Chapter 7 Outline
• 7.1 Linked Genes Do Not Assort Independently, 161
• 7.2 Linked Genes Segregate Together, and Crossing Over Produces Recombination between Them, 162
• 7.3 A Three-Point Testcross Can Be Used to Map Three Linked Genes, 175
• 7.4 Physical Mapping Methods Are Used to Determine the Physical Positions of Genes on Particular Chromosomes, 185
• 7.5 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
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
