A wild-type fruit fly (heterozygous for gray body color and normal wings was mated with a black fly with vestigial wings. The offspring had the following phenotypic distribution: wild type, 778; black-vestigial, 785; black-normal, 158; gray-vestigial, 162. What is the recombination frequency between these genes for body color and wing type.
First count the total number of offspring 778+785+158+162 = 1883
In all dihybrid test crosses (a cross between a known heterozygote for two wild type traits and a homozygous recessive individual for both traits) the expected ratio of phenotypes if the genes are on separate chromosomes must be:
wild type, 25%; black-vestigial, 25% black-normal, 25%; gray-vestigial, 25%. These results do not fit the experimental data above (778+785+158+162).
In fact the black-normal (158) and gray-vestigial (162) offspring represent recombinant individuals.
Calculation of recombination frequency:
|
778 – wild type 785 – black-vestigial |
778/1883 = 41.3% 785/1883 = 41.7% |
83% are non-recombinant |
|
158 – black-normal 162 – gray-vestigial |
158/1883 = 8.4% 162/1883 = 8.6% |
17% are due to recombination |
The generally accepted method of symbolizing the genotypes for a dihybrid cross of linked genes is as follows (pg. 264 – Campbell):
- b+ = wild-type (gray body)
- b = black body
- vg+ = normal wing shape
- vg = vestigial wings
The testcross is symbolized as follows: 
If no cross over occurs then only two phenotypes should be seen. That is 50% of the offspring should be dominant for both traits —- and the other 50% should be homozygous recessive —- just as in the parents above.
Cross over in the heterozygous parent results in 50% recombinants: