7.2: Some traits are controlled by a single gene.

Like father, like son. No one will ever seek a paternity test to prove that Jaden Smith is the son of Will Smith. All it takes is a quick look at the facial features shared by the two actors and we are sure they are related (FIGURE 7-4). We see all around us that offspring generally resemble their parents more than they resemble other random individuals in the population—a consequence of the passing of characteristics from parents to offspring through their genes. This is heredity.

Observing heredity is easy. Elucidating how it works is not. For thousands of years before the mechanisms of heredity were discovered and understood, plant and animal breeders recognized that there is a connection from parents to offspring across generations. In ancient Greece, for example, the poet Homer extolled the tremendous benefits to society that came from the skillful breeding of horses. The awareness of beneficial breeding practices enabled farmers to systematically create strains of crops, livestock, and even pets with desirable traits (FIGURE 7-5).

Once breeders recognized the existence of heredity, they began selecting individual plants or animals with the desired traits to breed with each other, in the hope that the offspring would also have those desirable traits. Since then, it has been possible to create a rich world of sweeter corn, loyal dogs, docile livestock, beautiful flowers, and more. And these breeders did it all without ever really understanding how heredity works.

Practical successes were many, but people didn’t understand exactly how the outcomes were achieved. They knew the result of selective breeding, but why and how it worked, or didn’t always work, was a mystery. Patterns of similarity among related individuals are impossible to make sense of without an underlying understanding of how heredity works. This understanding is the province of the field of genetics.

Mechanisms of heredity become a bit easier to grasp when we focus on visible traits with well-established inheritance patterns. Coat color and fur length in cats fits this bill well. Virtually every short-haired cat, for instance, has at least one short-haired parent. And virtually every completely white-haired cat has at least one white-haired parent. In humans, many genetic diseases have similarly simple patterns of inheritance. The neurodegenerative disorder known as Huntington’s disease, for example, follows such a pattern: individuals who develop the disease have a parent who developed the disease. Traits that are determined by the instructions a person carries on one gene are called single-gene traits (FIGURE 7-6).

It is important to note here that most human characteristics are influenced by multiple genes, as well as by the environment. However, because the mechanism by which single-gene traits pass from parent to offspring is the easiest pattern of inheritance to decipher, we first explore this process. Then we’ll expand our model of heritability to account for the passing on of more complex traits.