Question

In peas, tallness is dominant over dwarfness, and the red colour of flowers is dominant over the white colour. When a tall plant bearing red flowers was pollinated with a dwarf plant bearing white flowers, the different phenotypic groups were obtained in the progeny in numbers mentioned against them:

Tall, Red = 138
Tall, White =132
Dwarf, Red = 136
Dwarf, White = 128

Mention the genotypes of the two parents and of the four offspring types.

Answer 1

Genotype of parents

In peas, tallness is dominant over dwarfness, and the red colour of flowers is dominant over the white colour.

• 'R' the red flower allele, is dominant over 'r', the white flower allele.
• 'T', the tall allele is dominant over 't', the dwarf allele.

In the question, tall plants bearing red flowers were pollinated with dwarf plants bearing white flowers (ttrr) and resulted in the production of parent varieties and recombinants. So, the possible genotypes of the dominant parent can be TTRR, TTRr, TtRR and TtRr.

Test cross

The genotype of the plant can be identified by a test cross with a recessive parent.


Case 1: If the dominant parent is TTRR

If all plants in the F${}_1$ generation are tall with red flowers, then the genotype is TTRR.

Case 2: If the dominant parent is TTRr

The phenotypic ratio observed in this cross is tall, red flowered : tall, white flowered = 1:1

Case 3: If the dominant parent is TtRR

The phenotypic ratio observed is tall, red flowered: dwarf, red flowered = 1:1

Case 4: If the dominant parent is TtRr

The phenotypic ratio observed is, tall, red flowered: tall, white flowered: dwarf, red flowered: dwarf, white flowered = 1:1:1:1. Overall, 50$\%$ of plants are tall and 50$\%$ of plants have red flowers. Hence the genotype is TtRr.


The possible gametes and genotypes

The possible gametes and genotypes:

• In the experiment mentioned in question, the ratio of tall red: tall white: dwarf red: dwarf white = 138 : 132 : 136 : 128. This is approximately equal to the ratio 1:1:1:1.
• This ratio is fit for the case 4, therefore, the genotype of the parent which is tall with red flowers is TtRr.
• Since the genotypes of the parents are TtRr and ttrr, the genotypes of four offspring types are TtRr, ttRr, Ttrr and ttrr.

	TR	Tr	tR	tr
tr	TtRr (Tall plant with red flower)	Ttrr (Tall plant with white flower)	ttRr (Dwarf plant with red flower)	ttrr (Dwarf plant with white flower)