Modification toward dominance of a recessive lethal in the mouse.

The effect of the recessive siren (srn) gene singly in the presence of one Danforth's short tail (Sd) gene was to modify the expression of srn towards dominance with 46 percent penetrance. All mice that could be classified as sirens from srn/+ x Sd/+ matings had the Sd type short tail, indicating that interaction between Sd and srn was the principal cause for modification towards dominance of srn. External expressivity was variable in Sd srn pups, with true sirens, semi-sirens and apposed leg pups produced, each traceable to pelvic abnormalities. Variation in the urogenital system was similar to that found in siren pups with the srn/srn genotype.