Fruit shape in shephered purse (Capsella bursa) is of two types-triangular and top-shaped. Triangular fruit shape (T) is dominant over top-shape (t). Following table summarises the results of several crosses.
`{:("Cross","Result"),("Strain 1" xx "tt","All triangular"),("Strain 2" xx "tt","1 triangular : 1 top-shaped"),("strain 3"xx "tt","All top-shaped"),("Strain 4"xx "Tt","3 triangular : 1 top-shaped"):}`
Which pair of strains possess the genotyoe Tt ?

Options

A. Strains 2 and 3
B. Strain 2 and 4
C. Strains 1 and 3
D. Strains 1 and 4

Answer

To determine which pair of strains possess the genotype Tt (triangular fruit shape), we need to analyze the results of the crosses provided in the table.

### Step-by-Step Solution:

1. **Understand the Dominance**:
- Triangular fruit shape (T) is dominant over top-shaped fruit (t).
- Therefore, Tt (heterozygous) and TT (homozygous dominant) will both exhibit the triangular shape, while tt (homozygous recessive) will exhibit the top-shaped fruit.

2. **Analyze the Cross Results**:
- **Strain 1 (xx tt)**: All offspring are triangular. This indicates that the parent must be TT (homozygous dominant) since all offspring are triangular.
- **Strain 2 (xx tt)**: The ratio is 1 triangular : 1 top-shaped. This indicates that one parent must be Tt (heterozygous) and the other parent is tt (homozygous recessive).
- **Strain 3 (xx tt)**: All offspring are top-shaped. This indicates that the parent must be tt (homozygous recessive) since all offspring are top-shaped.
- **Strain 4 (xx Tt)**: The ratio is 3 triangular : 1 top-shaped. This indicates that one parent is Tt (heterozygous) and the other parent is Tt (heterozygous).

3. **Identify the Genotype Tt**:
- From the analysis, Strain 2 and Strain 4 both have Tt as one of their genotypes:
- Strain 2 has Tt (from the cross with tt).
- Strain 4 has Tt (since it is a cross between Tt and Tt).

4. **Conclusion**:
- Therefore, the pairs of strains that possess the genotype Tt are **Strain 2 and Strain 4**.