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Lecture 26 Mendelian Genetics Part II Notes

Lecture 26 Mendelian Genetics Part II Notes - Lecture 26...

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Lecture 26 Mendelian Genetics, Part 2 Campbell Chapter 14, 6 th Ed.; pp. 252-259: 7 th Ed. pp. 256-264: 8 th Ed.; pp. 268-275 Important Note Genetics is a problem-based science. It takes awhile to get familiar with the approach to be able to solve the puzzles. It's important to do the problems at the end of the chapter for practice. You should also go over the examples in the text to make sure you know how to get the answers. Finally, do the relevant problems from the old Exam 3 on the Blackboard. Monohybrid cross . The key part of Mendel’s experiment (discussed last time) on flower color was the cross of members of the F1 generation with each other. Remember that all F1 individuals were heterozygotes. Crossing them with each other gave the 3:1 ratio of purple to white-flowered peas in the F2 generation. This ratio gave Mendel the information he needed to figure out the laws we discussed last time. This type of cross; (crossing two individuals each heterozygous for one trait) is called a monohybrid cross . Mendel next extended his analysis by looking at inheritance of two traits at the same time. He found that the laws he’d discovered to describe inheritance of one trait still held. He also discovered one more important principle: the Law of Independent Assortment . This law says that different traits are inherited independently of each other. We’ll go through all this next. Dihybrid crosse s Mendel found that pea shape (round or wrinkled) and pea color (yellow or green) were inherited following the same rules as for flower color. Yellow pea color (Y) was dominant while green (y) was recessive. For shape, round (R) was dominant while wrinkled (r) was recessive. He then asked about the relationship between inheritance of the two traits. That is, would pea color have any effect on how pea shape was inherited? To answer this question, he did a similar experiment to the one he did on flower color. He started out with 2 true-breeding strains. One was true-breeding for BOTH dominant phenotypes (round and yellow), and the other was true-breeding for BOTH recessive phenotypes (wrinkled and green). Based on what he’d already learned, he knew that the true-breeding strains were homozygous for both traits. That is, the genotype of the round-yellow strain was RRYY, and the genotype of the wrinkled-green strain was rryy. He crossed these strains (the P generation) to give the F1 generation. All of the individuals in the F1 generation were heterozygous for both traits (RrYy) and had the dominant phenotype for both (round-yellow). He then did the crucial experiment: to cross members of the F1 generation with each other, and examine the phenotypes of the offspring (the F2 generation). This is an example of a dihybrid cross . This means crossing two individuals who are both heterozygous for two traits (RrYy x RrYy). The F2 generation offspring of this dihybrid cross had the following phenotypes. About 9/16 th of them had yellow and round peas (dominant phenotype for both traits). 3/16 th were green and round, and 3/16 th
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