LECTURE NOTES ON LETHAL GENES

There are some genetic factors or genes, when present in any organism cause its death during early stage of development. They may  cause death of the individual either in homozygous dominant or homozygous recessive condition.Lethal genes can also alter the basic Mendelian  ratio and lead to death of the possessor. A French geneticist L. Cuenot (1905) reported on the inheritance of coat colour in mice.

The yellow body colour in mice is dominant over brown, but the yellow mice are never true breeding. When yellow mice are inbred, the progeny consists of yellow and brown mice in the ratio 2 : 1 which does not fit any of the Mendelian result. Moreover, the size of litter after inbreeding is smaller by one-fourth as compared to litter size resulting from a cross between yellow and brown. When yellow mice were backcrossed to true breeding brown mice, only heterozygous yellow mice were obtained.

Yellow × Yellow                                                                 Yellow × Brown

Yy ↓ Yy Yy ↓ yy

2 Yellow : 1 Brown                                                           1 Yellow : 1 Brown

Yy yy                                                                                      Yy yy

Thus only heterozygous yellow and brown mice in the ratio 2 : 1 were being born. The ratio 1 : 2 : 1 expected when a cross between two heterozygotes is made was never obtained proving the lethal expression of the homozygous yellow gene.