A plant of genotype Cd/cD is crossed to cD/cD and the resulting F1 (Cd/cD) is testcrossed to cd/cd. If the distance between these two genes is 20 mapping units, the percentage of cd/cd in the progeny of the testcross will be

The percentage of cd/cd in the progeny will be 10%    

Explanation:

Available Data:

1st Cross:

Parental) Cd/cD    x    cD/cD

f1) Cd/cD

2nd Cross:

Parental)         Cd/cD              x              cd/cd

Gametes) Cd (Parental)                       cd

                cD (Parental)                      cd

                 CD (Recombinant)              cd

                 cd (Recombinant)               cd

Punnet square)         Cd          cD          CD          cd

                                 (P)           (P)          (R)        (R)

                    cd       Cd/cd     cD/cd      CD/cd    cd/cd

                    cd       Cd/cd     cD/cd      CD/cd    cd/cd

                    cd       Cd/cd     cD/cd      CD/cd    cd/cd

                    cd       Cd/cd     cD/cd      CD/cd    cd/cd

Now, to calculate the recombination frequency we make use of the next formula: P = Recombinant number / Total of individuals. The genetic distance will result from multiplying that frequency by 100 and expressing it in map units (MU).

The map unit is the distance between the pair of genes for which one of every 100 meiotic products results in a recombinant product.

Considering this, we can calculate the recombination frequency by clearing the next equation: genetic distance = recombination frequency x 100. So, if we have the genetic distance, we can calculate the recombination frequency like this:

genetic distance = recombination frequency x 100

20% = recombination frequency x 100

20/100 = recombination frequency

0.2 = recombination frequency.

From the total of the recombinants calculated, half correspond to each of the two types of possible recombinants. So, 0.1 correspond to CD/cd  genotype and 0.1 correspond to cd/cd.

The percentage of cd/cd in the progeny will be 10%

"do a testcross with a homozygous recessive plant."