[QUOTE=chestnutmarebeware;8988205]
I’ve never bred a horse, so I can’t speak to that specifically, but the concept he’s explaining is fairly common in other species bred for very specific inheritable traits (e.g. show dogs, sheep, singing canaries, etc.). It works, but you have to know what you are doing, especially in regard to recessive characteristics that may show up as an unpleasant surprise. GREAT record-keeping over multiple generations is a must.[/QUOTE]
True.
The ideas he is talking about, including the discussion of hybrid corn as a success story date back to the 1920s or so. The basic idea “works,” but 1) it’s expensive and slow (plus takes an incredible number of individuals bred and data recording)’ and 2) I’d explain how and why a system of inbreeding and outcrossing works differently than did the OP and the article. (I know some of this stuff in a scholarly way-- used to be a historian of science and this era of evolutionary biology/theoretical population genetics is what I studied.) Also, I want find a WB/Arabian cross and the biology plus economics of this is kicking my personal ass as the first generation of an outcross is a wildly variable thing!
So 2) first: How to conceptualize what line-breeding or out-crossing does, in Mendelian terms. Genes are nothing but positions on chromosomes and alleles are versions of genes. You inherit one allele from the male and one from the female parent. In the best and simplest scenario, one round of selective breeding works this way: If you have an allele that, by itself, produces a visible characteristic that you want, you “select” this individual for subsequent breedings; you do not breed the animals lacking this trait and, you infer, that allele.
In subsequent rounds of selective breeding, your cross animals who have that trait until the population is true-breeding for that trait. What that means in genetic terms is that every individual has that one allele corresponding with the trait you want. Thinking of one allele, this is the goal of inbreeding: To eliminate all other alleles such that you know the heritable trait you want will arrive in all offspring.
The “hybrid vigor” idea described here (one that goes back to at least Darwin and was understood in pre-Mendelian terms) is confusing. Leaving aside the common meaning (out-crossing so as to produce healthy individuals), it’s better to think of “hybrid vigor” for the purposes of creating a “breeding system” (the general plan for rounds of in-breeding, selection and out-crossing, then more in-breeding) in a different way. What I think breeders really hope to gain in that “F1” (“First Filial”) generation of crossing between two individuals from inbred and selected lines is to produce in an individual one that assuredly has the alleles you want; the ones that were “stabilized,” or bred into every individual in the respective populations containing the male and female parents.
This doesn’t always happen, but it is the purpose and hope of each out-crossing.
The theory continues that once you find an individual in that F1 group that has the combination of traits you want, you breed them within a population that has other traits you want. That could be going back to one of the parental populations, or founding a new line.
In this way, so breeders have long understood (before and after Mendel, I might add) this pattern of in-breeding, selection, cross-breeding and more selection, then in-breeding again modifies a population in a given direction over time.
Awesome… except that the biology is much more complicated. Alleles can have more than two options. Traits we were correlating with “one allele” may have more complicated genetic causes. (Worse, they can have epigenetic (developmental) or even “nuture” causes that we don’t appreciate quite yet). Also, it sucks that alleles come on chromosomes and whole chromosomes and/or whole sections of them are inherited as a package deal. This point is important: When we preserve one allele corresponding with a trait we want, we are also accepting a whole bunch of other alleles. E.g. (assuming that coat color and mane/tail hair have simple genetic causes) and those genes are close together on one chromosome: When you selected for a delightfully loud color pattern in your Appy, you get the allele for “shitty mane and tale”, too, because it sits next door on the same chromosome.
The reason, then, that in-breeding creates bad, heritable conditions that we’d like to correct is because we have been practicing “inadvertant selection” for alleles on sections of chromosomes that came along for the ride while we were selecting for their great neighbors. For a while, perhaps not all of these were expressed (and now we have to think about “dominance”-- the extent to which one version of an allele manifests its trait over another or incompletely). But if the “inadvertant selection continues” you can have deleterious versions of those alleles show up such that the individuals manifest traits we didn’t want.
So “hybrid vigor” in this second sense is what that out-crossing generates: The possibility that some of those deleterious alleles will be replaced by better ones. But you can see that out-crossing does the same thing as when we are talking about introducing a desired trait that’s true breeding in another line: It introduces a new allele to the inbred group.
- The reason this worked remarkably well for corn breeding was that plant breeding is cheap and fast in comparison to animals: Each generation is much shorter and you can produce many, many more individuals per generation for less money than you can with animals.
Well before Darwin theorized jack-diddly about selective breeding, or Gregor Mendel suggested a useful way to conceptualize the underlying units of inheritance and the way they get shared or moved around via inbreeding or outcrossing, animal breeders knew that it could a take one careful stockman’s career (and a more or less constant breeding ideal) in order to modify his herd in a reliable way.
And so to the Mennonite’s proposed “breeding system” and “breeding for the shed or the show ring.” As far as I can tell, what the cattleman in the article wanted to do was to create a true-breeding line of female animals. That in itself is a huge project. And really, he is just trying to refine a single population that, presumably, someone else would want to out-cross. For one livestock breeder to create two, true-breeding populations such that he could eventually make that F-1 cross and, hopefully, get a spectacular individual from that would take a huge amount of time and money. (And to be clear, every time this out-crossed to bring new traits into his “female line”-based true-breeding population he has, he’s creating an F-1 cross.) So inbreeding and outcrossing happen by degrees.
So the difference between breeding for the show ring or the shed needs refinement: I read this difference as one between “breeding for a spectacular individual and assuming that his phenotype corresponds well to his genotype and selecting him to breed next.” “Breeding for the shed,” in contrast means “sure, looking for phenotype in that individual, but also looking at her ancestors and her genetics so that I can estimate which alleles she carries with her.” In other words, “breeding for the shed” means overlooking appearances a bit and paying more attention to the pedigree behind those traits.
Kudos if you have read all this and follow it. I’m sure there are crucial pieces I have left out (e.g. the whole “dominance” issue). I hope this helps make sense of what the cattle and corn breeders are talking about. IMO, the genetics of animals and the traits we want are so complicated that this doesn’t work well for producing biologically complex traits.
