Chapter 7 Answers to Homework Problems

Chapter 7 Answers to Homework Problems - Chapter 7 Answers...

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Chapter 7 Answers to Homework Problems 1. Questions and Problems, 7.1 Mendel did not know of the existence of chromosomes. Had he known, what change might he have made in his Principle of Independent Assortment? Ans: If Mendel had known of the existence of chromosomes, he would have realized that the number of factors determining traits exceeds the number of chromosomes, and he would have concluded that some factors must be linked on the same chromosome. Thus, Mendel would have revised the Principle of Independent Assortment to say that factors on different chromosomes (or far apart on the same chromosome) are inherited independently. 2. Questions and Problems, 7.2 From a cross between individuals with the genotypes Cc Dd Ee × cc dd ee, 1000 offspring were produced. The class that was C- D- ee included 351 individuals. Are the genes c, d , and e on the same or different chromosomes? Explain. Ans: The class represented by 351 offspring indicates that at least two of the three genes are linked. 3. Questions and Problems, 7.3 If a is linked to b , and b to c , and c to d , does it follow that a recombination experiment would detect linkage between a and d ? Explain. Ans: No. The genes a and d could be very far apart on the same chromosome—so far apart that they recombine freely, that is, 50 percent of the time. 6. Questions and Problems, 7.6 If two loci are 10 cM apart, what proportion of the cells in prophase of the first meiotic division will contain a single crossover in the region between them? Ans: 20%. 7. Questions and Problems, 7.7 Genes a and b are 20 cM apart. An a + b + / a + b + individual was mated with an a b / a b individual. (a) Diagram the cross and show the gametes produced by each parent and the genotype of the F 1 . (b) What gametes can the F 1 produce, and in what proportions? (c) If the F 1 was crossed to a b / a b individuals, what offspring would be expected, and in what proportions? (d) Is this an example of the coupling or repulsion linkage phase? (e) If the F 1 were intercrossed, what offspring would be expected and in what proportions?
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Ans: (a) Cross: a + b + / a + b + X a b / a b Gametes: a + b + from one parent, a b from the other F 1 : a + b + / a b (b) 40% a + b + , 40% a b , 10% a + b , 10% a b + (c) F 2 from testcross: 40% a + b + / a b , 40% a b / a b , 10% a + b / a b , 10% a b + / a b (d) Coupling linkage phase (e) F 2 from intercross: Summary of phenotypes: a + and b + 66% a + and b 9% a and b + 9% a and b 16% 10. Questions and Problems, 7.10 A homozygous variety of maize with red leaves and normal seeds was crossed with another homozygous variety with green leaves and tassel seeds. The hybrids were then backcrossed to the green, tassel-seeded variety, and the following offspring were obtained: red, normal 124; red, tassel 126; green, normal 125; green, tassel 123. Are the genes for plant color and seed type linked? Explain. Ans:
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Chapter 7 Answers to Homework Problems - Chapter 7 Answers...

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