Chapter 13 Mendel and the Gene

A mating between parents that each carry two

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A mating between parents that each carry two different genetic determinants for the same trait is called a monohybrid cross Mendel designated wrinkled shape as a recessive trait relative to the round-seed trait. He referred to round seed as dominant to the wrinkled-seed trait. Individuals with the dominant phenotype do not necessarily have higher fitness than do individuals with the recessive phenotype. Nor are genetic determinants associated with a dominant phenotype necessarily more common than recessive ones. In genetics, the terms dominant and recessive identify only which phenotype is observed in individuals carrying two different genetic determinants for a given trait. Mendel did a reciprocal cross, in which the pollen from the wrinkled seeded plant was put in the female organ of the round seeded plant. The original experiment was the vice versa of this. The results were identical to the first experiment. The results from Mendel's Monohybrid Reciprocal Cross Experiments were general across the board. Particulate Inheritance To explain the patterns he observed, Mendel proposed particulate inheritance. He maintained that the hereditary determinants for traits do not blend together or acquire new or modified characteristics through use. Hereditary determinants maintain their integrity from generation to generation. Instead of blending together, they act like discrete entities or particles. Gene indicates the hereditary determinant for a trait. Mendel proposed that each individual can have two versions of any gene. Different versions of genes are called alleles. The alleles that are found in a particular individual are called the genotype. Genotype affects one's phenotype.
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The principle of segregation is the concept that each pair of hereditary elements separate from each other during the formation of offspring. Mendel used a notation of capital and lowercase letters to describe the genotype of the individuals in the round-seed pure line as RR and the genotype of the wrinkled-seed pure line is rr . RR and rr are homozygous. Rr is heterozygous. Because the R allele is dominant, all of the F 1 offspring produce round seeds. Punnett Squares help predict the genotypes and phenotypes of different crosses. Mendel's Experiments with Two Traits Mendel crossed a pure-line parent that produced round, yellow seeds with a pure-line parent that produced wrinkled, green seeds. According to his model, the F 1 offspring should be heterozygous for both genes. A mating between parents that are heterozygous for both traits is called a dihybrid cross. The F 1 offspring are expected to have the dominant round and yellow phenotypes. Truth. The phenotypes of the F 2 offspring conformed to the predictions of independent assortment, in which alleles of different genes don't stay together when gametes form.
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