题目内容

In a study of nineteen sets of twins who had been separated from birth, investigators found that in approximately two thirds of the sets there were no more significant differences than existed among ____(未分离的) pairs of twins.

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Her first foster parents had proved to be ____(情绪易变的), and Helen had been returned to the orphanage after a couple of years; after several months she was again adopted, by a farmer and his wife who lived in southeastern Michigan.

However, genes do not work in a ____(真空状态); as soon as we begin considering the role that they play in the development of the individual, we see that there can be no development without the interacting environment.

Important as they are, genes alone are never absolutely ____(是......的原因) any trait.

The following case illustrates what happens to identical twins when they are ____(抚养) in contrasting environments.