3 the carpel of the round seeded plant matured into a

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3 The carpel of the round-seeded plant matured into a pea pod containing all round peas. These peas were the seeds of the F 1 generation. 4 Mendel planted the F 1 peas and allowed them to grow into mature plants. Cross-fertilization Wrinkled pea Round pea Self- fertilization 5 These plants were allowed to self-fertilize. Pollen from the anthers of the F 1 plants fell directly onto the tops of the carpels of the same flower. 6 The carpels of the F 1 plants matured into pea pods containing peas—the seeds of the F 2 generation. About 75% of these peas were round and about 25% were wrinkled. F 1 (All round) F 2 (75% round 25% wrinkled) All round peas Figure 3-3 One of Mendel’s crosses between smooth round-seeded and wrinkled-seeded pea plants.
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68 C HAPTER 3 Mendelian Genetics: How Are Traits Inherited? When an organism has two identical factors for a trait, as in true-breeding varieties, it is said to be homozygous for that trait. (The prefix homo - means “the same.”) If the two factors are different, the organism is said to be heterozygous for that trait. (The pre- fix hetero - means “different.”) Mendel’s true-breeding varieties were homozygous; the hybrid offspring he created were heterozygous. Mendel’s results and interpretations are still relevant today,although our vocabulary is slightly different from his.We use the term gene to describe the hereditary information that determines a single trait. Seed texture in peas, for example, is a trait determined by a gene.Flower color is another trait determined by a gene.The different forms that a gene might take—what Mendel referred to as factors—are called alleles .Wrinkled and round are alternate alleles for the gene for seed texture; purple and white are different alleles for the gene for flower color. The relationship between genes and alleles is a fundamental concept of biology. In Mendel’s peas, the presence of the allele for round seeds was quite visible in the generation. Indeed all of the peas were round. But what of the allele for wrinkled seeds that later appeared in the Where was it hiding in the peas? Dominant Traits Mask the Presence of Recessive Alleles Let’s call the allele for round seeds R ,and the allele for wrinkled seeds r .Now,if both par- ents carry two identical alleles for wrinkled seeds, rr , each of the offspring will get an r allele for wrinkled seeds from mother and another wrinkled-seed r allele from father. All of the offspring will have a double dose of wrinkled-seed alleles, rr , and will have wrinkled seeds, just like the parents.These peas, as well as their parents, are homozygous for the gene for seed texture.But in Mendel’s first experimental cross,one parent had only wrinkled-seed alleles to give, rr ,and the other parent had only round-seed alleles, RR .One parent could contribute only r alleles and the other could contribute only R alleles to the offspring.When the peas reproduced, the generation ended up with two different alleles, Rr . They were heterozygous peas, with a wrinkled allele from one parent and a
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