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Principles of Inheritance and Variation - Mendel's Laws of Inheritance

Grade 12CBSEBiology

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

🔑Concepts

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Mendel's Laws are based on his experiments with the Garden Pea (Pisum sativumPisum\ sativum) where he studied 7 pairs of contrasting traits.

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Inheritance is governed by 'Factors' (now called Genes) which occur in pairs called Alleles. For example, TT and tt for height.

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Law of Dominance: In a monohybrid cross between two contrasting pure parents (TT×ttTT \times tt), only one form of the trait (Dominant, TT) appears in the F1F_1 generation (TtTt).

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Law of Segregation: During gamete formation, the alleles of a pair segregate from each other such that a gamete receives only one of the two factors (Purity of Gametes). This is a universal law.

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Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. This occurs during Metaphase I of meiosis.

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A Test Cross is used to determine the genotype of a dominant phenotype by crossing it with a homozygous recessive individual (Tt×ttTt \times tt or TT×ttTT \times tt).

📐Formulae

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

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

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

Total number of zygotes formed in F2=4n\text{Total number of zygotes formed in } F_2 = 4^n

Monohybrid Phenotypic Ratio=3:1\text{Monohybrid Phenotypic Ratio} = 3 : 1

Monohybrid Genotypic Ratio=1:2:1\text{Monohybrid Genotypic Ratio} = 1 : 2 : 1

Dihybrid Phenotypic Ratio=9:3:3:1\text{Dihybrid Phenotypic Ratio} = 9 : 3 : 3 : 1

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

💡Examples

Problem 1:

Determine the number of gametes produced by a plant with the genotype AaBbCcAaBbCc.

Solution:

Applying the formula 2n2^n, where nn is the number of heterozygous pairs. Here, n=3n = 3 (pairs AaAa, BbBb, and CcCc). 23=82^3 = 8.

Explanation:

The plant is trihybrid. According to the Law of Segregation and Independent Assortment, the alleles will combine in 2×2×2=82 \times 2 \times 2 = 8 possible ways.

Problem 2:

In a monohybrid cross between a tall (TTTT) and a dwarf (tttt) pea plant, what will be the phenotypic and genotypic ratio of the F2F_2 generation?

Solution:

F1F_1 is TtTt (Tall). F2F_2 (Selfing Tt×TtTt \times Tt): Phenotypic ratio = 3:13:1 (Tall:Dwarf); Genotypic ratio = 1:2:11:2:1 (TT:Tt:ttTT:Tt:tt).

Explanation:

The phenotypic ratio represents the physical appearance, where TTTT and TtTt are tall due to dominance. The genotypic ratio represents the actual genetic makeup.

Mendel's Laws of Inheritance - Revision Notes & Key Diagrams | CBSE Class 12 Biology