Diagram for inheritance of PRA (Progressive Retinal Atrophy)

reproduced with kind permission from Swedish Siamese Society (SES)

PRA is a hereditary eye disorder where the retina slowly degenerates, making the cat blind. This page illustrates the inheritance.

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= Normal - cat without the gene for PRA, does not pass on the gene for PRA


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= Carrier - cat with one gene for the disorder, can pass this gene on to progeny, will not develop PRA


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= Affected - cat with two genes for the disorder, will pass this gene on to progeny, will also develop PRA


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GreenCat46   +        GreenCat77

Normal cat mated with normal cat

GreenCat46 GreenCat46 GreenCat46 GreenCat46

100% of the progeny will be normal/without the gene for PRA

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GreenCat77   +        YellowCat77

Normal cat mated with carrier

GreenCat46 GreenCat46 YellowCat46 YellowCat46

50% will be carriers, 50% will be normal

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NB! The following combinations are not recommendable and should not be done

YellowCat77   +        YellowCat77

Carrier mated with carrier

RedCat46 YellowCat46 YellowCat46 GreenCat46

25% of the progeny will get PRA, 50% will be carriers and 25% will be normal

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YellowCat77   +        RedCat77

Carrier mated with affected cat (has PRA)

RedCat46 RedCat46 YellowCat46 YellowCat46

50% of the progeny will be affected and 50% will be carriers

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RedCat77   +        GreenCat77

Affected cat mated with normal cat

YellowCat46 YellowCat46 YellowCat46 YellowCat46

100% of the progeny will be carriers

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RedCat77   +        RedCat77

Affected cat mated with affected cat

RedCat46 RedCat46 RedCat46 RedCat46

100% of the progeny will be affected

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