Q.43

In a plant species, the genotype DDEE is crossed with the genotype ddee; and the F1 is

test crossed. Considering that the two genes are linked and 20 map unit (cM) apart in the

chromosome, the percentage (%) of the test cross progeny with the genotype ddee is

_______________. (answer in integer)

DDEE crossed with ddee produces F1 heterozygote DdEe, which is test-crossed to ddee. With genes linked at 20 cM apart, the ddee progeny in the test cross arise from recombinant gametes from the F1 parent. The percentage is 10%.

Problem Breakdown

The cross DDEE × ddee yields F1 genotype DdEe, where D and E are dominant alleles on one chromosome (cis or trans configuration, but typically cis for pure parents). The F1 is test-crossed to ddee, producing four progeny genotypes: DE/de (parental), de/de (parental), De/de (recombinant), and dE/de (recombinant). Since genes are linked 20 cM apart, recombination frequency equals map distance, so 20% recombinant gametes from F1 and 80% parental gametes.

Correct Answer Calculation

Recombination frequency (RF) = 20% = 0.20, so each recombinant class (De and dE gametes) is 10% of total progeny. The ddee genotype (de/de) matches one specific recombinant class from the dE gamete? No—wait, precise mapping: Assuming coupling phase (D E / d e in F1), parental gametes are DE (50%) and de (50%), recombinants De (10%) and dE (10%). Test cross progeny: DE/de, de/de (parentals, 40% each), De/de and dE/de (10% each). Thus, de/de progeny = 40%? Error in initial—standard for double recessive in test cross of dihybrid linked is parental if trans, but:

Correct: For DDEE × ddee, F1 is D E / d e (coupling). Parental progeny: D E / d e (D E d e phenotype) 40%, d e / d e (ddee) 40%. Recombinant: D e / d e 10%, d E / d e 10%. Hence, ddee progeny = 40% [ variant, adjusted for ccdd=cd/cd example where recessive recombinant=10%]. Wait, query specifies “percentage of test cross progeny with genotype ddee”.

Key Insight on Recombination

In coupling linkage (D E together), the de gamete from F1 is parental (no crossover needed), comprising (1 – RF)/2 = (80%)/2 = 40% of progeny. Crossover produces De and dE equally at 10% each. If repulsion (rare for pure cross), it flips, but standard assumption is coupling, making ddee parental.

Common Options Explained

• 40% (Correct): Parental class for coupling linkage; no recombination required for de gamete.

• 10%: Single recombinant class percentage; incorrect for ddee as it’s parental, not recombinant.

• 20%: Total recombinants; applies to sum of De/de + dE/de, not single ddee.

• 50%: Expected if unlinked (independent assortment); disproven by linkage.

• 0%: Complete linkage (no crossing over); not the case at 20 cM.

Exam Tip

Map units directly give RF%; progeny % for specific class = (1-RF)/2 for parentals, RF/2 for each recombinant. Answer: 40.