Concept 12 Evolution begins with the inheritance of gene variations.

HI! George Shull found that hybrids grew better than pure-breds or the openly pollinated plants. Let's explore the idea of "hybrid vigor." Let's start with two pure-breds, Strain 1 and Strain 2. If we assume that vigor is controlled by two genes, A and B, what are the possible genotypes of these pure-breds? AA/Bb and Aa/BB. No, these plants are not pure-bred for both genes. AA/bb and aa/BB. That is correct. Aa/Bb and Aa/Bb. No, these are the genotypes of the heterozygote, the hybrid. AA/bb and Aa/Bb. No, AA/bb is a pure-bred genotype, but Aa/Bb is a hybrid genotype. The possible genotypes of the pure-bred plants are AA/bb and aa/BB. Shull found that when he crossed the two pure-breds, the offspring grew much better. What is the genotype of the F1 hybrid offspring? AA/Bb. No, this plant is pure-bred for gene A. AA/bb. No, this is the genotype of a plant pure-bred for genes A and B. Aa/Bb. That is correct. Aa/BB. No, this plant is pure-bred for gene B. The genotype of the F1 hybrid offspring is Aa/Bb. The hybrid is heterozygous for both vigor genes. The recessive effects of aa and bb have been complemented in the hybrid by the inheritance of one dominant gene. What happens if the Aa/Bb hybrids are allowed to openly pollinate in the field? Eventually all the offspring will be pure-breds, and will grow poorly. No, draw a Punnett square and see how the genes segregate. Not all the offspring will be pure-breds. There will be a mixture of phenotypes. Some of the offspring will grow well, some won't. That is correct. All the offspring will be hybrids (Aa/Bb), and will grow well. No, draw a Punnett square and see how the genes segregate. Not all the offspring will be Aa/Bb. Shull found that if he let F1 hybrids, Aa/Bb, openly pollinate in the fields, the offspring were not all vigorous growers. Some of the offspring, those heterozygous for the vigor genes, grew well; some of the offspring, those homozygous for the vigor genes, grew poorly. In order to be sure of the crop, Shull maintained the inbred strains, and cross-fertilized them for the vigorous hybrid offspring. Farmers plant this type of hybrid seeds every year for their crops. CONGRATULATIONS!!! YOU'RE SO SMART!