0Class 12 Biology – Principles of Inheritance and Variation
1. Introduction
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Inheritance: The process by which traits are passed from parents to offspring.
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Variation: The differences observed among individuals of a species.
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These principles were first studied systematically by Gregor Mendel, known as the Father of Genetics.
2. Mendel’s Experiments
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Mendel studied pea plants to understand how traits are inherited.
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He focused on one or two traits at a time and counted offspring to study patterns.
3. Principles of Inheritance
a) Principle of Dominance
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When two contrasting traits are present, one may mask the other.
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The visible trait is called dominant, and the masked trait is recessive.
b) Principle of Segregation
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During gamete formation, the two alleles of a gene separate, so each gamete carries only one allele.
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Explains why offspring may show traits not visible in parents.
c) Principle of Independent Assortment
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Genes for different traits are inherited independently of each other.
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Explains why offspring may show new combinations of traits.
4. Variation
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Variation is the differences among individuals of a species.
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Types of variation:
a) Continuous Variation
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Traits vary gradually, e.g., height, weight, skin color.
b) Discontinuous Variation
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Traits occur in distinct categories, e.g., pea plant flower color, blood groups.
c) Causes of Variation
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Genetic: Mutations, recombination during reproduction.
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Environmental: Climate, diet, lifestyle.
5. Importance of Inheritance and Variation
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Inheritance ensures traits are passed to offspring.
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Variation is the basis of evolution.
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Helps in selective breeding, crop improvement, and medicine.
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Explains genetic disorders and diversity among organisms.
6. Key Points to Remember
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Traits are inherited according to Mendel’s principles.
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Dominant traits mask recessive traits.
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Alleles segregate during gamete formation.
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Different traits assort independently, producing new combinations.
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Variation arises from genetic and environmental factors.
