Gregor Mendel, one of the foundational thinkers in genetics, experimented with pea plants, breeding them for white or purple flowers, green or yellow peas and smooth or wrinkled peas. Whether by chance or by design, these traits are each coded for by a single gene and it is relatively easy to predict inheritance patterns. The effects of single genes can’t explain the many shades of human skin and hair color, however, and you may come from a family of thin people, but you won’t be thin if you eat junk food every day.
First Reason: Monogenic Traits Are Rare
Monogenic is the scientific term for traits that are controlled by a single gene. When more than one gene contributes to a trait it is called a polygenic trait. While it is difficult, if not impossible, to know the functions of, much less the interactions between all of the genes of the human genome, the number of traits that have been identified as monogenic is small. Even those traits that we think of as strictly monogenic, such as tongue rolling, may be influenced by other genes.
Genes Interact in Many Ways
In polygenic traits, also known as multifactorial traits, there are several ways in which the genes that influence the trait can interact. Genes interact with each other in a process known as epistasis. The individual genes can have an additive effect, with each gene contributing a small amount to the total trait expression. Genes can also mask or subtract from the effects of other genes. Some genes turn on or turn off other genes. Finally, one gene can modify the expression of another gene.
Second Reason: Genes Are Only Half of the Equation
You may have heard the phrase “nature vs. nurture.” It is used to describe the tension between explaining a trait as innate, or controlled by genes, or as a product of environmental influences. While there has been heated argument over the relative influence of the two factors, especially in the field of psychology, the truth is that genetics and the environment interact to produce the traits that are expressed by the individual.
The Concept of Heritability
To quantify the relative influence of genes and the environment, geneticists use heritability. Heritability explains the variance in a trait that is due to genetics. The values for heritability range from zero to one, corresponding to no genetic influence and no environmental influence respectively. Heritability is estimated by comparing the observed variability in the trait to the variability that would be expected if there was no environmental influence. When 20 percent of the variation in a trait is due to genetics, the heritability of the trait is 0.20.