Chapter 14: Mendel and Heredity
Introduction
- Focus on Mendel, the father of modern genetics.
- Mendel's experiments with pea plants (Pisum sativum) helped us understand heredity.
- Prior to Mendel, the blending hypothesis was a key theory of heredity.
Mendel's Background
- Studied at the University of Vienna under Christian Doppler.
- Became a monk and conducted genetic experiments in a monastery garden.
- Known for using scientific method and pea plants for genetic studies.
The Pea Plant
- Reasons for choice: Variety, short generation time, large numbers of offspring, controllable mating.
- Can self-fertilize or cross-fertilize (controlled with methods like removing anthers to prevent self-fertilization).
Key Terminology
- Character: A heritable feature (e.g., flower color).
- Trait: Variations of a character (e.g., purple or white flowers).
Mendel's Experiments
- True Breeders: Plants that consistently produce offspring with the same traits when self-fertilized.
- Non-True Breeders: Plants that produce varied offspring (sometimes different traits appear).
Mendel’s Findings
- Monohybrid Cross: Focused on one characteristic, such as flower color.
- P Generation: True breeder parents.
- F1 Generation: Offspring of P Generation, all purple, disproving blending hypothesis.
- F2 Generation: Result of self-fertilizing F1, showing a 3:1 phenotypic ratio (purple to white).
- Introduced concepts of dominant (e.g., purple) and recessive (e.g., white) traits.
Terminology Update
- Gene: Modern term for 'characteristic'.
- Allele: Modern term for 'trait'.
- Phenotype vs. Genotype: Phenotype is what you see; genotype is the allele composition.
Principle of Segregation of Alleles
- Each individual has two alleles for each gene, and pass one to offspring.
- Genotypic Ratio: 1:2:1 (homozygous dominant : heterozygous : homozygous recessive).
Dihybrid Cross
- Examined two characteristics simultaneously (e.g., seed color and shape).
- F1 Generation: All offspring showed dominant traits.
- F2 Generation: Showed new combinations, leading to the principle of independent assortment.
Principle of Independent Assortment
- Different genes (e.g., seed color and shape) assort independently during gamete formation.
- Phenotypic Ratio: 9:3:3:1 (dominant/dominant : dominant/recessive : recessive/dominant : recessive/recessive).
Conclusion
- Mendel's work laid the foundation for modern genetics.
- Predicted genetic principles before the discovery of genes and chromosomes.
- His experiments demonstrated principles that align with meiosis processes.
These notes capture the key contributions of Mendel to our understanding of heredity and genetics, using his experiments on pea plants to reveal principles still used today.