These are alleles which cause death in the individual possessing them, and they can be dominant or recessive. We usually find that they are recessive. The yellow coat gene in mice is a very good example. For years people tried to form a pure breeding strain of yellow-coated mice, but they could never do it. The reason they failed is because the mice with two yellow coat alleles always died before birth. This gene is in control of cartilage development and the mutant allele fails to cause cartilage manufacture but does alter the coat colour. This is also a good example of the fact that alleles can only be considered as dominant or recessive in respect of a particular characteristic. Thus the yellow coat mutant allele is a dominant as far as coat colour is concerned, but is a recessive as far as cartilage formation is concerned.

In some populations in which a particular frequency of phenotypes is present, the stability of the system is maintained by balanced lethal genes. Because the lethal genes are on the chromosomes they are linked to other normal genes. If a pair of different lethal genes are present in a population then their interaction can cause the linked genes to express themselves at a particular stable frequency in the population.

Further, consider the situation where c is linked to A and d is linked to B so closely that they are never recombined.

Then in this cross we have the gametes: Ac/Bd aC/bD Ac/bD aC/Bd

When we randomly mix these gametes we get:

The final outcome is that we never get the homozygous AA and BB individuals in the population.