Question

Explain Mendel’s dihybrid ratio with the help of any one cross.

Answer 1

• In the dihybrid cross, two pairs of contrasting characteristics are under consideration.
• Mendel performed more experiments on hybridization in which he considered more than one pair of contrasting characteristics. He brought about a cross between a pea plant producing rounded and yellow coloured seeds and a pea plant with wrinkled and green coloured seeds. In this cross, two pairs of contrasting characteristics were considered colour of seeds and shape of seeds. Hence, it is called a dihybrid cross.
• Parental generation (P₁): Mendel selected the pea plants producing rounded yellow seeds and wrinkled green seeds as parent plants, as shown in the chart. During gamete formation in P1 generation, the pair of gametes separate independently i.e. in RRYY plants, only RY type gametes are formed and not RR and YY. Similarly, in rryy plants, only ry gametes are formed. Thus we can say that each pair of genes is represented in the gamete by only one gene from that pair.
• Based on the conclusions from the monohybrid cross, Mendel expected that in the F₂ generation of dihybrid cross, plants would produce rounded-yellow seeds. He was proved right. Though the genotype of these plants was RrYy, their phenotype was like the parents producing rounded-yellow seeds, because yellow colour is dominant over green and round shape is dominant over wrinkled. Due to the combination of two different characteristics in the F₁ generation of the dihybrid cross, these plants are called dihybrid plants.
• Plants of the F₁ generation of dihybrid cross produce four types of gametes RY, Ry, rY, ry. Of these gametes, RY and ry are similar to those of the P₁ generation. The 16 different possible combinations through the union of 4 types of male gametes and 4 types of female gametes.

Mendel’s experiment of dihybrid cross
Parental Generation P1
Phenotype	Rounded-yellow seeds	Wrinkled-green seeds
Genotype	RRYY	rryy
Gametes	RY	ry
First Filial Generation F1	RrYy
	(Rounded-yellow seeds)
Phenotype P2	Selfing in F1
Phenotype	Rounded-yellow seeds	Rounded-yellow seeds
Genotype	RrYy	RrYy
Gametes	RY, Ry, rY, ry	RY, Ry, rY, ry
Second Filial Generation F2
Male gamete/Female gamete	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	RrYy	rrYy	rryy

• The proportions obtained in the F₂ progeny generation formed from the dihybrid experiment are as follows:
• Phenotypic ratio = 9 : 3 : 3 : 1
  1. Round-Yellow: 9
  2. Wrinkled-Yellow: 3
  3. Round-Green: 3
  4. Wrinkled-Green: 1
• Genotypic ratio: = 1 : 1 : 2 : 2 : 4 : 2 : 2 : 1 : 1
  RRYY = 1, RRyy = 1, RRYy = 2, RrYY = 2, RrYy = 4, Rryy = 2, rrYy = 2, rrYY = 1, rryy = 1

Answer 2

• In the dihybrid cross, two pairs of contrasting characteristics are under consideration.
• Mendel performed more experiments on hybridization in which he considered more than one pair of contrasting characteristics. He brought about a cross between a pea plant producing rounded and yellow coloured seeds and a pea plant with wrinkled and green coloured seeds. In this cross, two pairs of contrasting characteristics were considered colour of seeds and shape of seeds. Hence, it is called a dihybrid cross.
• Parental generation (P₁): Mendel selected the pea plants producing rounded yellow seeds and wrinkled green seeds as parent plants, as shown in the chart. During gamete formation in P1 generation, the pair of gametes separate independently i.e. in RRYY plants, only RY type gametes are formed and not RR and YY. Similarly, in rryy plants, only ry gametes are formed. Thus we can say that each pair of genes is represented in the gamete by only one gene from that pair.
• Based on the conclusions from the monohybrid cross, Mendel expected that in the F₂ generation of dihybrid cross, plants would produce rounded-yellow seeds. He was proved right. Though the genotype of these plants was RrYy, their phenotype was like the parents producing rounded-yellow seeds, because yellow colour is dominant over green and round shape is dominant over wrinkled. Due to the combination of two different characteristics in the F₁ generation of the dihybrid cross, these plants are called dihybrid plants.
• Plants of the F₁ generation of dihybrid cross produce four types of gametes RY, Ry, rY, ry. Of these gametes, RY and ry are similar to those of the P₁ generation. The 16 different possible combinations through the union of 4 types of male gametes and 4 types of female gametes.

Mendel’s experiment of dihybrid cross
Parental Generation P1
Phenotype	Rounded-yellow seeds	Wrinkled-green seeds
Genotype	RRYY	rryy
Gametes	RY	ry
First Filial Generation F1	RrYy
	(Rounded-yellow seeds)
Phenotype P2	Selfing in F1
Phenotype	Rounded-yellow seeds	Rounded-yellow seeds
Genotype	RrYy	RrYy
Gametes	RY, Ry, rY, ry	RY, Ry, rY, ry
Second Filial Generation F2
Male gamete/Female gamete	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	RrYy	rrYy	rryy

• The proportions obtained in the F₂ progeny generation formed from the dihybrid experiment are as follows:
• Phenotypic ratio = 9 : 3 : 3 : 1
  1. Round-Yellow: 9
  2. Wrinkled-Yellow: 3
  3. Round-Green: 3
  4. Wrinkled-Green: 1
• Genotypic ratio: = 1 : 1 : 2 : 2 : 4 : 2 : 2 : 1 : 1
  RRYY = 1, RRyy = 1, RRYy = 2, RrYY = 2, RrYy = 4, Rryy = 2, rrYy = 2, rrYY = 1, rryy = 1