An Explanation
Hartl pgs. 67-68
The maximum recombination frequency is 50%. This means
that there is an average of one crossover, between the
two loci, for each meiosis. There are three different
types of double crossover.
A two-strand double crossover is the result of two
chiasma between the two loci involving the same two
sister chromatids. These two events cancel each other
out and result in parental or non-recombinant progeny.
Three-strand double crossovers result when one chromatid
crosses over with two other chromatids. The result is
two parental and two recombinant progeny, the same as
a single crossover event.
The four-strand double crossover is a single crossover between two
chromatids and then a second single crossover between the remaining
chromatids. The result is that all four products are recombinant.
Type
of
crossover |
chromosomes
recovered |
| parental |
recombinant |
| Two-strand
double |
4 |
0 |
| 3-strand
double |
2 |
2 |
| 3-strand
double |
2 |
2 |
| 4-strand
double |
0 |
4 |
| Total |
8 |
8 |
The average number of recombinants per double crossover is 8/4 = 2
and the average number of parental types is 8/4=2. This is the same
as progeny from single crossover events. A maximum of 50% recombination
is obtained whether single or double-crossover events are considered.
Emerson and Rhodes show that regardless of whether single, double,
triple, etc. crossovers occur, the amount of recombination between two
loci will not exceed 50%.
See Emerson, R.A. and M.M. Rhodes. Relation of chromatid crossing over
to the upper limit of recombination percentages. Am. Naturalist 65:
375 - 376.
