Question

You are handed a mystery pea plant with tall stems and axial flowers and asked to determine its genotype as quickly as possible. You know that the allele for tall stems (T) is dominant to that for dwarf stems (t) and that the allele for axial flowers (A) is dominant to that for terminal flowers (a).

(a) Identify all the possible genotypes for your mystery plant.

(b) Describe the one cross you would do, out in your garden, to determine the exact genotype of your mystery plant.

(c) While waiting for the results of your cross, you predict the results for each possible genotype listed in part a. Explain how you do this and why this is not called “performing a cross.”

(d) Explain how the results of your cross and your predictions will help you learn the genotype of your mystery plant.

Short answer

(a) The possible genotypes are TTAA, TtAa, TTAa, and TtAA.

(b) A testcross is performed where the mystery plant is crossed with the homozygous recessive individual to determine the genotype of the mystery plant.

(c) The results of testcross for each possible genotype are:

TTAA X ttaa = TtAa

TtAa X ttaa = TtAa, Ttaa, ttAa, ttaa

TTAa X ttaa = TtAa, Ttaa

TtAA X ttaa = TtAa, ttAa

The results can be predicted by determining the gametes produced and working out the possible genotypes. It is not called ‘performing a cross’ because results are predicted from the genotype of gametes and not by performing a cross.

(d) By comparing the predictions and the phenotype of the offspring, one can determine the genotype of the mystery plant.

Step-by-step solution

Explanation for (a)

T is the dominant allele for tall stems, and t is the recessive allele for short stems. A is the dominant allele for axial flowers, and a is the recessive allele for terminal flowers. The phenotype of the mystery flowers is a tall stem with axial flowers.

Thus, the possible genotypes of the mystery plant can be TTAA, TtAa, TTAa, and TtAA because the phenotypes of these four genotypes are tall stems with axial flowers.

Explanation for (b)

A testcross is a dihybrid cross that is performed to determine the genotype of an organism. In the testcross, the organism with an unknown genotype is crossed with an organism homozygous recessive for both traits.

Thus, the mystery plant is crossed with a plant having a short stem and terminal flowers because the plant is homozygous recessive. The result of the cross helps to determine the genotype of the mystery plant.

Explanation for (c)

By determining the gametes produced by the mystery plant with each possible genotype and the homozygous recessive individual, the results of the testcross can be predicted.

The gamete produced by a homozygous recessive individual (ttaa) is ta.

• The gamete produced by the mystery plant with the possible genotype TTAA is TA. Thus, the genotype of the offspring from this cross would be TtAa.

• The gametes produced by the mystery plant with the possible genotype TtAa are TA, Ta, tA, and ta. Thus, the genotype of the offspring from the cross (TtAa × ttaa) would be TtAa, Ttaa, ttAa, and ttaa.

• The gametes produced by the mystery plant with the possible genotype TTAa are TA and ta. Thus, the genotype of the offspring from the cross (TTAa × ttaa) would be TtAa and Ttaa.

• The gametes produced by the mystery plant with the possible genotype TtAA are TA and tA. Thus, the genotype of the offspring from the cross (TtAA × ttaa) would be TtAa and ttAa.

Thus, without performing across, the results of the testcross can be predicted with the genotype of the gametes produced by the parent plant.

Explanation for (d)

The genotype of the mystery plant can be determined by comparing the predicted result of the testcross with the phenotype of the offspring produced by the dihybrid cross. The genotype can be determined as follows:

• The mystery plant is homozygous dominant for both T and A (TTAA) if they produce offspring with only tall stems and axial blooms.

• The mystery pea plant is homozygous dominant for T and heterozygous for A (TTAa) if half of their progeny are tall and have axial flowers and half are tall and have terminal flowers.

• The pea plant is heterozygous for T and homozygous dominant for A (TtAA) if half of their offspring are tall with axial flowers and the other half are short with axial flowers.

• The mystery pea plant is heterozygous for both T and A (TtAa) if one-fourth of their progeny are tall with axial, tall with terminal, short with axial, or short with a terminal.