Genes are not Paint Pots: Part 2 Polygenic Traits

Articles on genetics make it seem that every trait is a single gene trait. Single gene traits are easier to understand and seem easier to manipulate. I imagine that most geneticists would be thrilled if all our traits were caused by single genes. Unfortunately for them, that is not the case. In fact, for most traits of interest multiple genes are involved. They are polygenic traits: the result of the interaction of multiple forms of multiple genes with the environment in which that individual and/or his/her ancestors live/lived. The proportion of genes to environment varies with each trait. In most cases, we really have no idea of the true proportion. Regardless, it is important to remember that these traits are not simple and, therefore, we are a long ways from being able to fully understand and manipulate them.

Polygenic traits exhibit continuous variation: there are no distinct boundaries between one form of the trait and another. Unlike the blood type example from the Part 1 blog [below] where one is either Type A or Type O, polygenic traits can seem like blends. Some examples of polygenic traits include: height, weight, intelligence, hair color, and eye color. Adults are not 5’, 5.5’, or 6’ tall with no heights between. Height is continuously distributed. Although we say a woman is blond or brunette, we know that the range of variation within each category is large. Another example of a polygenic trait is skin color.

Geneticists have spent years trying to figure out the genetics of human skin color. They’ve found genes that affect skin color, but they have not yet been able to fully explain the genetics that produce the variation we see. Nor can they confidently predict what color the offspring will be of parents with markedly different skin colors. If genes were paint pots, this should be easy to do. Take one very dark parent and one very light parent, and the offspring should be halfway between. Sometimes this looks to be the case, but the children can range in skin color from light to dark. There is no way to tell ahead of time. I know a couple where the wife has very dark skin color and black hair, while the husband has very light skin color and blond hair. One child has medium brown skin color and wavy brown hair while the other child has very light skin color and tightly curled blond hair.

Some geneticists have talked about parents choosing their baby’s traits. If traits were of the single gene variety this might barely be possible. But with polygenic traits, it is not going to happen. At least, not in the foreseeable future. If you look a great deal like your mate, your child may look a great deal like you. But it isn’t guaranteed. [See the Part 1 blog below.] If you and your mate are quite dissimilar in looks, each birth will be an unexpected mix of traits. Your child will not be a blend. Genes are not paint pots. And that is a great thing because we do not know what the future will bring. The more variation there is in your offspring, the better the chance that they will do well in that unknown future.