bicd 100 - lecture 9

bicd 100 - lecture 9 - LECTURE 9 1 Dihybrid crosses 2...

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Unformatted text preview: LECTURE - 9 1. Dihybrid crosses 2. Branch diagrams 3. Binomial Expansion Dihybrid Cross Rr R RR Rr r Rr rr Y y Y YY Yy y Yy yy Branch Diagrams Rr r Rr rr r Rr rr Y y y Yy yy y Yy yy Dihybrid Testcross INCOMPLETE DOMINANCE Probabilities and Binomial Expansion A A Aa a Aa ¼ albino a Aa aa ¾ normal pigmentation If the heterozygous patients (Aa) have 3 children, what is the probability that 1 child has albinism? There are 3 possible scenarios (events): Child 1 Child 2 Child 3 Albino Pigmented Pigmented Pigmented Albino Pigmented Pigmented Pigmented Albino Probability for event 1 (first child is albino AND second child is pigmented AND third child is pigmented) to occur is the product of the individual probabilities (muliplication rule) 1/4x3/4x3/4=9/64 : event 1 Similarly, for events 2 and 3, the probabilities are: 3/4x1/4x3/4=9/64: event 2 3/4x3/4x1/4=9/64: event 3 Only one of these scenarios will actually occur, event 1 OR event 2 OR event 3. Hence, the probability of having one child with albinism and two without (in any order) is determined by adding the probabilities of events 1, 2 and 3 (addition rule). 9/64 + 9/64 + 9/64 = 27/64 Binomial Expansion 1 1 1 1 1 1 2 3 4 5 1 1 3 6 10 1 4 10 1 5 1 Pascal's triangle is a geometric arrangement of the binomial coefficients in a triangle. When progeny of crosses segregate into two distinct classes, we can calculate the binomial probability of any particular combination in another way using the following formula: s individuals fall into the class with probability a t individuals fall into the class with probability b In the albinism example (two heterozygous parents with three children, one of whom has albinism), s (= 1) would be the number of children with albinism, with probability a (= ¼) t (= 2) would be the number of children with normal pigmentation with a probability b (= ¾) For our other albinism example: five children three of whom have albinism: ...
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This note was uploaded on 01/25/2011 for the course BICD 100 taught by Professor Nehring during the Spring '08 term at UCSD.

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