Gregor Johann Mendel: Life and Work

Biography: Gregor Johann Mendel was born on July 22, 1822, in the village of Heinzendorf, Moravia (now part of the Czech Republic), into a farmer’s family. He is widely known as the father of modern genetics due to his groundbreaking work on heredity using garden pea plants.

Brief Chronology of Mendel’s Scientific and Literary Works:

• 1822: Born in Heinzendorf, Moravia (Brunn), Austria.
• 1840: Graduated from Gymnasium in Troppau.
• 1847: Ordained as a priest.
• 1849: Appointed as substitute science teacher in Znaim High School.
• 1851-1853: Sent by the monastery authorities to the University of Vienna for higher studies in science.
• 1854: Joined the Brunn Modern School as a teacher of physics and natural history.
• 1857: Began to collect pea seeds for his experiments from all over Europe.
• 1862: Became one of the founding members of the Natural History Science Society, Brunn.
• 1865: Delivered his first two lectures on his pea experiments (on February 8 and March 8) in the Brunn Natural Science Society.
• 1866: Published the research paper titled “Experiments on Plant Hybridization” in Volume 4 of the “Proceedings of the Natural History Science Society, Brunn”.
• 1868: Elected as the abbot of the Monastery of St. Thomas. Delivered a lecture on his Hieracium experiments at the Natural History Science Society.
• 1870: Papers related to Hieracium and the Brunn tornado published in the proceedings of the society.
• 1873: Wrote his last letter to Professor Nageli.
• 1884: Died on January 6, likely due to Bright’s disease.

Rediscovery of Mendel’s Work: In 1900, Mendel’s work was rediscovered by Hugo de Vries in the Netherlands, Carl Correns in Germany, and Erich von Tschermak in Austria, sparking the foundation of modern genetics.

Mendel’s Work on Genetics:

Mendel conducted his experiments on Pisum sativum (garden pea) due to its ideal properties for genetic study:

• Ease of cultivation: Can be grown in both fields and pots.
• Short life cycle: Fast generations for studying inheritance.
• Self-pollination: Ensured controlled mating of plants.
• Contrasting heritable characters: Easy identification of genetic traits.
• Emasculation and hybridization: Allowed for crossbreeding of plants.

Mendel selected seven pairs of contrasting traits for his experiments:

Reasons for Mendel’s Success:

Mendel’s success in his genetic experiments was due to several key factors:

1. Single Trait Focus: He focused on one trait at a time rather than considering all traits together, which simplified his observations.
2. Contrasting Pairs: He used pairs of contrasting characters (dominant and recessive) to track inheritance.
3. Independent Assortment: The traits studied were independent of each other (no genetic linkages).
4. Chromosome Independence: The characters were carried on different chromosomes.
5. Accurate Experimentation: Mendel kept detailed records, ensuring precision in his experiments.
6. Use of Symbols: He adopted clear symbols, signs, and terminology, allowing for easy communication of his findings.

Mendel’s Laws of Inheritance

Mendel’s work was rediscovered in 1900 by Hugo de Vries (Dutch), Carl Correns (German), and Erich Von Tschermark (Austrian), leading to the universal acceptance of three fundamental principles of genetics:

1) Law of Segregation (Purity of Gametes)

In a heterozygous state, the dominant and recessive alleles remain distinct throughout the life of the individual. These alleles segregate or separate during gamete formation, ensuring that each gamete carries only one allele for each gene.

Principles of the Law of Segregation:

• a) Hereditary characters are determined by factors (now called genes).
• b) These factors occur in pairs, meaning each gene has two or more alternative forms called alleles.
• c) During gamete formation, these alleles segregate so that only one allele from each pair is passed into a gamete.
• d) When male and female gametes unite during fertilization, the chromosome number is restored in the zygote.

Homologous and Heterozygous States:

• Homozygous: Individuals with identical alleles for a particular gene (e.g., TT or tt).
• Heterozygous: Individuals with different alleles for a gene (e.g., Tt).
• Hemizygous: Individuals having only one allele for a gene (e.g., males with X-linked traits).

Example:

• In Mendel’s pea plants, tall plants (TT) produce only T-type gametes, and dwarf plants (tt) produce only t-type gametes.
• Heterozygous (Tt) plants produce both T-type and t-type gametes.

2) Law of Dominance (Uniformity of First Filial Generation) Mendel observed that when two different varieties of plants with contrasting traits are crossed, only one of the traits appears in the F1 generation. This trait is termed dominant, while the other is recessive.

Explanation:

• For example, in a cross between a tall plant (Tt) and a dwarf plant (tt), the F1 generation will consist of all tall plants (because tall is dominant over dwarf).
• The F1 plants will be uniform (100% tall), confirming the law of dominance.

Genotypic and Phenotypic Ratios:

• Genotypic ratio: 1 TT : 2 Tt : 1 tt
• Phenotypic ratio: 3 Tall : 1 Dwarf

3) Law of Independent Assortment This law states that the alleles of different genes segregate independently of one another during gamete formation. This law applies only to genes located on different chromosomes or genes that are not linked.

Explanation:

• Mendel crossed yellow, round-seeded plants (YYRR) with green, wrinkled-seeded plants (yyrr).
• The F1 generation had all yellow, round seeds (YyRr).
• In the F2 generation, four combinations of seed traits were observed in a 9:3:3:1 ratio.

Example of Dihybrid Cross:

• Parents: Yellow, Round seeds (YYRR) × Green, Wrinkled seeds (yyrr)
• F1 Generation: YyRr (All Yellow and Round)
• F2 Generation:
  • Phenotypic ratio: 9 Yellow Round : 3 Yellow Wrinkled : 3 Green Round : 1 Green Wrinkled

This outcome shows that the alleles for seed color and seed shape assort independently, which confirms the law of independent assortment.

Checkerboard for Dihybrid Cross:

• The phenotypic ratio is 9 Yellow, Round : 3 Yellow, Wrinkled : 3 Green, Round : 1 Green, Wrinkled.

Dihybrid Cross (Four Types of Traits)

• Phenotypic ratio: 9 : 3 : 3 : 1
• Genotypic ratio: 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 (9 types)

Trihybrid Cross (Three Pairs of Contrasting Traits)

• Phenotypic ratio: 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1

F1 Selfing, Backcross, and Test Cross

• When the F1 individual is crossed with either of the two parents, such a cross is called a back cross.
• When the F1 individual is crossed with a recessive parent, it is called a test cross.

Test Cross Example:

• Tall plant (X) × Dwarf plant (tt) If all progeny are tall, the parent plant X is homozygous (TT). If 50% of the progeny are tall, the parent plant X is heterozygous (Tt).

Back Cross: When the F1 individual is crossed with the dominant parent (tall), the result would be:

• 50% Pure Tall (TT)
• 50% Hybrid Tall (Tt)