Look around and you’ll see that most traits are not discrete – people are not either short or tall, obese or skinny, fast runners or slow; instead, such traits fall into a continuous range of values, such as a bell curve pattern. Traits with continuous values are called quantitative traits, and the genetics behind these traits is much more complex than discrete traits, because these continuous traits are not produced by variants of just one gene. Quantitative traits are controlled by multiple genes, and thus there is no simple 1:1 correlation between a trait and a particular genetic variant.
Why is it important to understand the genetics behind quantitative traits? Other than the fact that almost all interesting traits are quantitative, scientists are interested in quantitative traits because they are key to our understanding of both evolution and many common human diseases. To understand the evolution of quantitative traits and complex diseases, ones that are affected by variants of multiple genes, we need to answer a common set of questions: How many genes (and which variants of those genes) have an impact on the trait? How large of an impact does each gene have? How does a variant of one gene impact the effect of another gene on the trait? Without knowing the answers to questions like these, we can’t accurately predict your genetic risk of getting diabetes, for example, and the growing field of personalized medicine will have little hope of success.
Quantitative traits can be built with variants in only a few genes, each with large effects, instead of dozens with tiny effects, which means that there might be hope yet for personalized medicine. And many of the critical genetic variants will probably be found in regulatory genes, meaning that the physiological diversity in a species is in large part due not to differences in the molecular machinery responsible for physiology; it’s due to differences in how that machinery is regulated.
When we started looking at genetics, we looked at DNA like a laundry list: if you check item A, you get trait A1, and so on.
People were even talking about how race “didn’t exist” and was a “social construct” because they couldn’t find a single gene for race. That’s hilarious!
Much of the debate over the existence of human races stems from how one chooses to define ‘race’ (or ‘subspecies’). No realistic definition can avoid using qualitative terms, yet these invariably invite disagreement in their application: “a group of individuals in a species showing closer genetic relationships within the group than to members of other such groups”; “essentially discontinuous sets of individuals”; “conspecific populations that differ from each other morphologically”; “genetically non-discrete (confluent) populational entities”; “geographically circumscribed, genetically differentiated populations”; or groups identified “by the usual criterion that most individuals of such populations can be allocated correctly by inspection.” Compounding the confusion, still others employ the term ‘race’ in a way more akin to ‘species’ than to ‘subspecies.’
In response to questionable interpretations of the U.S. Endangered Species Act, and to help ensure the evolutionary significance of populations deemed ‘subspecies,’ a set of criteria was outlined in the early 1990s by John C. Avise, R. Martin Ball, Jr., Stephen J. O’Brien and Ernst Mayr  which is as follows: “members of a subspecies would share a unique, geographic locale, a set of phylogenetically concordant phenotypic characters, and a unique natural history relative to other subdivisions of the species. Although subspecies are not reproductively isolated, they will normally be allopatric and exhibit recognizable phylogenetic partitioning.” Furthermore, “evidence for phylogenetic distinction must normally come from the concordant distributions of multiple, independent genetically based traits.” This is known as the phylogeographic subspecies definition, and a review of recent conservation literature will show that these principles have gained wide acceptance.
Now we’re seeing it’s more like computer code. Maybe five genes regulate height. The first determines a loop; the second the chemicals involved; the third a stop point; the fourth a test; and the fifth regulates an increment to that test.
What a welcome change.