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Principles of Inheritance and Variation - Pleiotropy

Grade 12CBSEBiology

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

🔑Concepts

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Pleiotropy is a genetic phenomenon where a single gene influences multiple, seemingly unrelated phenotypic traits.

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The underlying mechanism of pleiotropy usually involves a gene product (enzyme or protein) that contributes to a metabolic pathway common to different physiological processes.

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A classic example is Phenylketonuria (PKU) in humans, caused by a mutation in the gene that codes for the enzyme phenylalanine hydroxylase. This single gene mutation results in mental retardation, reduction in hair, and skin pigmentation.

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In Garden Peas, starch synthesis is controlled by a single gene with two alleles (BB and bb). This gene exhibits pleiotropy by affecting both the size of starch grains and the shape of the seed.

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The relationship of dominance is not an autonomous feature of a gene; it depends on which phenotypic trait of the gene is being observed. For example, in BbBb pea seeds, 'roundness' is dominant, but 'starch grain size' shows incomplete dominance.

📐Formulae

1 Gene→PleiotropyPhenotype1+Phenotype2+Phenotypen\text{1 Gene} \xrightarrow{\text{Pleiotropy}} \text{Phenotype}_1 + \text{Phenotype}_2 + \text{Phenotype}_n

Genotype Bb (Pea)  ⟹  {Round shape (Dominance)Intermediate starch size (Incomplete Dominance)\text{Genotype } Bb \text{ (Pea)} \implies \begin{cases} \text{Round shape (Dominance)} \\ \text{Intermediate starch size (Incomplete Dominance)} \end{cases}

Phenotypic Ratio for Starch Grain Size (F2)=1:2:1\text{Phenotypic Ratio for Starch Grain Size (F}_2\text{)} = 1 : 2 : 1

💡Examples

Problem 1:

In Mendel's pea plants, starch grain size and seed shape are controlled by the same gene. If a cross is made between two heterozygous plants (Bb×BbBb \times Bb), what will be the phenotypic ratio regarding starch grain size?

Solution:

The cross Bb×BbBb \times Bb results in the following genotypes:

  • 11 BBBB (Large starch grains)
  • 22 BbBb (Intermediate starch grains)
  • 11 bbbb (Small starch grains)

The ratio is 1:2:11:2:1.

Explanation:

This demonstrates pleiotropy because the BB gene affects two traits. For seed shape, BbBb is round (showing dominance). However, for starch grain size, BbBb is intermediate, meaning the gene shows incomplete dominance for that specific phenotype.

Problem 2:

How does the disease Phenylketonuria (PKU) illustrate the principle of pleiotropy?

Solution:

Mutation in a single gene →\rightarrow Deficiency of phenylalanine hydroxylase →\rightarrow Accumulation of phenylalanine →\rightarrow Multiple effects: Mental retardation, Skin depigmentation, Hair loss.

Explanation:

Because one single metabolic defect (lack of one enzyme) leads to multiple distinct clinical symptoms across different organ systems, it is a primary example of pleiotropy.