Chapter 14: Mendel and the Gene Idea

Gregor Mendel and the Study of Inheritance

Gregor Mendel, often called the "father of genetics," developed foundational concepts of heredity through experiments with garden peas (Pisum sativum). His work established the principles that govern the inheritance of traits from parents to offspring.

• Gregor Mendel: Austrian monk who used garden peas to study inheritance.

• Experimental Approach: Mendel used a quantitative method, collecting numerical data to support his hypotheses.

• Characters: Inheritable features (e.g., flower color).

• Traits: Variants of a character (e.g., purple or white flowers).

• Strength of Approach: Mendel's use of statistics and controlled breeding allowed him to identify patterns in inheritance.

• Example: Mendel observed that crossing true-breeding purple and white flowered plants produced all purple offspring in the first generation (F1).

Hypotheses of Inheritance

Before Mendel, two main hypotheses attempted to explain how traits were inherited:

• Blending Hypothesis: Genetic material from parents mixes in offspring, resulting in intermediate traits.

• Particulate Hypothesis: Parents pass discrete units (particles) of inheritance to offspring, which remain separate and can be recombined in future generations.

• Mendel's Support: Mendel's experiments supported the particulate hypothesis, now known as the gene theory.

• Example: The reappearance of the white flower trait in the F2 generation disproved the blending hypothesis.

The Law of Segregation

Mendel's experiments led to the discovery of the Law of Segregation, which explains how alleles are separated during gamete formation.

• True-Breeding Parents: Plants that consistently produce offspring with the same trait.

• Hybridization: Crossing two true-breeding parents with different traits produces hybrids (F1 generation).

• Dominant Trait: The trait that appears in hybrids.

• Recessive Trait: The trait that is masked in hybrids but reappears in the F2 generation.

• F1 Generation: First filial generation; all hybrids show the dominant trait.

• F2 Generation: Second filial generation; recessive trait reappears, demonstrating segregation of alleles.

• Example: Crossing F1 hybrids yields a 3:1 ratio of dominant to recessive traits in F2.

Genes and Heritable Factors

Mendel referred to the units of inheritance as "heritable factors," now known as genes. Each gene exists in different forms called alleles.

• Gene: A segment of DNA that codes for a specific character.

• Allele: Different versions of a gene (e.g., purple vs. white flower allele).

• Example: Each parent contributes one allele for each gene to their offspring.

Mendel's Model of Inheritance

Mendel proposed four postulates to explain the patterns of inheritance observed in his experiments:

1. Alleles Account for Variation: Genes exist in different forms (alleles), which cause variations in traits.

2. Two Alleles per Gene: Each individual inherits two alleles for each gene, one from each parent.

3. Dominance: If alleles differ, one (dominant) masks the expression of the other (recessive).

4. Segregation: The two alleles for a gene separate during gamete formation, so each gamete carries only one allele.

Example: In pea plants, the allele for purple flowers is dominant, while the allele for white flowers is recessive.

Summary Table: Mendel's Postulates

Genotype and Phenotype

The genotype is the genetic makeup of an organism, while the phenotype is the observable trait.

• Genotype: The combination of alleles (e.g., PP, Pp, pp).

• Phenotype: The physical expression of the genotype (e.g., purple or white flowers).

• Example: Both PP and Pp genotypes result in purple flowers, while pp results in white flowers.

Key Formula: Probability of Trait Appearance

Mendel used probability to predict trait ratios in offspring.

• Monohybrid Cross Ratio:

Additional info: Mendel's principles laid the foundation for modern genetics, including the understanding of DNA, chromosomes, and molecular inheritance.