A maternal effect on the frequency of spontaneous cleft lip in the A/J mouse

Female mice of the inbred C57BL/6 strain were crossed to males of the A/J strain. F 1 females were backcrossed to A/J males, the backcross females were crossed to A/J males, and so on for six backcross generations. In each generation a reciprocal backcross was obtained by crossing the hybrid males with A/J females. The following conclusions were drawn: (1) Spontaneous resorption in the C57BL/6 strain is determined in part by genes, recessive to their alleles in the A/J strain, which act in the mother, rather than the fetus. (2) The predisposition to spontaneous cleft lip in the A/J strain shows multifactorial inheritance. In the backcrosses, where the picture is not complicated by the maternal effect, the frequency of cleft lip reached the A/J level only in the sixth backcross. (3) In each backcross generation (hybrid mother) the cleft lip frequency was lower than in the genetically similar reciprocal backcross (A/J mother); thus the A/J mother contributes to the fetal susceptibility to cleft lip, either by providing an appropriate uterine environment or by not providing a factor, present in the C57BL strain, that decreases susceptibility to cleft lip.