Read the following passage and choose the best answer to each of the following questions.
BASIC PRINCIPLES OF HEREDITY

Over 150 years ago, in the middle of the nineteenth century, the Austrian Monk Gregor Mendel provided us with the first scientific explanation for why children look like their parents. By experimenting with different straits of peas in his garden, he happened to discover the laws of heredity.

Mendel bred tall pea plants with short pea plants, expecting to get medium-height pea plants in his garden. However, mixing tall and short “parent” plants did not produce medium-sized “children” as a result. Instead, it produced some generations that were tall and others that were short.

This led Mendel to hypothesize that all traits (such as eye color or height) have both dominant or recessive characteristics. If the dominant characteristic is present, it suppresses the recessive characteristic. For example, tallness (T) might be dominant and shortness (t) recessive. Where there is a dominant T, offspring will be tall. Where there is no dominant T, offspring will be short.

Imagine, for example, that each parent has two “markers" for height: TT, Tt or tt. The child inherits one marker from each parent. If both parents have full tallness (TT and TT), the child will definitely be tall; any marker the child could receive is the dominant marker for tallness. If both parents have full shortness (tt and tt), then the child will likewise be short; there are no dominants Ts to suppress the shortness. However, if both parents have a mix of markers (Tt and Tt), then there are four possible combinations: TT, Tt, tT and tt. Of course, TT will result in a tall child and tt in a short child. If the child receives one T and one t, the child will also be tall, since tallness is dominant and will suppress the marker for shortness. Thus, if both parents have a mix (Tt and Tt), the child has a 75% chance of being tall and a 25% chance of being short.

This is an over-simplification, but it is the basis of Mendel’s theory, which was later proven by the discovery of genes and DNA. We now know that characteristics such as height are determined by several genes, not just one pair. Still, Mendel’s contribution to the world of science is immeasurable.

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The main idea of this passage is that…………