Question

A homozygous plant having round (R) and yellow (Y) seed is crossed with a homozygous plant having wrinkled (r) and green (y) seeds.

1. Draw a Punnett square to show F₁ generation. 
2. Give the genotype of F₁ generation 
3. Give the dihybrid phenotypic ratio and the phenotype of the offspring of the F₂ generation when two plants of the F₁ generation are crossed. 
4. Name and state the law which explains the dihybrid ratio. 
5. Mention the phenotype and genotype ratios of the offsprings in F₂ generation.

Answer 1

Let R = round, r = wrinkled, Y = yellow, and y = green.

P generation:
RRYY × rryy

Gametes:
RY × ry

(i) Punnett square for F₁ generation

	ry
RY	RrYy

Thus, all F₁ plants are RrYy and have round yellow seeds.

(ii) Genotype of F₁ generation

RrYy

(iii) F₂ generation

F₁ cross:

RrYy × RrYy

Gametes produced by each parent: RY, Ry, rY, ry

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

F₂ phenotypic ratio:
9 Round yellow : 3 Round green : 3 Wrinkled yellow : 1 Wrinkled green

(iv) Law explaining the dihybrid ratio

Law of Independent Assortment:
During gamete formation, the alleles of one pair of characters assort independently of the alleles of another pair of characters.

(v) Phenotypic and genotypic ratios of F₂

Phenotypic ratio:
9 : 3 : 3 : 1

Genotypic ratio:
1 RRYY : 2 RRYy : 1 RRyy : 2 RrYY : 4 RrYy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy