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) What are 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. How do you do this? Why is this 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.

c) Genotype of plant is TTAA if all progeny from test cross is tall with axial flowers. Genotype of the plant is TtAa if the progeny express both dominant and recessive traits.

This prediction is based on the test cross and can be done with help of punnet square without performing a cross.

d) Test cross determines the genotype of the dominant organisms based on appearance of only dominant or both dominant and recessive traits in the progeny.

Explanation:

a) Since the phenotype of the mysterious plant is dominant, it could be homozygous or heterozygous dominant. So, the possible genotype for the plant: TTAA and TtAa

b) Test cross between possible genotypes and double homozygous recessive can be done to test its genotype.

Test cross 1 = TTAA x ttaa

Test cross 2 = TtAa x ttaa

c) Genotype of plant is homozygous dominant (TTAA) if all progeny from test cross is tall with axial flowers (TTAA x ttaa= TtAa).

Genotype of the plant is TtAa if the progeny is obtained exhibits following phenotypic ratio=

1 tall plant with axial flower :1 tall plant with terminal flower :1 dwarf plant with axial flower:1 dwarf plant with terminal flower.

This prediction is based on the test cross and can be done with help of punnet square without performing a cross.

d) The homozygous dominant plant does not have recessive alleles and therefore, all the progeny of the test cross express only dominant traits.

On the other hand, if the genotype of the mystery plant was heterozygous dominant, it would form four types of gametes in 1:1:1:1 ratio which in turn would give four types of phenotype in test cross progeny.