Question

Identify a, b, c and d in the table given below:

| TR| Tr| tR| tr
---|---|---|---|---
TR| Tall Red| a| Tall Red| Tall Red
Tr| Tall Red| b| Tall Red| Tall White
tR| c| Tall Red| d| Dwarf Red
tr| Tall Red| Tall White| Dwarf Red| Dwarf White

A. a-Tall Red, b-Tall Red, c-Dwarf Red, d-Dwarf White
B. a-Tall Red, b-Tall White, c-Tall Red, d-Dwarf Red
C. a-Dwarf Red, b-Dwarf White, c-Tall Red, d-Tall White
D. None of the above

Answer 1

This is the table showing the F2 generation of the cross between a pure tall red plant and pure white dwarf plant. Here, Tall (T) is dominant over dwarf (t) and red (R) is dominant over white (r). Here in the F2 generation, tall red, tall white, dwarf red, and dwarf white will be in the ratio of 9:3:3:1.

Complete answer:
- The above chart is the example of a pea plant showing dihybrid and monohybrid cross.
- In monohybrid, one trait is taken and the cross is performed.
- In dihybrid, two traits are taken together. A cross between a pure tall red plant and the pure dwarf white plant is performed.
- In the F1 generation, all the plants have similar genotypes which are heterozygous red and heterozygous tall (TtRr).
- Then these plants are allowed to intercross between themselves to produce the F2 generation. In the F2 generation, the phenotypic ratio is 9:3:3:1, as the F2 generation is in the same ratio.
- So, tall red plants will have 4 genotypes which are TTRR, TTRr, TtRR, ttRr. Tall white will have 2 genotypes which are TTrr, Ttrr. Dwarf red will have 2 genotypes which are ttRR, ttRr. Dwarf white plants will be have1 genotype which is ttrr.

Hence, the correct answer is option (B), where a - Tall red (TTRr), b - Tall white (TTrr), c - Tall red (tTRr) and d - Dwarf red (ttrr).

Note: In the pea plant tall and dwarf are the two alleles representing height, in which tall (T) is dominant over dwarf (t). Similarly, red and white are also two alleles representing the colour of a flower, in which red flower colour (R) is dominant over white flower colour (r).