15. Genetic mapping reveals that distance between two genes ‘A’ and ‘B’ is 10 cM. What is chance of getting Aabb progeny if AaBb is test crossed?
(1) 5 &           (2) 10 %
(3) 45 %       (4) 90 %

Introduction:
Genetic mapping helps us understand the distance between genes on a chromosome, measured in centimorgans (cM), which correlates with recombination frequency. This article explains how to calculate the chance of obtaining a specific genotype, Aabb, when an AaBb individual is test crossed, considering a genetic distance of 10 cM between genes A and B.

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Understanding Genetic Mapping and Distance

Genetic distance between two genes, measured in centimorgans (cM), indicates the likelihood of recombination occurring between those genes during meiosis. A 10 cM distance corresponds to a 10% recombination frequency, meaning there is a 10% chance that a crossover will separate the two genes. The remaining 90% reflects parental allele combinations without recombination.

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Genotypes and Gametes in AaBb Test Cross

• The heterozygous individual AaBb can produce four types of gametes if genes are unlinked: AB, Ab, aB, and ab.

• Since genes A and B are 10 cM apart (linked), the recombinant gametes Ab and aB appear at a frequency of 10%, split equally (5% each).

• The parental gametes AB and ab occur at a combined frequency of 90%, also split equally (45% each).

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Calculating the Chance of Aabb Progeny

When AaBb is test crossed with aabb (homozygous recessive for both genes), the possible progeny genotypes depend on gametes from AaBb and the only gamete from aabb (ab).

• To get Aabb progeny, the AaBb parent must contribute the A allele from gene A and the b allele from gene B.

• The A allele comes from parental gamete AB (45%) or recombinant gamete Ab (5%).

• For Aabb progeny, the gamete must be Ab (because b is recessive and must come from the ‘b’ allele).

• Therefore, the chance of producing Aabb progeny equals the frequency of the recombinant gamete Ab, which is 5%.

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Explanation of Options

(1) 5% – Correct answer: This matches the frequency of the Ab recombinant gamete producing Aabb progeny.

(2) 10% – Incorrect: This is the total recombination frequency combining Ab and aB; Aabb requires only the Ab gamete.

(3) 45% – Incorrect: This frequency corresponds to one of the parental gametes (AB) and produces a different progeny genotype (AaBb).

(4) 90% – Incorrect: This is the combined frequency of parental gametes (AB and ab), not related to the Aabb progeny frequency.

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This detailed breakdown shows that with genes 10 cM apart, the chance of obtaining Aabb progeny from an AaBb test cross is 5%, reflecting the recombination frequency divided equally between Ab and aB gametes.