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
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
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 TwoPoint 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 doublecrossover 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 doublecrossover 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