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Unformatted text preview: Answers to Problem Set 1 1. Gregor Mendels less well-known brother Fritz also performed some pioneering experiments on heredity in Pisum sativum . Fritz Mendel crosses a round yellow pea plant with a wrinkled green pea plant. Fritz counts up the F1 peas and gets the following results: 470 round yellow : 467 wrinkled yellow. State the likely genotypes of the parents and the F1 progeny classes. Fritz then takes one of the F1 wrinkled yellow seeds and allows it to grow and self to give the F2 generation. What do you predict he should see in the F2 self progeny, and why? (A dihybrid test cross problem) Lets denote yellow as Y and round as R. Round R is dominant to wrinkled r, Yellow Y is dominant to green y. All F1 were yellow so the yellow parent must have been homozygous YY. The F1 were round:wrinkled in approximately 1:1 ratio. So: YY Rr x yy rr giving F1 Yy Rr and Yy rr in 1:1 ratios Selfng wrinkled yellow Yy rr should give 3:1 yellow:green in F2. All the F2 should be wrinkled. 2. As a keen amateur geneticist and much to the dismay of your housemates you maintain a colony of mice. The normal wild type mouse has 5 toes per paw. You nd an unusual variant mouse with six toes on each paw. When this six-toed mouse is crossed with a wild type male mouse, its F1 progeny consist of four six-toed mice and three ve-toed mice. a. Form a hypothesis for the genetic basis of the six-toed trait. How would you generate a true breeding stock of six-toed mice? State the crosses you would perform, giving genotypes and phenotypes in each generation. The six-toed trait seems to be a single dominant mutation. Denote it T and the normal fve-toed as recessive t. The six toed mouse was Tt and when crossed with tt gave Tt and tt in about a 1:1 ratio. To get a true breeding stock o six-toed mice, we need to identiy the homozygous TT males and emales and cross them together. How do we distinguish homozygous TT rom heterozygous Tt, both o which could be six-toed? (Its possible homozygous TT might look dierent rom Tt but we dont know that). To distinguish TT rom Tt you need to do test crosses to recessive tt. b. A friend alerts you to the existence of a true-breeding strain of mice with three toes per paw. You cross such a three toed mouse with your ve-toed wild type and nd that all the F1 have four toes per paw. Formulate a hypothesis to explain these data. Based on your hypothesis, if two four- toed (F1) mice were crossed, what would you predict to occur in the F2 and why? The three toed trait could be a single allele with incomplete dominance. Denote it A and the wild type a. The parental three toed animal must be homozygous (you are told true breeding) AA. The F1 are all Aa and have the our toed phenotype....
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- Winter '08