In the 19th century, an Austrian monk named Gregor Mendel performed a series of experiments in breeding pea plants. He was able to identify what are now thought of as genes by observing the patterns of inheritance. He succeeded because pea plants are relatively simple, with one gene directly controlling one physical feature. Genetic inheritance works the same way in humans, except that we're much more complicated than pea plants, and almost no human traits are controlled by a single gene.
Mendel measured a variety of characteristics of pea plants. For example, plants could come in two versions, tall or short. He reasoned that the plants transmitted height information from one generation to the next, telling the next generation whether to be tall or short. That is, the plant characteristic is height, and the values are either tall or short. His experiments demonstrated that each plant has two copies of of a height gene, and each copy could take one of the two values, tall or short. His work demonstrated that if both copies of the height gene were for tall, the plant would be tall. If both copies were short, the plant would be short. But if one copy was tall and the other short, the plant would still be tall.
Genes and Proteins
Today biologists understand that genes correspond to regions on a strand of DNA, and further, that the regions generally carry the information necessary to build proteins. In the particular case of Mendel's pea plant height, for example, the dominant gene builds an enzyme -- a protein that speeds up chemical reactions -- that turns on a plant growth hormone. One gene defines one protein that controls one physical feature. Human inheritance is more complex because just about every human trait is the result of the interaction among many different proteins, each built or controlled by a different gene.
Look at the example of skin color. In humans, at least 14 different proteins are involved in creating, transporting and expressing melanin -- the pigment responsible for skin color. You can think of the system for distributing melanin as something like an old-fashioned fire bucket brigade. Someone pumps water, others pass the buckets and someone sprays the water. If any one of them doesn't do their job then you don't put much water on the fire. If the fire brigade was your body expressing melanin, each person would be a protein built by a completely separate gene, and they all work together in a sequence called the signalling chain.
Almost every human physical trait is the result of a long and complex signalling chain requiring the involvement of many genes. The strength of the expression of a human trait depends upon the strength of each protein in the signalling chain, and that shows up in a couple ways. First, most human traits are not as simple as a choice between tall and short -- the traits exist on a continuum. For example, you can have very short, short, average, tall and very tall people (and everything in between). Second, humans can exhibit such things as patterns called incomplete dominance and codominance. Incomplete dominance is when neither parents' trait appears in the child. For example, if one parent is pointy-nosed and the other round-nosed, the child could end up with an in-between nose. Codominance is when both parents' traits show up in the child. For example, a mother with blood type A and a father with blood type B can have a child with blood type AB.
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