Heredity and Mendel’s Experiments
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For thousands of years, people have understood that many characteristics of plants or animals are transmitted from parent to offspring. Because of this understanding, farmers have been able to create better varieties of crops and livestock, by allowing the reproduction of only those individuals that have the desired characteristics. As a result, we now have domestic plants and animals that provide us with much more food than their wild ancestors ever did. But even though people have long possessed some vague appreciation for the principles of heredity, it has only been since the late 19th century that a systematic understanding of those principles has been gained.
The first scientist to discover the laws of heredity was an Austrian monk, Johann Gregor Mendel. Mendel’s investigations of heredity made use of the pea plant. He studied several characteristics of peas that are transmitted by heredity, such as the color of the peas (green versus yellow), the texture of the peas (smooth versus wrinkled), and the height of the plant (tall versus short). Mendel studied heredity by first finding plants that had shown the same characteristic for several generations?that is, plants that were “pure-bred” for certain traits.
He then crossed, or hybridized, pairs of pea plants that had different colors, different textures, and different heights. Mendel found that the offspring of these crossings did not show characteristics intermediate between those of the parents, but instead resembled only one of the parents, with respect to a particular characteristic. For example, when yellow pea plants and green pea plants were crossed, all of the offspring had yellow peas.
In this sense, the yellow color is said to be dominant, and the green color is said to be recessive. Mendel also found that the recessive characteristics could re-emerge in later generations. When he crossed different hybridized plants, he found that one-quarter of these second-generation offspring would show the recessive trait, such as the green color. From these results, Mendel deduced that traits are transmitted by discrete particles. (Nowadays, these are called “genes”.) For a given characteristic, an offspring inherits two of these, one from each parent. If the offspring inherits different genes from each parent, then one of the genes is “dominant” over the other, and the dominant trait emerges in the offspring. However, the recessive trait may emerge in a later generation.
Mendel also found that different characteristics are inherited independently. For example, the inheritance of color (green versus yellow) did not depend on the inheritance of texture (smooth versus wrinkled). Each characteristic, such as color or texture, was inherited separately, without influence due to any other characteristic. As a result, any combination of color and texture could occur. Mendel’s work was published in 1866, but it was ignored by other scientists until about 1900, when other scientists independently re-discovered Mendel’s findings, and brought attention to his work.
When scientists began studying heredity more widely, they noticed many cases that were exceptions to Mendel’s rules. For example, there are cases in which two or more different kinds of characteristics tend to be inherited together. Nevertheless, Mendel’s basic principles provided the first insight into the mechanisms of heredity, and his work is recognized as the beginning of the scientific study of genetics.