Short direct answer: The botanist can determine if the green-pod plant is homozygous or heterozygous by performing a test cross with a plant that has yellow pods (the recessive phenotype). The pattern of offspring phenotypes will reveal the genotype. Method and reasoning
- Define the trait: pod color is governed by a single gene with green being dominant (G) and yellow recessive (g). The green-pod parent is either GG (homozygous dominant) or Gg (heterozygous); the yellow-pod plant has genotype gg.
- Cross with a recessive phenotype partner (gg):
- If the green-pod plant is GG, all offspring will be green-pod (genotypes GG or Gg, but all phenotypes green) because GG × gg yields all Gg offspring.
- If the green-pod plant is Gg, half of the offspring will be green-pod (Gg) and half yellow-pod (gg) in a 1:1 ratio, because Gg × gg yields 50% Gg and 50% gg.
- Observations:
- All green-pod offspring → the green-pod parent is homozygous GG.
- Approximately 1:1 ratio of green-pod to yellow-pod offspring → the green-pod parent is heterozygous Gg.
Practical notes
- Ensure a sufficiently large progeny sample to clearly observe the ratio; small samples can mislead due to sampling error.
- Self-pollination or crossing with a green-pod plant could sometimes be inconclusive, especially if the green-pod plant is GG or Gg with limited offspring, so the test cross with a recessive partner is the most definitive.
- This approach is a classic test-cross guided by Mendel’s law of segregation and is widely used for determining genotype in simple dominance systems.
