The maternal DDK syndrome phenotype is determined by modifier genes that are not linked to Om

The DDK syndrome is a polar, early embryonic lethal phenotype caused by inc ompatibility between a maternal factor of DDK origin and a paternal gene of non-DDK origin. Both maternal factor and paternal gene have been mapped to the Om locus on mouse Chromosome (Chr) 11. The paternal contribution to th e syndrome has been shown to segregate as a single locus. Although the inhe ritance of the maternal contribution has not been characterized in depth, i t as been assumed to segregate as a single locus. We have now characterized the segregation of the DDK fertility phenotype in over 240 females. Our re sults demonstrate that females require at least one DDK allele at Om to man ifest the syndrome. However, the DDK syndrome inter-strain cross-fertility phenotype of heterozygous females is highly variable and spans the gamut fr om completely infertile to completely fertile. Our results indicate that th is phenotypic variability has a genetic basis and that the modifiers of the DDK syndrome segregate independently of Om.