Decoding Hybrid Vigor: Why Mutts Are Healthier Than Purebreds

Posted by TF Oren

We've all heard that mutts are usually healthier than purebreds, but here's why. 

This claim is rooted in a genetic phenomenon called hybrid vigor. Hybrid vigor, or heterosis, refers to the increased hardiness and improved function of the biological properties of a hybrid organism due to genetic diversity.

Heterosis is the counterpart to what's called "inbreeding depression," which is when a lack of genetic diversity among breeding stock results in offspring with traits that hinder their overall fitness. Genetic diseases are an example of the perils of inbreeding.

Outcrossing, which is the practice of introducing novel genetic material into a breeding line, often, but not always, corrects for inbreeding depression. However, in some cases, a hybrid offspring can inherit incompatible traits from genetically diverse parents, and that incompatibility can negatively affect that hybrid's fitness. This is called outbreeding depression.

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There are two hypotheses that help explain hybrid vigor, and although they are competing hypotheses that affect gene expression differently, they're not mutually exclusive. The first of these is the dominance hypothesis, which suggests that the genetic superiority of hybrids owes to the suppression of one parent's disadvantageous, recessive alleles by the other parent's more desirable, dominant ones.

An allele is one version of a gene, and in human cells, for example, there are two versions of each gene, or two alleles. A dominant allele will overpower a recessive allele, and in such a case, the effect of the dominant allele is the one that will be expressed (i.e. as a physical trait). Only when both alleles are recessive will the organism exhibit the effect of the recessive allele (i.e. as genetic disease).

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The second hypothesis is the overdominance hypothesis. It suggests that certain combinations of alleles that result from crossing two inbred parents positively impact the offspring because some of the recessive alleles, which are harmful to the homozygotic parents (parents with two copies of the recessive allele), are in fact advantageous to the heterozygote (an organism with two different alleles for a gene). This is what's known as heterozygote advantage.

In simpler terms, two negatives (unrelated, inbred parents) make a positive (a hybrid offspring).

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So, now you have a quick and dirty explanation for why your funny little Heinz 57 pooch might actually be healthier than the beautiful Golden Retriever your neighbor paid big bucks for.

There are no guarantees for any pet owners, but chances are when you adopt a mutt, you'll not only be saving a life, you'll be saving yourself a lot of money in vet bills too.

In-text photos courtesy of Eric Oren, Nancy Oren, and J. Daley

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Decoding Hybrid Vigor: Why Mutts Are Healthier Than Purebreds