Quantitative Trait Breeding

[Weston Adams]

Most of my breeding work has been with traits that are more-or-less qualitative, but recently I've been digging into the mechanics of quantitative traits. Quantitative geneticists have complicated software for their work, but I'm hoping to distill the concept down to its simplest form.

When I work with quantitative traits, I'm usually wanting to generate something that is improved over ANY of the varieties currently available. I'm wanting to generate something extraordinary from something ordinary.

To my current understanding, the best way to accomplish this is through the action of additive genes from diverse sources. For simplicity, let's say there are 5 genes controlling sugar content in Cucumis melo, and that the dominant version of the genes increases sugar content. Suppose I have an individual melon plant producing super-sweet of fruit. It has the following genotype: eeFFiiLLoo. It's true-breeding and homozygous for two dominant alleles. These dominant alleles are responsible for its elevated sugar content.

And let's say that I find another melon in the same population that is also super sweet. I could attempt to intercross these, but since they came from the same population, it's quite likely that this second plant also has the genotype eeFFiiLLoo. If I intercross the plants, the best I will be able to do is retain the existing levels of sugar content.

But I'm wanting to RAISE the sugar content, not maintain existing levels.

So let's say that on a another continent there's another population of melons. These melons have the exact same sugar content as the ones I was initially working with. But if I genotype them I find that their genotype looks more like EEffIIlloo. They still have two sets of homozygous dominant alleles raising the sugar content of the fruit, but they are a DIFFERENT SET OF DOMINANT ALLELES.

Even though the two populations have SIMILAR phenotypes, the genetic basis is DIFFERENT. If I intercross these and sort through the F2 offspring, I could potentially come up with an EEFFIILLoo genotype, which would be dramatically sweeter than either parent.

I would love to take a formal course in quantitative genetics sometime, but from my understanding of the subject so far, the key to making significant quantitative improvement above what is currently available is this: 1) Find more than one progenitors from UNRELATED populations that display the best phenotype for your desired trait. These will likely have DIFFERENT ALLELES contributing to the SAME PHENOTYPE. 2) Cross these selections. 3) Grow out a large f2 population. 4) Carefully screen the population for those recombinants that display a phenotype that is improved over either parent. 5) Move forward with these improved selections.

I wish I could say I had lots of experience with this, but I can't. However, if I were to tackle a quantitative trait, this is how I would currently do it. I'd love to hear any additional experience or tips that others have for dealing with quantitative traits.

Thanks so much.

[Joseph Lofthouse]

I deal with genetic selection by recurrent mass selection. I can't see, or measure genes, therefore in practice, I don't think about them.

It's really hard to stabilize dominant traits, and a joy to stabilize recessive traits.

[Weston Adams]

True.

Some traits resist stabilization 😆