Heterosis - Controversy

Controversy

The term heterosis often causes confusion and even controversy, particularly in selective breeding of domestic animals, because it is sometimes claimed that all crossbred plants and animals are "genetically superior" to their parents, due to heterosis. However, there are two problems with this claim:

• First, "genetic superiority" is an ill-defined term and not generally accepted terminology within the scientific field of genetics. A related term fitness is well defined, but it can rarely be directly measured. Instead, scientists use objective, measurable quantities, such as the number of seeds a plant produces, the germination rate of a seed, or the percentage of organisms that survive to reproductive age. Within this perspective, crossbred plants and animals exhibiting heterosis may have "superior" production on these scales, but this does not necessarily equate to evidence of "genetic superiority". Use of the term "genetic superiority" is a value judgement, generally in the realm of politics, and is not science.
• Second, not all hybrids exhibit heterosis (see outbreeding depression).

A clearly ambiguous counter-example to any value judgement on hybrids and hybrid vigor is the mule. While mules are almost always infertile, they are valued for a combination of hardiness and temperament that is different from either of their horse or donkey parents. While these qualities may make them "superior" for particular uses by humans, the infertility issue implies that these animals would most likely become extinct without the intervention of humans through animal husbandry, making them "inferior" in terms of natural selection.

Some modern geneticists refrain from even using the terms inferior and superior due to the association of these words with political movements that espouse genocide.