When two closely related organisms are crossed, the offspring produced will be more likely to pick up pairs of recessive alleles and these may well be genetic combinations which reduce the vigour of the offspring. Consider the following cross:

This would give rise to numerous combinations since the gametes could have various combinations of dominant and recessive alleles. Thus from the above parents we could derive the following gamete genotypes:

Thus we could end up with offspring with large numbers of paired recessive alleles, in the extreme case we could end up with offspring with the genotype: aabbccddeeffgghhiijjkkll.

There are a number of recessive alleles which are referred to as lethal. If the offspring receives a homozygous lethal recessive genotype then it dies. Many recessive alleles fail to produce a functional product and the lack of this product in a recessive homozygote may not be lethal but may well reduce the efficiency of the overall system in the organism. If many loci are homozygous recessive then the organism may fail to function adequately showing reduced growth, fertility or resistance to disease. Such recessives may be referred to as sub-lethals and are probably responsible for the reduced vigour found in highly homozygous populations.