A tall pea plant with axial flowers was crossed to a dwarf plant with terminal flowers. Tall plants and axial flowers are dominant traits. The following offspring were obtained: 27 tall, axial flowers; 23 tall, terminal flowers; 28 dwarf, axial flowers; and 25 dwarf, terminal flowers. What are the genotypes of the parents?
The Correct Answer and Explanation is:
Correct Answer:
The genotypes of the parents are:
TtAa × ttaa
Explanation:
This genetics problem involves dihybrid inheritance, where we are tracking two traits:
- Plant height:
- Tall (T) is dominant
- Dwarf (t) is recessive
- Flower position:
- Axial (A) is dominant
- Terminal (a) is recessive
The observed offspring are:
- 27 tall, axial
- 23 tall, terminal
- 28 dwarf, axial
- 25 dwarf, terminal
These numbers are roughly equal, suggesting a 1:1:1:1 phenotypic ratio. This pattern is a classic outcome of a dihybrid test cross, which is used to determine the genotype of an individual with dominant traits.
Let’s break it down:
Step 1: Determine phenotype and possible genotype of parents
- One parent is tall with axial flowers — since these are dominant traits, the genotype could be either homozygous (TTAA) or heterozygous (TtAa).
- The other parent is dwarf with terminal flowers, which are recessive traits, so the genotype must be ttaa.
We test which genotype of the dominant parent (tall with axial flowers) could produce the 1:1:1:1 offspring ratio when crossed with ttaa.
Step 2: Try TtAa × ttaa
Gametes from TtAa = TA, Ta, tA, ta
Gametes from ttaa = ta (only one type)
Crossing them yields:
- TA × ta → TtAa (Tall, Axial)
- Ta × ta → Ttaa (Tall, Terminal)
- tA × ta → ttAa (Dwarf, Axial)
- ta × ta → ttaa (Dwarf, Terminal)
Each genotype occurs in equal proportion (1:1:1:1), and that exactly matches the observed data:
- 27 tall, axial
- 23 tall, terminal
- 28 dwarf, axial
- 25 dwarf, terminal
Conclusion:
This confirms that the dominant parent is heterozygous for both traits (TtAa), and the recessive parent is ttaa.
So, the genotypes of the parents are:
👉 TtAa × ttaa.