(A)
P – 25
Q – 25
R – 25
S – 25

(B)
P – 25
Q – 50
R – 45
S – 38

(C)
P – 50
Q – 50
R – 90
S – 76

(D)
P – 25
Q – 50
R – 10
S – 24

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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.

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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)

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Test Cross

GgHh × gghh

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

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What We Are Asked

Percentage (%) of test cross progeny with genotype gghh

This happens only when the F₁ produces a gh gamete.

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Case-wise Explanation

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(P) Genes Unlinked

🔹 Independent assortment
🔹 Four types of gametes in equal proportion

✅ gghh progeny = 25%

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(Q) Completely Linked (No Crossing Over)

🔹 Only parental gametes formed

✅ gghh progeny = 50%

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(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%

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(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%

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Final Answer Summary

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Correct Option

✅ Option (B)

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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

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Exam Tip

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