Fundamental principles governing the transmission of genetic traits, as discovered by Gregor Mendel. Mendelism is the term used to describe the basic laws of genetic inheritance (Fig. 1). These operating laws were formulated by Gregor Johann Mendel (1822–1884), an Augustinian monk, who conducted a number of notable experiments on plant hybrids. Mendel published his scientific work in 1866; however, it went largely unheralded until 1900, when other investigators performed similar hybridization experiments and brought his work to the attention of the scientific world. The rediscovery of his work is regarded as the beginning of genetics as an organized discipline. Since then, genetic crosses and breeding endeavors with many different organisms have confirmed the fundamental nature and significance of Mendel's work. See also: Animal breeding; Genetics; Plant breeding
In 1856, Mendel performed his first set of hybridization experiments with pea plants in the monastery garden at Brno, Moravia (now part of the Czech Republic). Although the pea plant is normally self-fertilizing, it can be easily crossbred and grows to maturity in a single season. True breeding strains, each with distinct characteristics, were available from local seed merchants. For his experiments, Mendel chose seven sets of contrasting characters or traits—stem height, seed shape and color, pod shape and color, flower color, and flower location on the plant stem (Fig. 2). See also: Pea
The most simple crosses performed by Mendel involved only one pair of traits; each such experiment is known as a monohybrid cross (Fig. 1). The plants used as parents in these crosses are known as the P1 (first parental) generation. For example, when plants yielding yellow seeds (peas) were crossed with plants yielding green seeds (peas), all of the resulting offspring (called the F1 or first filial generation) yielded yellow peas. When members of the F1 generation were self-crossed, approximately three-fourths of the resulting F2 (second filial generation) plants yielded yellow peas and one-fourth yielded green peas (a 3:1 ratio). The yellow-colored trait is expressed in both the F1 and F2 generations, whereas the green-colored trait disappears in the F1 generation and reappears in the F2 generation. The trait expressed in the F1 generation is called the dominant trait; in contrast, the recessive trait is unexpressed in the F1, but reappears in the F2. Mendel made similar crosses with plants exhibiting each of the other pairs of traits that he studied; in each case, all of the F1 offspring showed only one of the parental traits and, in the F2, three-fourths of the plants showed the dominant trait and one-fourth exhibited the recessive trait. In subsequent experiments, Mendel found that the F2 recessive plants bred true; among the dominant plants, one-third bred true and two-thirds behaved like the F1 plants. See also: Dominance
Law of segregation
To explain the results of his monohybrid crosses (Fig. 1), Mendel derived several postulates. First, he proposed that each of the traits is controlled by a factor (later to be called a gene). Because the F1 plants in the aforementioned example produce both yellow- and green-colored seed offspring, they must contain a factor for each; thus, Mendel proposed that each plant contains a pair of factors for each trait. Second, the trait that is expressed in the F1 generation is controlled by a dominant factor, whereas the unexpressed trait is controlled by a recessive factor. To prevent the number of factors from being doubled in each generation, Mendel postulated that factors must separate or segregate from each other during gamete (sex cell) formation. Therefore, the F1 plants can produce two types of gametes—one type containing a dominant factor and the other type containing a recessive factor. At fertilization, the random combination of these gametes can explain the types and ratios of offspring in the F2 generation. See also: Fertilization (animal); Fertilization (plant); Gametogenesis; Gene
Mendel extended his experiments to examine the inheritance of two characters simultaneously. Such a cross, involving two pairs of contrasting traits (such as seed color and seed shape), is known as a dihybrid cross (Fig. 3). For example, Mendel crossed pea plants yielding yellow-colored and round-shaped seeds with plants yielding green-colored and wrinkled-shaped seeds. All of the F1 offspring yielded yellow-colored and round-shaped seeds. When the F1 individuals were self-crossed, four types of offspring were produced in the following proportions—9/16: yellow and round; 3/16: green and round; 3/16: yellow and wrinkled; 1/16: green and wrinkled. This 9:3:3:1 ratio is known as the dihybrid ratio.
On the basis of similar results in other dihybrid crosses, Mendel proposed that, during gamete formation, segregating pairs of factors assort independently of one another. As a result of segregation, each gamete receives one member of every pair of factors [this assumes that the factors (genes) are located on different chromosomes]. As a result of independent assortment, all possible combinations of gametes will be found in equal frequency. In other words, during gamete formation in the aforementioned example, round and wrinkled factors segregate into gametes independently of whether they also contain yellow or green factors. Thus, the 9:3:3:1 ratio is the result of segregation, independent assortment, and random fertilization. See also: Chromosome