Question

In the sweet-pea plants, the gene A/a controls flower colour. The dominant allele gives purple flower and the recessive allele red flower. A second gene, B/b controls the shape of the pollen grains. The dominant allele gives elongated grains and the recessive allele spherical grains. A plant with the genotype AaBb was test crossed by interbreeding it with a plant with red flowers and spherical pollen grains.

Copy and complete the table to show the expected ratio of phenotypes of the offspring of this cross. The gametes from one parent are already in the table.

Accepted Answer

A test cross is the crossing of the individuals of unknown genotype with homozygous recessive genotype for the same gene. The genotype of an individual with the dominant phenotype for a particular gene can find out with a test cross. In the given example, the sweet-pea plant produces two types of flowers, purple and red that is controlled by the gene A/a. The dominant allele (A) gives purple flowers and the recessive allele (a) red flowers. A second gene, B/b controls the shape of the pollen grains. The dominant allele (B) gives elongated grains and the recessive allele (b) spherical grains. Here, a plant with the genotype AaBb was test crossed by interbreeding it with a plant with red flowers and spherical pollen grains (aabb). The parents produce the following types of gametes:

Here, the tested parent is heterozygous. Hence, the phenotypic ratio of the test cross is 1 : 1 : 1 : 1.

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