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CHAPTER 11.2 AP Biology Fall 2010 Mendel’s Theory of Segregation Monohybrid Experimental Predictions Mendel suspected that every plant inherits two “units” (genes) of information for a trait One from each parent on pairs of homologous chromosomes Monohybrid Experimental Predictions Mendel’s first experiments were monohybrid crosses Monohybrid crosses have 2 parents that are truebreeding for contrasting forms of a trait White vs. purple flowers Monohybrid Experimental Predictions One form of the trait (white) disappears in the first generation offspring (F1), only to show up in the second generation (F2) We know that all members of the F1 offspring are heterozygous (Aa) because one parent could only produce an A gamete and the other could produce only an a gamete Monohybrid Experimental Predictions Mendel had a strong mathematical background knew about sampling error so he crossed thousands of offspring F2 Generation In the F2 generation, the white flowers reappear The numerical ratios of crossing suggested that genes do not blend For example: F2 offspring show a 3:1 phenotypic ratio of purple to white F2 Generation Mendel assumed that each sperm has an equal probability of fertilizing an egg This can be seen most easily by using a Punnett square Thus, each new plant as three chances in 4 of having at least one dominant trait Punnett Square Page 173, figure 11.7 True breeding homozygous recessive parent x true breeding homozygous dominant parent a a A Aa Aa A Aa Aa F1 Phenotypes: 4 purple plants F1 Genotype: 4 Aa (100%) Punnett Square Page 173, figure 11.7 F1 x F1 A a A AA Aa a Aa aa F2 Phenotypes: 3 purple (75%), 1 white (25%) F2 Genotypes: 1 AA (25%), 2 Aa (50%), 1 aa (25%) Testcrosses Testcross: an organism shows dominance for a specified trait but its genotype may be unknown, so it is crossed with a homozygous recessive individual Test results reveal whether it is homozygous dominant or heterozygous Draw an example of a test cross…. Testcrosses To support his concept of segregation, Mendel crossed F1 plants with homozygous recessive individuals to reveal whether F1 plants were homozygous or heterozygous dominant A 1:1 ratio of recessive and dominant phenotypes support his hypothesis Testcrosses Mendel’s theory of segregation states that diploid organisms inherit pairs of genes on pairs of homologous chromosomes Each gene segregates from the other during meiosis so that each gamete will receive only one gene per trait Mendel’s Theory of Segregation Diploid cells have pairs of genes, on pairs of homologous chromosomes. The two genes of each pair are separated from each other during meiosis, so they end up in different gametes Review 1. 2. 3. What is a testcross? What is Mendel’s theory of segregation? Construct a Punnett square of testcross of F1 purple flowered plants (Aa) with true breeding white flowered plants (aa) Answers 1. 2. Testcross: an organism shows dominance for a specified trait but its genotype may be unknown, so it is crossed with a homozygous recessive individual Mendel’s Theory of Segregation: Diploid cells have pairs of genes, on pairs of homologous chromosomes. The two genes of each pair are separated from each other during meiosis, so they end up in different gametes Answers 3. Aa x aa a a A Aa Aa a aa aa Phenotype: 2 purple (50%), 2 white (50%) Genotype: 2 Aa (50%), 2 aa (50%)