Mendel discovered that traits are inherited in

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Mendel Discovered That Traits Are Inherited in Discrete Units Gregor Mendel, born Johann Mendel, was the son of peasant farmers. From a child- hood spent on a farm, Mendel understood the value of plant breeding in developing productive varieties of crops. This undoubtedly contributed to his lifelong interest in gardening and horticulture.At the age of 21, Mendel entered the priesthood, taking the clerical name of Gregor by which he is now remembered. In the remote monastery in what is now the Czech Republic, Mendel had time to indulge his love of plant breeding. He read widely, especially the natural sciences. He was quite aware of the controversial new theory of evolution proposed by his British contemporary, Charles Darwin. He knew of the unanswered questions about heredity arising from Darwin’s theory, the very topic Mendel sought to understand. He made the fortuitous choice to study the pea, which is an organism that can be easily manipulated in breeding experiments. Even in Mendel’s time, pea plants came in many distinct strains or varieties. Mendel studied traits that each occur in two distinct forms (Figure 3-1). The color of the pea flower, for example, is either purple or white, never purplish white. The shape of the pea pod is either puffy and inflated or narrow and constricted, never partly puffy, and so on with each of the seven traits he studied.He began by developing true-breeding varieties for each of the seven traits; that is, when bred among themselves, all of the off- spring of a given variety were identical to the parent for that trait.For example,one variety produced only purple blooms for many generations, another only white blooms. One va- riety produced only puffy pods for many generations,another only constricted pods.There were 14 varieties in all.When he was certain that all of his varieties bred true,he carefully engineered matings between pairs of plants showing different forms of each trait. In addition to this focused approach, Mendel added yet another new twist: Mathe- matical analysis. He counted the number of young plants that developed the parental forms of each trait and calculated the numerical ratios of offspring showing each form of a trait. In Mendel’s day, this application of mathematics to plant breeding experi- ments was uncommon, to say the least. Let’s start with just one trait that Mendel stud- ied and carefully follow his reasoning. Mendel’s First Discovery When organisms reproduce sexually, both parents produce specialized reproductive cells called gametes . Male gametes are sperm , and female ga- metes are eggs . When egg and sperm fuse—a process called fertilization —a new indi- vidual is produced. In flowering plants such as the garden pea, sperm are contained in pollen . Eggs are contained in ovules , which when fertilized, mature into seeds. Ovules are contained within a structure in the flower called the carpel (Figure 3-2). Pea plants are often (but not always) self-fertilizing ; that is, the sperm-carrying pollen usually lands on the top of the egg-carrying carpel of the same plant. If the flowers are covered to en-
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