An individual with the double heterozygous genotype (AaBb) is used in a test cross. Assuming genes A and B assort independently, what is the expected phenotypic ratio in the offspring?

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BlackTom AI · Accepted Answer

When analyzing a dihybrid cross with independent assortment, it helps to break down the parental genotype and the resulting gametes.
The individual is AaBb, and in a test cross the partner is aabb (assuming independent assortment for A and B).
From AaBb, the possible gametes are AB, Ab, aB, and ab, each with probability 1/4.
Crossing those with the aabb partner yields:
- AB × ab → AaBb (dominant A and dominant B phenotypes)
- Ab × ab → Aabb (dominant A, recessive b)
- aB × ab → aaBb (recessive a, dominant B)
- ab × ab → aabb (recessive for both A and B)
Each of these four outcomes occurs with equal probability, giving a phenotypic set of four equally likely classes.
Now evaluating the options:
- 3:1: This ratio corresponds to a single-gene monohybrid cross with complete dominance, not a dihybrid test cross. Here we are tracking two genes, so a 3:1 split is inappropriate.
- 1:1:1:1: This matches the four equally likely phenotypes (A_B_, A_bb, aaB_, aabb) arising from the AaBb × aabb cross under independent assortment. This option aligns with the expected outcome.
- 9:3:3:1: This is the classic phenotypic ratio for a dihybrid cross AaBb × AaBb with independent assortment, not for a test cross against aabb. The presence of the parental AaBb × AaBb cross changes the proportions, so this is not the correct scenario here.
- 9:3:4: This ratio describes epistasis situations where one gene masks another, not the straightforward independent two-gene dihybrid test cross. It does not fit the AaBb × aabb test-cross outcome.

Putting it together, the cross produces four equal phenotypic classes, corresponding to the 1:1:1:1 pattern. Therefore, the statement 1:1:1:1 accurately reflects the expected offspring phenotypes for this test cross scenario.

AGRI30048_2025_SM2 AGRI30048 Mid Semester Test

An individual with the double heterozygous genotype (AaBb) is used in a test cross. Assuming genes A and B assort independently, what is the expected phenotypic ratio in the offspring?

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