A plant of genotype C/C ; d/d is crossed to c/c ; D/D and an F1 testcrossed to c/c ; d/d. If the genes are linked, and 20 map units apart, what will be the percentage of c/c ; d/d recombinants

10%

Explanation:

The parental crossings = Cd/Cd   x  cD /cD

The distance the genes C and D = 20 map units

Determine the percentage of  cd / cd recombinants that will be formed

the distance shows that there is only 20% recombinant progeny

The recombinants are : CD/CD and cd/cd = 20%

i.e. each recombinant progeny = 20% / 2 = 10%

hence % of cd/cd recombinant = 10%

it takes hundreds of years to become stable.

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