Mendel studied the inheritance of seven traits in the pea plant. For example, one trait was seed shape. Mendel had a variety that produced either round or wrinkled seed. He started with two sets of true-breeding plants, one that produced only round seeds and another that produced only wrinkled seeds. When he crossbred them with each other, all of the offspring had round seeds (see Figure 6.1). It would seem that the ability to produce wrinkled seeds had been lost. However, if a number of plants produced in this way were allowed to self-fertilize (self-pollination is the normal mode of fertilization in the pea plant), one-fourth of the offspring had wrinkled seeds!
The explanation for this result is as follows. True-breeding plants are those that have been self-bred for many generations. As Problem 6.1 shows, this results in plants that are homozygous in all their genes. If we represent the gene for seed shape by R for round seed and r for wrinkled seed, the true-breeding round-seed plant would have its two alleles represented by RR, and the homozygous wrinkled-seed plant would have genotype rr. The round-seed plant could produce only one type of gamete: R. Similarly, the wrinkled-seed plant could only produce r gametes. Thus, the offspring produced by mating these two plants could only be heterozygous: Rr. The fact that all of the offspring of this generation had round seeds demonstrates that R is the dominant allele and r is the recessive.
The offspring, being all heterozygous, produce gametes half of which are R and half r. Randomly mating these would produce genotypes of about 25% RR, 50% Rr, and 25% rr. Since R is dominant, both RR and Rr, or about 75% of the offspring, would have round seeds. The remaining 25% are homozygous recessive and would express the gene for wrinkled seeds. The reappearance of the wrinkled phenotype and the proportions involved proved Mendel's hypothesis of the particulate transmission of hereditary traits, the principles of paired alleles, the existence of dominant and recessive alleles, and the existence of segregation of alleles in gametes.
Mendel demonstrated the principle of independent assortment by examining the phe-notypes for two genes at a time in similar experiments. Another phenotype of Mendel's pea plants had to do with seed color. Yellow seeds were dominant over green seeds in his variety. True-breeding plants with yellow round seeds would be homozygous for both genes (YYRR). True breeding plants with green wrinkled seeds would also be homozygous, but in this case, they would be double recessive for both traits (yyrr). A cross between these two plants could only be YyRr, heterozygous for both traits, and therefore expressing the dominant yellow, smooth seed phenotype.
However, the gametes of these offspring could be of four types: YR, Yr, yR, or yr. When allowed to self-fertilize, the possible crosses can be found by constructing a Punnett square (Figure 6.2). The possible gametes are listed along the top and left. Each type has equal probability of occurring. Thus, each cross represented by a box within the square occurs in the same proportion of the population. Just as expected from the single- trait experiment, one-fourth of the population will have wrinkled seeds
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