The effects of litter size on the expressiviity of the jaundice genotype in Gunn rats.

In 2 studies Gunn rats carrying 2 recessive alleles for jaundice (jj) were compared with their heterozygous littermates (Jj) under varied litter size (4 vs 12 pups) and handling conditions. Handling did not produce long-term effects. Litter size interacted with Genotype to increase the differential response of jj rats by 21, 23, and 50 days of age. A decrease in survival rates and weight gain was found in Jj and especially jj rats from large litters. For jj rats only, being reared in a large litter increased the severity of ataxia. Jaundiced rats from large litters showed less rearing and tended to enter fewer squares than those from small litters whereas Jj littermates showed an equivalent amount of rearing and number of squares entered in large as compared to small litter sizes. Differences between Jj and jj rats in frequency of pivoting were greater in those from large litters than in those from small litters.