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section16_ak - MIT Department of Biology 7.014 Introductory...

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MIT Department of Biology 7.014 Introductory Biology, Spring 2005 Recitation Section 16 Answer Key April 6-7, 2005 Recombination and Pedigrees A. Chromosomes and Recombination 1. What is the physical basis of the genetic phenomenon of gene recombination? Recombination of genes occurs because of the physical swapping of pieces of chromosomes during meiosis. 2. Why is the recombination frequency between two genes correlated with the physical distance between the two genes? Recombination frequency (genetic distance) is determined by the frequency of the recombination events between the two genes in meiosis. The greater the physical distance between the two genes, the more likely are they to recombine during any given meiosis event. 3. Recombination can occur anywhere along the length of the chromosome. However, we have been relying on the fact that genes are inherited as discreet units. How do we reconcile these two things? (Hint: think about what usually is the difference between two alleles of the same gene.) Recombination of genes occurs because of the physical swapping of pieces of chromosomes during meiosis. The swapping can occur anywhere along the chromosome, including in any part of a gene. However, the differences between alleles are generally as small as a difference of a single base pair (or as large as a deletion of a region of a gene). As a result, the only thing that matters for the purposes of determining the genotype (and phenotype) of the offspring is whether as the result of the recombination event, the site of the difference between alleles moved to the other chromosome or not. 4. Why is the highest possible recombination frequency 50%? The recombination frequency between two genes is equal to the proportion of offspring in which a recombination event occurred between the two genes during meiosis. The recombination frequency between two genes cannot be greater than 50% because random assortment of genes generates 50% recombination (non-linked genes produce 1:1 parental to non-parental. Thus, the recombination frequency would be non-parental/total Æ 1/(1+1) = 50%).
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