One theory is that different alleles at the same locust are more rubust than the identical allele in the homozygous state (which is common in inbred lines). A simple way to think about this is that if a gene needs to be in the homozygous state to express as a weakness, this is less likely in a hybrid between two lines that do not contain the same alleles at this locust. A simplistic example is if a random mutation in one allele causes a phenotypic deficiency only in the homozygous state, then it can be passed from generation to generation within the inbred line, being maintained in the heterozygous state. These types of mutations can occur randomly among the many thousands of genes. When one population is crossed with a second distant population, they are less likely to be carrying the same defects, but rather an accumulation of different defects; so when they are crossed, one good copy is available to express in the hybrid. Genes produce proteins (mostly), so one good version of the protein is produced. This is way over-simplified, but irregardless of the mechanism, the observations from both crops and livestock illustrate the pattern of vigor. Even if two good, but different, proteins are produced from two different alleles, each may be adapted to deal with a different environment, allowing the hybrid to cope with a more diverse habitat. I hope that this helps, and sorry to the genetists out there for leaving out the greater complexity. This explaination suggests that genes and proteins do not interact, which they commonly do.
Last edited by RAH; 02/21/10 07:40 AM.
