In Drosophila, occasional females regularly produce female and male progeny in 2 : 1 ratio, which means that half of male progeny is missing. In small populations this can be explained on the basis of sampling error. However, in Drosophila, large populations can be handled and therefore the difference becomes rather significant. In such cases, it can be explained to be due to presence of sex linked lethal (recessive) in heterozygous condition in female parent (Fig. 16.7).