Heritable determinants of left-right bias in the rat

The offspring of matings of rats having opposite or same-sided turning biases were tested for turning biases as adults and the degree of similarity to the parents' biases assessed. There were significant and equivalent tendencies for the male offspring to have the same bias as the male parent and the opposite bias as the female parent. Although, overall, female offspring were distributed randomly with respect to the parents' biases, a significant tendency for female offspring to have biases opposite those of the female parent was apparent in litters having more males than females. Based on reports indicating a relationship between the sex ratio of a litter and levels of testosterone in female fetuses, it was suggested that in utero exposure to testosterone reverses the coding of a heritable female influence and induces a tendency for the offspring to have biases opposite those of the female parent. The origins of sidedness in the rat appear to involve a complex interaction between heredity and hormones.