Hybrid vigor, technically called heterosis, is a phenomenon that occurs in the offspring of unrelated parent plants or animals.  These hybrid offspring are generally considered to posses a number of superior traits.  For example:

• Hardiness.
• Longevity.
• Fertility.
• Rapid growth.

The term hybrid vigor is typically applied to the offspring of crosses between entirely different breeds or varieties.  However, it can also be applied to the offspring of a cross between two family lines within the same breed, a common practice when breeding commercial White Leghorn hens.  In some cases it can even refer to the results of a cross between two different species, such as when a horse and a donkey are crossed to produce a mule.

Causes of Hybrid Vigor

There are two traditional theories about the causes of hybrid vigor:

1. Beneficial genes from one parent mask the harmful genes from another.
2. Certain combinations of unlike genes just naturally work well together.

While plant and animal breeders have used these basic principles to their advantage for centuries, scientists are just beginning to realize that hybrid vigor is quite complicated.  MicroRNA, very small molecules of RNA, appear to be involved in determining which genes are allowed to control the characteristics of the hybrid and to what extent; and it appears that other factors are at play, as well.

So although we know that the two basic principles above are true to some degree, at present we have to admit that we simply don’t have a full understanding of what causes hybrid vigor.

Controversy

Hybrid vigor is not unchallenged by skeptics.  This is partly because some adherents to the theory have used it as an excuse to keep inferior breeding animals.  If an animal that is deficient in health, temperament, or performance traits is mated to an unrelated animal, the offspring will nevertheless be superior, right?  Not necessarily.  The results are much more predictable and desirable when the cross is made between two good-quality parents.

Another common objection is that hybrid vigor is not sustainable.  An Angus and a Hereford can be mated to create a Black Baldy with its associated benefit of hybrid vigor, but then what can you do with the Black Baldy besides finish it for beef?  Some people create complicated breeding plans involving crossing hybrids back to first one of the parent breeds and then the other, but it is generally recognized that hybrid vigor mostly occurs in the first generation.  This is why most of the hybrid seeds for sale are categorized as F1 (first-generation hybrid).

For this reason, critics of hybrid vigor would argue that crossbreeding is a “quick fix.”  To maximize hybrid vigor, some breeders have to raise purebreds anyway.  Why not focus on breeding desired traits into the purebreds?

The fact remains, however, that hybrid vigor is a real phenomenon.  Farmers buy hybrid seeds that are particularly adapted to meet the challenges of their unique growing conditions.  Hybrids also dominate the livestock industry, with Baldies reigning in the sale barns, Cornish crosses in the poultry realm, and various combinations of chickens in the egg-laying business, all because of their superior ability to produce.