Lethal genes are genes which control certain phenotypic traits, and at the same time also influence the viability of the individuals. For instance, white eyes and vestigial winged flies have lower viabilities than the wild types. There are still other genes which have no effect on the phenotype but influence the viability. This influence on viability may be of such an order that the individual may fall to survive. Such genes which cause the death of the individual may fall to survive. Such genes which cause the death of the individual carrying it are known as lethal genes.
If the lethal effect is dominant over its normal allele, all individuals carrying it will die and the gene cannot be transmitted to next generation. Such dominant lethal genes will, therefore, be lost in the same generation. On the other hand, recessive lethals are carried in heterozygous condition, and will express themselves only in the homozygous condition.