Qualitative vs. Quantitative Traits

Breeding can be summarized as the creation of and selection for phenotypic diversity. Modern or ‘capitalist’ breeders tend to select for homogeneity because it is either more productive or it’s demanded by the consumer. Can diversity be all that bad?

As a backyard seed saver you will, without a doubt, be taking part in some sort of breeding. When selecting your traits of interest two simple yet infinitely complex terms should be at least acknowledged: Qualitative and Quantitative. Mendelian genetics deals primarily with qualitative traits, the things we can see and characterize. Mendel’s peas were revolutionary and still remain valid today for understanding discontinuous dominant and recessive alleles. However, we must remember that there are few, although important, traits that are single-gene mediated and classifiable into discrete categories…another post for another time :)

The limitations of Mendelian traits were acknowledged by the biometricians of the early 20th century. They sought to develop statistical formulas and theories to understand quantitative traits. Yield is the classic quantitative trait because it is multiallelic or polygenic (many genes directly influence and interact to produce the phenotype) in addition to being heavily influenced by the environment. Now I’m not trying to discount the research of the past 100 years, because we know more now than ever, but sometimes, the more you learn the less you actually know. I suspect this old adage will hold true for the genetic revolution we are now finding ourselves in. Genetic markers currently work very well for qualitative and quantitative traits that are controlled by a few alleles, but lack the precision to detect all the small effect genes and interactions within the plant system. Even if one day they can say for certain that trait X is better than Y, the end result is a homogenized product that has no intrinsic value. And what about terroir? Society continues to question homogenization because life is inherently creative and diverse. This is where an evolutionary approach to breeding becomes important, because it serves to produce heterogeneous populations that are stable (resistance to environmental aberration), yet aesthetically, nutritionally, and ethically desirable.

Stability through diversity is not only important for productivity, due to abiotic factors of the environment, but also for biotic resistance. Insects and disease can contribute significantly to what geneticists call environment variance. An evolutionary approach builds on what is called horizontal resistance. Modern plant breeding often employs molecular or applied conventional techniques to isolate and incorporate specific resistance gene(s) with varying levels of efficacy into a single cultivar. However, this method of vertical resistance has been known for some time, and durability in the long-term, even when multiple resistance genes are stacked is risky, as we have seen with herbicide resistant and Bt transgenics. While some single resistant genes have been more successful than others, these remain limited, and in some cases no major resistance genes remain. This is a race I’m not sure we can win. Natural selection for durable resistance, which currently cannot be measured with modern techniques, may be a less destructive path. Minor effect genes, interacting within a heterogeneous population, allow genetic shifts over time. This evolution with the antagonist (i.e., the pathogen) can result in plant-pathogen self-regulation.