Mendelian Principles of Heredity

Introduction

• The Mendelian Principles of Heredity are the fundamental laws that explain how hereditary characters are transmitted from one generation to the next. These principles were discovered by Gregor Johann Mendel, an Austrian monk and scientist, through a series of carefully designed experiments on the garden pea (Pisum sativum) between 1856 and 1863. His findings were published in 1866 in the paper “Experiments on Plant Hybridization”.
• Mendel demonstrated that hereditary characters are controlled by discrete units called factors, now known as genes, which are transmitted unchanged from parents to offspring. His experiments disproved the Blending Theory of Inheritance and established the foundation of classical genetics.
• Although many complex inheritance patterns have been discovered since Mendel’s time, his principles remain the basis of modern genetics and are widely applied in plant breeding, animal breeding, medicine, and biotechnology.

Why Did Mendel Choose Garden Pea (Pisum sativum)?

Mendel selected the garden pea because it possessed several characteristics that made it ideal for genetic experiments.

Advantages of Garden Pea

• Easy to cultivate and maintain.
• Short life cycle (about 3–4 months).
• Naturally self-pollinated, allowing the maintenance of pure lines.
• Artificial cross-pollination is simple and easy to control.
• Produces a large number of seeds.
• True-breeding varieties were readily available.
• Possesses several easily distinguishable contrasting traits.

Mendel’s Seven Contrasting Characters

Important Genetic Terms

Before understanding Mendel’s Principles of Heredity, it is essential to become familiar with some basic genetic terms. These terms form the foundation of classical genetics and help explain how hereditary traits are transmitted from one generation to the next.

1. Character

• A character is a heritable feature or attribute of an organism that can be transmitted from parents to offspring. A character may exist in different alternative forms called traits.
• Example: Plant height, Seed colour, Flower colour, Pod shape
• Plant height is a character because it is inherited and can occur in different forms such as tall or dwarf.

2. Trait

• A trait is an alternative expression or form of a character.
• For every character, there are usually two or more contrasting traits.

Examples

Thus, Tall and Dwarf are two contrasting traits of the character plant height.

3. Gene

• A gene is the basic physical and functional unit of heredity. It is a specific segment of DNA located on a chromosome that controls the expression of a particular character by directing the synthesis of proteins.
• Each gene occupies a definite position, known as a locus, on a chromosome.

Examples

• Gene controlling plant height
• Gene controlling seed colour
• Gene controlling flower colour

4. Alleles

• Alleles are alternative forms of the same gene that occupy the same position (locus) on homologous chromosomes and control the same character.
• One allele is inherited from the father and the other from the mother.

Example: For plant height:

• T = Tall allele
• t = Dwarf allele

Both T and t are alleles of the same gene controlling plant height.

5. Dominant Trait

A dominant trait is a trait that is expressed in both homozygous and heterozygous conditions. The dominant allele masks the expression of the recessive allele in a heterozygous individual.

Example: Tall (T) is dominant over Dwarf (t).

Genotypes expressing the dominant trait:

• TT → Tall
• Tt → Tall

6. Recessive Trait

• A recessive trait is expressed only when both alleles are recessive (homozygous condition).
• Its expression is masked by the dominant allele in the heterozygous condition.
• Example: Dwarf (t) is recessive.
• Genotype expressing the recessive trait: tt → Dwarf

7. Homozygous

• An organism is said to be homozygous when it possesses two identical alleles for a particular gene.
• Homozygous individuals are also called pure or true-breeding.

Types

• Homozygous Dominant → TT
• Homozygous Recessive → tt

Characteristics

• Both alleles are identical.
• Produce only one type of gamete.
• Breed true for a particular trait.

8. Heterozygous

An organism is heterozygous when it possesses two different alleles for the same gene.

Example: Tt

Characteristics

• Contains one dominant and one recessive allele.
• Produces two different types of gametes.
• Usually expresses the dominant trait.

9. Genotype

• The genotype is the genetic constitution or genetic makeup of an organism.
• It refers to the combination of alleles possessed by an individual, regardless of whether they are expressed.

Examples: TT, Tt, tt

All three are different genotypes for plant height.

10. Phenotype

• The phenotype is the observable physical appearance or expression of an organism resulting from the interaction of its genotype and the environment.
• Examples: Tall plant, Dwarf plant, Yellow seed, Green seed, Purple flower, White flower

Different genotypes may produce the same phenotype.

For example:

• TT → Tall
• Tt → Tall

Both have the same phenotype but different genotypes.