Harelip in the House Mouse

HE character “harelip and cleft palate” is definitely inherited T (REED and SNELL, 1931). Though there are various degrees of expression of harelip and cleft palate, the general condition, regardless of the types of clefts, will henceforth be designated simply as “harelip.” Harelip is not inherited as a simple dominant or a simple recessive but seems to depend upon either one recessive gene with modifinrs or the cooperation of a small number of cumulative genes. If the first hypothesis is correct, it must be assumed that when the recessive gene for harelip is homozygous, the mouse may show any grade of harelip from the most extensive bilateral clefts to a normal mouth without clefts, depending upon whatever genetic and environmental modifiers had an effect upon the embryo. The phenotypically normal, but genetically harelip, animals will be designated “normal overlaps.” It is usually easy to determine at birth whether a mouse is phenotypically harelip. Occasionally the cleft or clefts are so slight that they are hardly noticeable. Probably in several cases there have been slight lip and palate defects in genetically harelip animals which were not noticed. Such animals would have been erroneously classified as “normal.” Environmental influences greater than ordinary may be suspected in connection with variable characters such as harelip. These effects may be detected if found to occur in correlation with differences in litter size, weight, age of mother, and the like.