Chapter 7 Outline 7.1 Linked Genes Do Not Assort Independently, Linked Genes Segregate Together, and Crossing Over Produces Recombination between Them, A Three-Point Testcross Can Be Used to Map Three Linked Genes, Physical Mapping Methods Are Used to Determine the Physical Positions of Genes on Particular Chromosomes, Recombination Rates Exhibit Extensive Variation, 188 7.1 Linked Genes Do Not Assort Independently 7.2 Linked Genes Segregate Together and Crossing Over Produces Recombination between Them Notation for Crosses with Linkage Complete Linkage Leads to Nonrecombinant Gametes and Nonrecombinant Progeny Crossing Over with Linked Genes Lead to Recombiant Gametes and Recombinant Progeny 7.2 Linked Genes Segregate Together and Crossing Over Produces Recombination between Them Calculating Recombination Frequency Recombination frequency = (number of recombinant progeny / total number of progeny) 100% Coupling and Repulsion Configuration of Linked Genes Coupling (cis configuration): Wild type alleles are found on one chromosome; mutant alleles are found on the other chromosome. Coupling and Repulsion Configuration of Linked Genes Repulsion (trans configuration): Wild-type allele and mutant allele are found on the same chromosome. Testing for Independent Assortment Concept Check 2 The following testcross produces the progeny shown: AaBb aabb 10 AaBb, 40 aaBb, 40 aaBb, and 10 aabb. What is the percentage of recombination between the A and B loci? Were the genes in the AaBb parent in coupling or repulsion? Concept Check 2 The following testcross produces the progeny shown: AaBb aabb 10 AaBb, 40 aaBb, 40 aaBb, and 10 aabb. What is the percent recombination between the A and B loci? Were the genes in the AaBb parent in coupling or repulsion? % recombination: 20%; genes in the AaBb parent were in repulsion Gene Mapping with Recombination Frequencies Genetic maps are determined by recombinant frequency. Map unit and centiMorgans Constructing a Genetic Map with Two- Point Testcrosses 7.3 A Three-Point Testcross Can Be Used to Map Three Linked Genes Constructing a Genetic Map with the Three-Point Testcross Determining the gene order Determining the location of crossovers Concept Check 3 Write the genotypes of all recombinant and nonrecombinant progeny expected from the following three-point cross: Concept Check 3 Write the genotypes of all recombinant and nonrecombinant progeny expected from the following three-point cross: Answer: Concept Check 4 A three-point test cross is carried out between three linked genes. The resulting nonrecombinant progeny are s + r + c + and s r c, and the double- crossover progeny are s r c + and s + r + c. Which is the middle locus? Concept Check 4 A three-point test cross is carried out between three linked genes. The resulting nonrecombinant progeny are s + r + c + and s r c, and the double- crossover progeny are s r c + and s + r + c. Which is the middle locus? the C locus Calculating the recombination frequencies Interference and coefficient of coincidence Effect of multiple crossovers Mapping human genes 7.4 Physical Mapping Methods Are Used to Determine the Physical Positions of Genes on Particular Chromosomes Deletion Mapping Somatic Cell Hybridization