Principles of Inheritance and Variation

Notes

Principles of Inheritance and Variation

Introduction

• The study of heredity, or how the characteristics of living things are transmitted from one generation to the next, is called genetics.
• Heredity is the process of transmission of traits from parents to their offspring, either through sexual reproduction or asexual reproduction.
• Individuals of the same species have some differences. These differences are called variations.
• An autosome is a chromosome that is not a sex chromosome.
• Genes are found on structures called chromosomes. Long pieces of DNA are wound around proteins.
• An allele is one of the possible forms of a gene. Most genes have two alleles: a dominant allele and a recessive allele.
• Homologous chromosomes are pairs of chromosomes containing corresponding maternal and paternal chromosomes.
• Heterologous chromosomes differ in shape, size or function and do not belong to the same pair.

Gregor Mendel

• Gregor Mendel is known as the Father of Genetics.
• He studied inheritance using pea plants.
• He performed experiments with pea plants for about seven years, from 1856 to 1863.
• He proposed the laws of inheritance.
• He used statistical analysis and mathematical logic.
• He selected the pea plant Pisum sativum for his experiments.

Why Did Mendel Select Pea Plants for His Experiments?

• Pea plants were easy to cultivate and grew well in his garden.
• The flowers were hermaphrodite.
• Pea plants are naturally self-pollinated.
• Cross-pollination can be performed artificially with ease.
• The plant completes its life cycle in one season.
• Pea plants have many sharply distinct traits, with each trait having two clear-cut alternative forms.

Seven Different Traits Studied by Mendel

1. Form of ripe seed (R): Smooth or wrinkled
2. Colour of seed albumen (Y): Yellow or green
3. Colour of flower (P): Purple or white
4. Form of ripe pods (I): Inflated or constricted
5. Colour of unripe pods (G): Green or yellow
6. Position of flower (A): Axial or terminal
7. Length of stem (T): Tall or dwarf

Mendel's Observation

• Cross-pollination was carried out between tall and dwarf pea plants.
• In the F₁ generation, all the offspring were tall.
• Phenotypically, the pea plants were tall.
• The genotypes of the offspring were different.