Progressive Retinal Atrophy comes in two forms. rcd2-PRA, is a dysplasia of the rods and cones in the eye which leads to early onset night blindness. It is already apparent when puppies are 6 weeks of age. It leads to complete blindness by 1 year of age. It is not present in UK dogs, but when importing from other countries (especially the US), the dog should be tested for PRA to ensure it is not introduced. The form of PRA that could be found in UK dogs is thought to have been eliminated with advances in diet and nutrition.

GENETIC TEST
rcd2-PRA is an autosomal recessive disease which means a dog requires two copies of the mutation for the disease to occur. A genetic test is available to determine carrier status.


The below chart is a theoretical breeding calculator for rcd2-PRA. Results in bold are results where the puppy will display the disease. Results that are 'around' a percentage means that each puppy has the quoted percentage chance of that result. It does not mean that EXACTLY that percentage of the litter will have the result. 
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RESULTS IN PRACTICE
Although it can seem like the most ethical thing to do would be to never breed from carrier dogs, erasing this portion of the gene pool would have harmful effects (please see Genetic Diversity section). Instead, ensure your breeding stock is tested, and work towards developing non carriers of the condition with subsequent generations of dogs, by breeding carrier to non carrier. 

Good practice would be to ensure dogs you import from other countries are PRA genetically tested.

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FURTHER READING

Collie Health Foundation - Test your Collie 'PRA-rcd2'
Animal Labs - PRA-rcd2 (Progressive Retinal Atrophy) - more detailed information on causation and development