Test of recessive lethals in the mouse

The presence of spontaneous lethal mutations in inbred strains is discussed with special reference to their variation and influence on estimates of induced mutations. A model is presented that will facilitate classification of lethal-free and lethal heterozygotes. The model is used in classification of sons to lethal heterozygous males carrying a spontaneous mutation. The observed results are in good agreement with the model. From experience it is concluded that the most efficient way to use the facilities in lethal tests it to examine 10 or more full brothers to the P parents. By doing so pre-existing spontaneous lethals can be excluded by eliminating families in which any of the P parents were lethal heterozygous. The observed total rate of recessive lethals gives slight over-estimation of the induced rate of mutations, as spontaneous mutations in the gametes forming the F1 cannot be excluded.