Introduction

Problems based on linkage, recombination frequency, and test crosses are high-yield topics in NEET and other competitive biology exams. This question tests your understanding of:

• Independent assortment

• Complete linkage

• Genetic map units (m.u.)

• Calculation of parental vs recombinant gametes

Let’s solve this step by step and evaluate all four options logically.

Given Cross

Parental Cross

GGHH × gghh

➡️ Produces F₁ genotype: GgHh

Since both dominant alleles come from one parent, the F₁ arrangement is:

GH / gh (coupling or cis configuration)

Test Cross

GgHh × gghh

👉 The phenotype and genotype of offspring depend only on the gametes produced by the F₁ parent.

What We Are Asked

Percentage (%) of test cross progeny with genotype gghh

This happens only when the F₁ produces a gh gamete.

Case-wise Explanation

(P) Genes Unlinked

🔹 Independent assortment
🔹 Four types of gametes in equal proportion

✅ gghh progeny = 25%

(Q) Completely Linked (No Crossing Over)

🔹 Only parental gametes formed

✅ gghh progeny = 50%

(R) 10 Map Units Apart

🔹 Recombination frequency = 10%
🔹 Parental gametes = 90%

Each parental type:

902=45%\frac{90}{2} = 45\%290​=45%

✅ gghh progeny = 45%

(S) 24 Map Units Apart

🔹 Recombination frequency = 24%
🔹 Parental gametes = 76%

Each parental type:

762=38%\frac{76}{2} = 38\%276​=38%

✅ gghh progeny = 38%

Final Answer Summary

Correct Option

✅ Option (B)

Why Other Options Are Incorrect

Option (A)

• Assumes no effect of linkage or distance

• Ignores recombination frequency

Option (C)

• Overestimates parental combinations

• Biologically impossible percentages

Option (D)

• Miscalculates recombination values

• 24 m.u. cannot give only 24% gghh

Exam Tip

% Parental gametes = 100 − recombination frequency
Each parental type = (100 − RF) / 2