Double Crossovers
Two-Strand Double Crossover
A
a
A
a
Four-Strand Double Crossover
A
A
CO
a
a
CO
CO
CO
B
B
b
b
B
B
b
b
A
a
A
a
A
a
A
a
B
b
B
b
b
B
b
B
100% parental type
0% recombinant type
0% parental type
100% recombinant type
Three-Strand Double Crossover (I)
Three-Strand Double Crossover (II)
A
a
A
a
A
A
CO
CO
CO
CO
a
a
B
B
b
b
B
B
b
b
A
a
A
a
A
a
A
a
b
B
B
b
B
b
b
B
50% parental type
50% recombinant type
Genetics Handout 17.1
50% parental type
50% recombinant type
Multiple Crossovers
• The average number of crossovers between two loci depends on the distance between
the loci.
• The percentage of meioses with no crossovers and the percentage of meioses with one
or more crossovers can be calculated from the average number of crossovers between
two loci (using the Poisson distribution).
• The percentage of recombinants is one half of the percentage of meioses with one or
more crossovers between the two loci.
• For linked loci, the percentage of recombinant type gametes produced approaches, but
never exceeds, 50%.
• Short map distances add up; long map distances do not.
Genetics Handout 17.2
Recombination Mechanism
• The diagrams show both DNA strands from chromatids undergoing crossing-over.
• The DNA strands from the chromatid from one homolog are shown in red.
• The DNA strands from the chromatid from the other homolog are shown in blue.
1
•
•
•
•
•
2
3
4
5
Chromatids from homologous chromosomes are brought into close contact (1).
A nick is made in one strand of one chromatid (2).
The nicked strand invades the other chromatid (3).
The displaced strand is nicked and moves over to the other chromatid (4).
Gaps and nicks are repaired, linking the two chromatids together (5).
or
6a
7a
6b
7b
• The strands that were nicked originally are cut and joined (6a).
• The resulting chromatids have a heteroduplex region but are not recombinant (7a).
or
• The strands that were left intact originally are cut and joined (6b).
• The resulting chromatids have a heteroduplex region and are recombinant (7b).
Genetics Handout 17.3