14. A plant with genotype r⁺h⁺/ r^– h^– was test crossed. Out of total 280 progeny 260 are r⁺h⁺/ r^–h^– and r^–h^–/r^–h^–. The recombination frequency will be
(1) 92.8       (2) 46.4
(3) 7.2       (4) 3.6

The recombination frequency in the given genetics problem can be calculated using the formula:

Recombination frequency (%) = (Number of recombinant progeny / Total number of progeny) × 100

Here, the plant genotype is r+h+/ r- h- and was test crossed. Out of 280 total progeny, 260 are parental types (r+h+/ r- h- and r- h-/ r- h-), implying the remaining are recombinants. To find recombinants:

Recombinants = Total progeny – Parental progeny
Recombinants = 280 – 260 = 20

Now calculate recombination frequency:

Recombination frequency = (20 / 280) × 100 = 7.14%

Among the options given:
(1) 92.8 – Too high, incorrect for observed data
(2) 46.4 – Far too high, incorrect
(3) 7.2 – Closest to calculated value 7.14%, likely correct
(4) 3.6 – Less than half the correct value, incorrect

Hence, the correct answer is option (3) 7.2. This shows a low recombination frequency, indicating the genes are closely linked on the chromosome.

Introduction

Understanding recombination frequency helps geneticists determine the distance between genes on a chromosome. This article explains step-by-step how to calculate recombination frequency from a test cross using a practical example of plant genotypes.

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Explanation of Options

• Option 1 (92.8): This would mean almost all offspring show recombination, which contradicts the data showing most progeny are parental types.

• Option 2 (46.4): A recombination frequency near 50% suggests independent assortment, but here, recombinant individuals are far fewer.

• Option 3 (7.2): Matches the calculation of 7.14%, indicating closer linkage and fewer recombinants, making it the correct choice.

• Option 4 (3.6): Half of the correct recombination frequency, not matching observed recombinant numbers.

Using the recombination frequency formula ensures clear understanding of gene linkage and crossing over effects observed in progeny from test crosses.