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Breeding recommendations

Having one or more variants does not automatically exclude an animal from breeding. The goal is to step-wise reduce the frequency of P-variants in a breed population without affecting genetic diversity or eliminating desired traits.


  • Animals with clinical symptoms of MIM (PSSM2) should not be used for breeding for animal welfare reasons. 

  • Animals with one or more variants which have no clinical symptoms can be considered for breeding if the breeding partner is tested normal for all variants. 

    If it is planned to breed horses which both have variants, it is recommended to pair animals which have different variants to limit the production of possible homozygotes.

  • The impact of the Px variant is still unclear. For this reason, EquiSeq does not recommend considering the Px variant in breeding decisions. It appears that the Px variant is may be a greater problem when it is found in combination with other variants. For example, a horse with the profile n/P2 will usually have less severe symptoms than a horse that is n/P2 n/Px.

  • It is recommended to limit the breeding of homozygous animals (P*/P*) because all offspring will inherit at least one variant from that parent.