Pure-breeding pea plants with the dominant long-stem trait were crossed with pure-breeding pea plants with short stems. The F1 generation was allowed to self-pollinate. If the long-stemmed offspring of the F2 generation are test crossed, what fraction of their offspring are expected to have short stems? Select one: a. 3/4 b. 1/2c. 1/3 d. 1/4

b

Explanation:

First, let us assume the long-stem trait to be represented by the allele A and its short stem counterpart by the allele a.

Pure-breeding long-stem = AA

Pure-breeding short-stem = aa

                AA    x    aa

F1:       Aa    Aa    Aa    Aa

The F1 generation was allowed to self-pollinate

                Aa    x    Aa

F2:       AA    Aa    Aa    aa

Test-crossing means crossing an organism of doubtful zygosity with a phenotypically recessive version of the same organism.

From the resulting F2, the long-stemmed offspring are of 2 genotypes - AA and Aa. If AA is test-crossed with a phenotypically recessive version:

            AA    x    aa

        Aa    Aa    Aa    Aa

all the offspring are expected to have long stems.

If the Aa offspring is test-crossed with aa:

             Aa    x    aa

         Aa    Aa    aa    aa

Half or 1/2 of the offspring are expected to have short stems.

Considering the second cross, the correct option is, therefore, b.

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