单项选择题

Our physical development is largely directed by the genes that we inherit from our parents. At one time, psychologists waged vigorous debates about how much of our behavior is genetically determined and how much of it is learned. This was known as the nature-nurture controversy. We now know that this "either-or" question is largely meaningless, since behavior depends on the interaction between heredity (遗传) and environment. However, it is also dear that genetic factors do set limits on our behavioral capabilities, as illustrated most dramatically in certain genetically baaed birth disorders. Psychologists working in the field of behavior genetics study how behavior is influenced by biological factors and how favorable or unfavorable environmental conditions can affect an organism’ s genetically inherited potential.
Genetic theory began around 1865, when Gregor Mendel reported on his experiments with garden peas. Mendel showed that heredity involved the passing on of specific organic factors, not the simple blending of the parents’ characteristics. These specific factors might produce visible characteristics in the offspring(后代), or they might simply be carried for possible transmission to another generation. In any case, the offspring of one set of parents did not all inherit the same traits.
Early in the twentieth century, geneticists made an important distinction between genotype and phenotype. Genotype refers to the specific genetic makeup of an individual, while phenotype refers to the observable characteristics produced by that genetic endowment(才能). Genotype and phenotype axe not identical because not every gene is expressed in a visible trait and bemuse the expression of a genotypic trait may be modified by the environment.
An individual’s genetic coding is like the catalog of a giant library. Some of the coded directives are used on one occasion, some on another. Some are never used at all, either, because they are contradicted by stronger genetic directives or because the environment never calls them forth. For example, geneticists have discovered that chickens have retained the genetic code for teeth, yet, because the code is prevented from being expressed, hens’ teeth remain a cliche for scarcity.

The 15est title of this passage might be （）.

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