BICD 100 - Week 4

# BICD 100 - Week 4 - BICD 100 Week 4 Handout Pedigrees...

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BICD 100 Week 4 Handout: Pedigrees, Isolating Mutants This week’s section will cover some of the more challenging types of pedigree problems, along with an introduction to isolating mutants. There is less material this week, but spend some time carefully going over what has been covered in class so far. Some of the upcoming topics (recombination, linkage, chromosome mapping) will require a solid understanding of the previous material! Advice : • There are three kinds of pedigree problems that involve probability calculations: 1) Carrier Probability This kind of problem will ask for the probability of a specific individual on a pedigree of being a carrier. To solve this problem (1) determine the mode of inheritance of the allele (this might be indicated in the problem), (2) calculate the probability of each prior relative inheriting the allele, starting at the affected relative. (3) Then, use the product rule to determine P(carrier), since each inheritance of each allele is an independent event. Example: In this pedigree, individual a has a rare autosomal recessive disorder (genotype tt ). What is the probability that individual i will be a carrier? Answer: In order for individual i to be a carrier, she must inherit a t allele from individual g, who inherits the t allele from individual d, who inherits the t allele from individual a. So to solve this problem, calculate the probability of each prior individual inheriting the t allale, then use the product rule. P(individual d inheriting the t allele) = 1 (she must inherit it, since individual a is homozygous recessive) P(individual g inheriting the t allele) = ½ P(individual i inheriting the t allele) = 1 x ½ x ½ = ¼ *a more difficult question will require probability calculations of two parents being carriers, then finding the probability of their child having the disease* 2) Probability of Multiple Cases: This kind of problem asks for multiple scenerios to be analyzed, then the probabilities are

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summed together. Example: This pedigree shows inheritance of a rare autosomal dominant disorder. Those individuals suffering from the disease are shown in black, and they are heterozygous for the disease (genotype Aa). However, some of the kids in generation III will have the mutant allele and will develop the disease, but they have not yet shown the symptoms.
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