dog these receptors continually renew and are replaced by new cells. In affected dogs this process of renewal and replacement slows down and then eventually stops altogether. Night vision receptors are the first to be affected, and after time, the day vision receptors start to degenrate too.
XL-PRA is a "late-onset" form of PRA. It usually isn't until dogs are three to five years old that the first clinical signs of disease start to manifest.
The disease is described as X-linked, which means that the faulty gene that causes this type of PRA is found on the X-chromosome.
Male dogs only have one X-chromosome, which they always inherit from their mother, while females have two X-chromosomes, which they inherit from both their mother and father.
In females, the disease acts in a recessive way, i.e., a female must inherit two copies of the faulty gene (one from her mother and one from her father) to be affected.
In males, because they only have a single copy of the X chromosome, the condition behaves as if it were dominant and male dogs are either healthy (they only have a healthy copy of the gene) or affected (they only have the faulty copy of a gene).