Mendel's Experiments on Inheritance

Alternative forms of the same gene controlling a pair of contrasting traits are called alleles.

An organism having identical alleles for a character, such as TT or tt, is called homozygous.

An organism having unlike alleles for a character, such as Tt, is called heterozygous.

The trait expressed in a heterozygous condition is called dominant.

The trait that remains unexpressed in a heterozygous condition but appears in homozygous form is called recessive.

Gregor Mendel carried out hybridisation experiments on pea plants between 1856 and 1863.​ He carefully selected true-breeding lines, performed controlled crosses, studied many offspring over several generations, and analysed the results statistically.​

Revolutionary Impact of Mendel's Work:

• He used a scientific experimental method rather than simple observation.​
• He worked with clear contrasting traits in pea plants.​
• He counted large numbers of progeny and drew conclusions from numerical ratios.​
• He proposed that inheritance depends on particulate factors, now called genes.

The garden pea (Pisum sativum) was an ideal experimental plant for Mendel’s studies.​

Reasons for Selection:

• Pea plants have several clear, easily observable contrasting traits.​
• They normally undergo self-pollination, which helps maintain pure lines.​
• Artificial cross-pollination can also be carried out easily.​
• The plant has a short life cycle and produces many seeds, allowing repeated observation.​
• True-breeding varieties were available for several contrasting characters.

Mendel selected seven pairs of clearly contrasting characters in pea plants.

These contrasting traits were suitable for observing inheritance patterns clearly.

Stepwise Method Used by Mendel

1. Mendel selected true-breeding pea plants showing opposite forms of one character.​
2. He removed the anthers of the flower selected as the female parent to prevent self-pollination.​
3. He collected pollen from the chosen male parent and transferred it to the stigma of the female flower.​
4. He allowed seeds to form and then observed the traits in the offspring.​
5. He self-pollinated the hybrid offspring and studied the next generation.​
6. He counted the number of plants showing each trait and analysed the pattern.

When Mendel crossed two true-breeding parents with contrasting traits, only one of the two traits appeared in the first filial generation (F1).​ The other trait, though absent in F1, reappeared in the second filial generation (F2).​

Example: Tall × Dwarf Cross

• Parent generation: Tall plant × Dwarf plant.​
• F1 generation: All plants were tall.​
• F2 generation: Both tall and dwarf plants appeared.​

This showed that traits do not disappear permanently; they remain as discrete hereditary units.​

Symbolic Representation

• Tall allele = T; dwarf allele = t.​
• Parent cross: TT × tt.​
• F1 generation: All Tt (tall).​
• F2 generation after selfing of F1: TT, Tt, Tt, tt.

Mendel’s experiments proved that inheritance is particulate and not blending in nature.​ His work established that hereditary factors occur in pairs and separate during gamete formation.​

Reliability of His Results

• He studied one trait at a time in the beginning.​
• He used pure lines for controlled experiments.​
• He observed many offspring, increasing reliability.​
• He applied counting and statistical analysis to biological data.​

These features make Mendel’s work a model of scientific investigation in biology.

• Gregor Mendel is known as the Father of Genetics; he worked on pea plants (1856–1863).
• He used true-breeding pea plants and studied inheritance using cross-pollination experiments.
• Mendel selected 7 pairs of contrasting traits (e.g., tall/dwarf, round/wrinkled, yellow/green).
• He introduced the concepts of dominant and recessive traits.
• His experiments had a large sample size and statistical analysis, making the results reliable.
• Mendel’s work formed the basic laws of inheritance, explaining how traits pass from parents to offspring.
• His findings were confirmed by repeated experiments across generations.