Why We are Different

Biologists use two fancy words to describe the relationship between your genes and your physical traits. The first word is genotype. Your genotype is your genes for a given trait. In most cases, you've got two copies of a gene - one from your mother and one from your father.

The second word is phenotype. Phenotype is what you actually turn out to be, the way these genes get expressed. Biologists have a saying involving these two fancy words: "Genotype determines phenotype."

Let's take eyelashes, for example. There are 2 kinds of eyelashes in people - long and short. Maybe you've got a short lashes version of a gene from your father, and a long lashes version of a gene from your mother. That's your genotype. And what length of lashes do you actually have? Long - that's your phenotype.

The eyelash example makes an important point. Some genes are dominant, and others are recessive. When you have two different genes for the same trait, and one is dominant (long lashes) while the other is recessive (short lashes), it's the dominant trait that wins out in the phenotype.

But not all genes follow this dominant/recessive model. For example, the gene for blood type is codominant; if you get a gene for type A blood from one parent and type B blood from the other, neither dominates. Instead, you wind up with type AB blood.

Other human traits are polygenic, which means that they are controlled by several genes that contribute in an additive fashion. Skin color is believed to be polygenic. Scientists also think that polygenic inheritance is responsible for inherited predispositions to certain diseases, such as heart disease, arteriosclerosis, and some cancers.

Pass the Peas, Please

Gregor Mendel was an Austrian monk who did extensive breeding research on pea plants. In doing so, he overturned our understanding of heredity.

At the time of Mendel's work, in the 1860's, most people believed in the blending theory of heredity, the idea that offspring were born with traits constituting a blend or average of the two parents. By this theory, when a red and a white flower are bred together, or crossed, their offspring should all be blended, that is, pink.

Mendel's results suggested otherwise. For example, when he crossed pure-bred tall pea plants with pure-bred short pea plants, he got all tall pea plants-no blending there. He concluded that every organism possesses two "factors" (we now call them genes) for a given trait, and passes on just one of these factors-at random-to its offspring.