Some sources of nongenetic variability in steroid‐induced cleft palate in mice

C57BL/6JKt female mice mated to A/JKt males were each injected im with 50 mg/kg triamcinolone 13 days post-VP, and the roles of a number of nongenetic factors underlying variability in the frequency of cleft palate (CP) induced in the fetuses were analyzed. Litter size and number of young per uterine horn, overall, were not significantly related to the CP rate. But nearly one-quarter of litters (almost all larger-sized ones) contained CP rates deviating from-mostly less than-expected on a random basis. Analysis of litters with deviant CP distribution, as well as of all suitable litters, indicated that only maternal treatment-day weight was significantly associated, inversely, with CP frequency-in spite of the treatment dosage having been weight-based. There was no clear relation between the CP and resorption rates; but litters with deviant CP distribution had a significantly greater resorption rate than the others. Fetal weight was inversely related to CP rate even in litters of uniform size. Fetuses occupying uterine positions immediately caudal to those with CP had a significantly higher CP rate than those next to normals, and conceptuses caudal to resorptuses a significantly higher resorption rate than those next to surviving fetuses; but the reverse was not true: CP and resorption rates were not greater in those next to resorptuses and surviving fetuses, respectively.