Question
A botanist has acquired a group of pea plants. All of the plants have yellow pea pods (the recessive trait), except for one plant with green pea pods (the dominant trait). Pea pod color is caused by a single gene.How could the botanist best determine whether the genotype of the green-pod plant is homozygous or heterozygous?- Cross the green-pod plant with a yellow-pod plant.- Cross the green-pod plant with another green-pod plant.- Self-pollinate the green-pod plant.
Answer
The best choice is:
Cross the green-pod plant with a yellow-pod plant.
This is a test cross. Let represent the dominant green-pod allele and the recessive yellow-pod allele. A yellow-pod plant must be , while the green-pod plant could be either or .
If the unknown plant is homozygous dominant
All offspring are expected to have green pods.
If the unknown plant is heterozygous
About half the offspring are expected to have green pods and half yellow pods. Any yellow-pod offspring must have received from the unknown green-pod parent, proving that parent was heterozygous.
With a small number of offspring, an all-green result is still possible by chance from a heterozygous parent. For independent offspring, that probability is , so more offspring provide stronger evidence for .
Evidence boundary
The predicted outcomes assume one gene, complete dominance, equal allele segregation, random fertilization, and no genotype-dependent survival. A yellow-pod offspring establishes heterozygosity under this model; a finite all-green sample does not prove homozygous dominance with certainty.
Sources
These references support the concepts and methods used in the explanation above.