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Unformatted text preview: Lecture 8 : Study Question Answers 1. What does the term recombination mean? What are two causes of recombination? Recombination means that meiosis generates gametes with different allelic combinations than the original gametes the organism inherited. If the organism was created by the fusion of an egg bearing AB and a sperm bearing ab, recombination generates gametes that are Ab and aB. Recombination may be caused by loci on different chromosomes that sort independently or by a physical crossing over between two loci on the same chromosome, with breakage and exchange of strands of homologous chromosomes paired in meiotic prophase I. 2. In a testcross for two genes, what types of gametes are produced with (a) complete linkage, (b) independent assortment, and (c) incomplete linkage? (a) Complete linkage of two genes means that only nonrecombinant gametes will be produced; the recombination frequency is zero. (b) Independent assortment of two genes will result in 50% of the gametes being recombinant and 50% being nonrecombinant, as would be observed for genes on two different chromosomes. Independent assortment may also be observed for genes on the same chromosome if they are far enough apart that one or more crossovers occur between them in every meiosis. (c) Incomplete linkage means that greater than 50% of the gametes produced are nonrecombinant and less than 50% of the gametes are recombinant; the recombination frequency is greater than 0 and less than 50%. 3. What effect does crossing over have on linkage? Crossing over generates recombination between genes located on the same chromosome, and thus renders linkage incomplete.4. Why is the frequency of recombinant gametes always half the frequency of crossing over? 5. What is the difference between genes in coupling configuration and genes in repulsion? What effect does the arrangement of linked genes (whether they are in coupling configuration or in repulsion) have on the results of a cross? Genes in coupling configuration have two wild-type alleles on the same chromosome and the two mutant alleles on the homologous chromosome. Genes in repulsion have a wildtype allele of one gene together with the mutant allele of the second gene on the same chromosome, and vice versa on the homologous chromosome. The two arrangements have opposite effects on the results of a cross. For genes in coupling configuration, most of the progeny will be either wild type for both genes, or mutant for both genes, with relatively few that are wild type for one gene and mutant for the other. For genes in repulsion, most of the progeny will be mutant for only one gene and wild-type for the other, with relatively few recombinants that are wild-type for both or mutant for both....
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This note was uploaded on 05/07/2011 for the course BIOLOGY 352 taught by Professor Townsend during the Spring '08 term at San Diego State.
- Spring '08