Fundamental principles governing the transmission of genetic traits, discovered by an Augustinian monk Gregor Mendel in 1856. Mendel performed his first set of hybridization experiments with pea plants. 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. For stem height, the true breeding strains tall (7 ft or 2.1 m) and dwarf (18 in. or 45 cm) were used. He also selected six other sets of traits, involving the shape and color of seeds, pod shape and color, and the location of flowers on the plant stem.
The most simple crosses performed by Mendel involved only one pair of traits; each such experiment is known as a monohybrid cross. The plants used as parents in these crosses are known as the P1 (first parental) generation. When tall and dwarf plants were crossed, the resulting offspring (called the F1 or first filial generation) were all tall. When members of the F1 generation were self-crossed, 787 of the resulting 1064 F2 (second filial generation) plants were tall and 277 were dwarf. The tall trait is expressed in both the F1 and F2 generations, while the dwarf trait disappears in the F1 and reappears in the F2 generation. The trait expressed in the F1 generation Mendel called the dominant trait, while the recessive trait is unexpressed in the F1 but reappears in the F2. In the F2, about three-fourths of the offspring are tall and one-fourth are dwarf (a 3:1 ratio). Mendel made similar crosses with plants exhibiting each of the other pairs of traits, and 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, while among the dominant plants one-third bred true and two-thirds behaved like the F1 plants. See Dominance
Law of segregation
To explain the results of his monohybrid crosses, Mendel derived several postulates. First, he proposed that each of the traits is controlled by a factor (now called a gene). Since the F1 tall plants produce both tall and dwarf offspring, they must contain a factor for each, and thus he proposed that each plant contains a pair of factors for each trait. Second, the trait which is expressed in the F1 generation is controlled by a dominant factor, while 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 formation. Therefore, the F1 plants can produce two types of gametes, one type containing a factor for tall plants, the other a factor for dwarf plants. At fertilization, the random combination of these gametes can explain the types and ratios of offspring in the F2 generation (see illustration). See Fertilization, Gene
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Independent assortment
Mendel extended his experiments to examine the inheritance of two characters simultaneously. Such a cross, involving two pairs of contrasting traits, is known as a dihybrid cross. For example, Mendel crossed plants with tall stems and round seeds with plants having dwarf stems and wrinkled seeds. The F1 offspring were all tall and had round seeds. When the F1 individuals were self-crossed, four types of offspring were produced in the following proportions: 9/16 were tall, round; 3/16 were tall, wrinkled; 3/16 were dwarf, round; and 1/16 were dwarf, wrinkled. 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, round and wrinkled factors segregate into gametes independently of whether they also contain tall or dwarf factors. See Gametogenesis, Meiosis
It might be useful to consider the dihybrid cross as two simultaneous and independent monohybrid crosses. In this case, the predicted F2 results are 3/4 tall, 1/4 dwarf, and 3/4 round, 1/4 wrinkled. Since the two sets of traits are inherited independently, the number and frequency of phenotypes can be predicted by combining the two events:
This 9:3:3:1 ratio is known as a dihybrid ratio and is the result of segregation, independent assortment, and random fertilization. See Genetics
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