A STUDY OF THE EFFECT OF HETEROZYGOSIS ON LITTER SIZE IN THE MOUSE MUS MUSCULUS

The experiment reported here was undertaken to test whether litter size in the mouse Mus musculus is a trait which is exclusively determined by the mother's heredity and condition, or whether perhaps the heredity of the zygotes, aside from homozygosity for lethal genes, might also have some effect. Since any increase in litter size due to the embryos ' heredity might be slight, care was taken to design an experiment with maximum possible sensitivity for this effect. The method used was to compare litter sizes of sister mice from inbred strains one of which was mated to her brother while the other was out-crossed. Appropriate statistical analysis of the data secured showed a significant increase in litter size where the young born were heterozygous. It seems justifiable therefore to con-clude that heterozygosity of the young can increase litter size. The character "litter size " may be denned as the number of young born, whether alive or dead. It is not a simple character but a function of at least three variables, viz., fecundity, number of eggs fertilized, and amount of pre-natal mortality. Fecundity, the number of eggs liberated by the ovaries at a