Thursday, November 22, 2007

The Myth of the Myth of Hybrid Vigor

(This blog originally posted on 6/20/2005).

If you happen to mention 'hybrid vigor' in the wrong place, at the wrong time, you will likely find yourself attacked by the myth-ites. Their advances follow a consistent template- a split hair, some red herrings, the occassional split herring...

"WHAT" my ever observant readers demand, "Is a Split Herring"? Well that's easy. You take about 4 large herrings, split them, remove the backbone. Add a little salt and pepper and grill until nicely roasted. Then, you add a lovely concoction of pototoes, cauliflower, gherkins... Hey wait a minute! Stop Distracting me! This is important - people are spreading il-logic and it has to be stopped!

Okay, that's better. Still with me?

Let's start here - with Karen Peak. She opens with a split hair. Since dogs are of the same species she says, crossing them does not create a hybrid therefore no hybrid vigor. And if that's not enough, she says even in recognized 'hybrids', the vigor is questionable. Carefully ignoring the beefalo, yak crosses and the mule, she finds the health problems of ligers and tions and the tempermental quircks of wolfdogs all the evidence required to challenge hybrid vigor.

But hold on! It gets better! Darlene Arden doesn't waste time splitting hairs, she heads straight for the herrings. According to Ms. Arden because a dog could potentially inherit the genetic diseases of it's parents, hybrid vigor is a myth in dogs.

You see what we're up against! Now I know those of you who have raised cattle and pigs and crops are jumping up and down screaming right now. Hold your horses! I know what you're saying, but we have to quote some real experts.

“Heterosis (hybrid vigor) can improve the performance of crossbred animals, relative to the average of their parental breeds.” - Dr. Kent Weigel, Department of Dairy Science, University of Wisconsin

"Heterosis, known as hybrid vigor, is the added performance you get when mating parents of different breeds. It is free and nothing is spared to achieve it." - John Hough, Ph.D. Chief Science Officer, EPD International, Inc.

"Heterosis, often referred to as "hybrid vigor," measures the difference between average performance of crossbred animals and average performance of the breeds that were crossed to produce them." - T. A. Olson, associate professor of Animal Science, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.

"B-b-b-utt I thought a hybrid had to come from two different species! What's all this nonsense about crossbred hybrids??"

Common mistake. Let's let Wikipedia do the talking.

In biology, hybrid has three meanings.
-The first meaning is either the offspring of two different species, or of two different genera.
-The second meaning of "hybrid" is crosses between populations or cultivars ("cultivated varieties") of a single species. This second meaning is often used in plant breeding.
-The third meaning is in molecular biology.

Seems it's all Mendels fault. He, like most plant breeders, called his heterozygous crosses 'hybrids'. And, since he was popular and did lots of research that had implications for breeding far beyond mere plants, people went with what he said. No surprise there!

Only the people stuck in high school biology are stuffing herrings. Certainly Ms. X, her intelligent readers and the real experts quoted above understand this complexity.

Now let's get out of the bean fields and find the dogs. Since Ms. Arden didn't trip over the definition of hybrid, but moved straight into the test for vigor, we should explain that.

Simply, hybrid vigor is defined by it's success. (Re-read the real experts quoted above). If the offspring has improvement in performance over it's purebred parents, hybrid vigor has surely occurred.

"Wow, so hybrid vigor doesn't mean the dogs will be physically and tempermentally perfect and never get sick?" Funny how the only people saying that are the zealots trying to disprove the scientific phenomenon of hybrid vigor. It's easy to disprove something that isn't true, now, isn't it?

Here are a few things to know about hybrid vigor (or heterosis) in animals.

"Level of heterosis tends to be inversely proportional to heritability. In moderately to highly heritable traits, such as carcass characteristics, level of heterosis is low. On the other hand, in traits having low heritability, such as fertility and livability, heterosis is high." -Crossbreeding Systems for Beef Cattle by Harlan Ritchie, B. Dennis Banks, Daniel Buskirk and Joel Cowley Michigan State University - East Lansing

"We found our F1 females became much superior mothers compared to the purebred animals, producing large quantities of milk and giving excellent care to their offspring, so that the F2's had a better start in life. These genetic and environmental advantages of the F1's and F2's are reflected in decreased mortality." - Genetics and the Social Behavior of the Dog by John Paul Scott and John L. Fuller

Scott and Fuller did the only scientific study I know of that looked at hybrid vigor in cross breed dogs. It wasn't the purpose of the study, but it was one of their oft noted observations.

And a final word about hybrid vigor- don't forget the key word is "hybrid"! By the time you get to the F9 and F10 generation, you're losing heterozygosity.

"The extrapolation of logic regarding color has led many beef producers to first make their cattle black, then to work on making their cattle genetically superior in regard to production, carcass merit and reproduction. Producers should not be criticized for this decision, they are simply responding to a market reality. By this response many commercial herds have been drained of nearly all of the hybrid vigor that was once there in their crossbred cows and calves. Three generations of Angus bulls on F1 Angus-Hereford cows results in 15/16 Angus cows and loss of 87% of the hybrid vigor, particularly in important reproduction & production traits. Hardest hit are the lowly heritable traits (such as conception, survival, fitness) which do not respond well to selection for the more highly heritable carcass and growth traits." - Balancing Angus Genetics and Hybrid Vigor in Breeding Black Cattle, Jim Gosey, Beef Specialist, University of Nebraska, Lincoln

Well, Ms. X's fingers are getting tired, but this last last thought just couldn't be left out. While the topic is beef, the implication is dog and in a nutshell it is the reason crossbreeds are sooo very popular today. Let's just hope those breeders take notes from industries that have been capitalizing on that 'hybrid vigor myth' for a long time and do dogs some good.'


  1. This deserves a whole post (and I've been kicked off of a show dog list for explaining it). But since I really have to go eat some Thanksgiving goodness, I'll be brief.

    The other argument the idiots who think inbreeding (they call it linebreeding to avoid the negative associations with banjo players in Appalachia) is that outcrossing or hybrids create twice the disease because you're combining all the diseases of both parents.

    Brilliant stupidity this. I guess it comes from their notion that if they inbreed and limit the breeding pool enough they can actually get rid of a disease or two here and there.

    Sadly, there are only a handful of DNA tests for obscure diseases and a few for common ones. There are hundreds of thousands of diseases that still have no test and potentially millions of mutations out there that could become harmful if they are ever expressed homozygously. And yet these people carry on, making dogs the most homozygous species on the face of the earth.

    What they don't get is that while you might be bringing in twice the number of potential diseases into a breed (minus the huge number that are common), you are also greatly decreasing the chance that any of those genes will ever meet up in homozygous pairs and cause the disease.

    A one-in-a-million disease suddenly becomes one-in-four affected and two-in-four carrier (wow, that's one in a million to 75%) the second you start inbreeding.

    I'll take my chances with out crosses.

  2. Well stated Christopher. I would only add that for some of the identified real genetic diseases, the same disease (at least the same set of symptoms) can be sparked by completely different genes, in different breeds. Certainly crossing two such breeds with the same affliction, but associated with different genes is not going to increase the likelihood of the offspring being afflicted.

    And this -

  3. A very good synopsis of the hybrid vigor debate. I like how you offered alternative points of view on the debate.