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