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Principles of Inheritance and Variation - Inheritance of Two Genes (Dihybrid Cross)

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A Dihybrid Cross is a cross between two individuals that differ in two pairs of contrasting characters, such as seed shape (Round/Wrinkled) and seed color (Yellow/Green).

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The Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters during gamete formation.

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In the PP generation, a cross between homozygous dominant (RRYYRRYY) and homozygous recessive (rryyrryy) individuals results in an F1F_1 generation with the genotype RrYyRrYy and phenotype 'Round Yellow'.

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The F1F_1 hybrid (RrYyRrYy) produces four types of gametes: RYRY, RyRy, rYrY, and ryry, each with a frequency of 25%25\%.

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The F2F_2 generation is produced by selfing F1F_1 plants (RrYy×RrYyRrYy \times RrYy), resulting in 1616 possible combinations in the Punnett square.

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The Phenotypic Ratio of the F2F_2 generation is 9:3:3:19:3:3:1 (9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green).

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The Genotypic Ratio of the F2F_2 generation is 1:2:1:2:4:2:1:2:11:2:1:2:4:2:1:2:1.

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Recombinant phenotypes (Round-Green and Wrinkled-Yellow) appear due to the independent assortment of alleles, provided the genes are not linked.

📐Formulae

Number of types of gametes=2n\text{Number of types of gametes} = 2^n

Number of F2 phenotypes=2n\text{Number of } F_2 \text{ phenotypes} = 2^n

Number of F2 genotypes=3n\text{Number of } F_2 \text{ genotypes} = 3^n

Dihybrid Phenotypic Ratio=(3:1)2=9:3:3:1\text{Dihybrid Phenotypic Ratio} = (3:1)^2 = 9:3:3:1

Dihybrid Genotypic Ratio=(1:2:1)2=1:2:1:2:4:2:1:2:1\text{Dihybrid Genotypic Ratio} = (1:2:1)^2 = 1:2:1:2:4:2:1:2:1

Where n=number of heterozygous gene pairs\text{Where } n = \text{number of heterozygous gene pairs}

💡Examples

Problem 1:

A dihybrid plant with genotype RrYyRrYy is crossed with a double recessive plant rryyrryy (Test Cross). What will be the ratio of the resulting phenotypes?

Solution:

1:1:1:11:1:1:1

Explanation:

This is a dihybrid test cross. The parent RrYyRrYy produces four types of gametes: RYRY, RyRy, rYrY, and ryry. The parent rryyrryy produces only one type of gamete: ryry. The fusion results in four genotypes: RrYyRrYy, RryyRryy, rrYyrrYy, and rryyrryy in equal proportions (25%25\% each), leading to a phenotypic ratio of 1:1:1:11:1:1:1.

Problem 2:

In an F2F_2 population of a dihybrid cross (RrYy×RrYyRrYy \times RrYy), how many plants out of 16001600 are expected to be wrinkled and green?

Solution:

100100 plants

Explanation:

The phenotypic ratio for a dihybrid cross is 9:3:3:19:3:3:1. The proportion of wrinkled green plants (rryyrryy) is 116\frac{1}{16} of the total population. Therefore, 116×1600=100\frac{1}{16} \times 1600 = 100.

Inheritance of Two Genes (Dihybrid Cross) Revision - Class 12 Biology CBSE