Chapter 14: Mendel and the Gene

Introduction to Mendelian Genetics

Gregor Mendel's experiments with garden peas in the mid-19th century established the foundational principles of genetics. His work addressed the fundamental question: How does heredity work? Mendel's systematic approach and use of model organisms led to the discovery of predictable patterns in the inheritance of traits.

Main Topics

Mendel’s Experimental System

Mendel's research focused on understanding how traits are passed from parents to offspring. He used the garden pea (Pisum sativum) as his model organism due to its advantageous characteristics for genetic studies.

• Trait: Any characteristic of an individual organism (e.g., flower color, seed shape).

• Heredity: The transmission of traits from parents to offspring.

• Two main hypotheses before Mendel:

  • Blending inheritance: Parental traits blend together in offspring, resulting in intermediate traits.

  • Inheritance of acquired characteristics: Parental traits are modified through use and passed on to offspring.

The Garden Pea as a Model Organism

The garden pea was chosen for its suitability in controlled genetic experiments.

• Inexpensive and easy to grow

• Short generation time

• Produces large numbers of seeds

• Crosses can be controlled (self- or cross-fertilization possible)

• Exhibits several polymorphic traits (traits with at least two different forms)

• These features make peas a classic model organism in genetics research.

Self-Fertilization and Cross-Fertilization in Peas

Mendel could control the breeding of peas by manipulating their fertilization process.

• Self-fertilization: Pollen from a flower fertilizes eggs in the same flower.

• Cross-fertilization: Pollen is transferred from one plant to the female organ of another plant.

• Cross-fertilization allows for the study of inheritance patterns between different parental lines.

Traits Studied by Mendel

Mendel focused on seven distinct traits in peas, each with two contrasting forms (phenotypes).

• Trait: General characteristic (e.g., seed color)

• Phenotype: Observable form of the trait (e.g., green or yellow seed color)

• Each trait had two phenotypes (e.g., round vs. wrinkled seeds)

• Mendel used pure (true-breeding) lines—plants that consistently produce the same phenotype when self-fertilized.

Mendel’s Experiments with a Single Trait (Monohybrid Crosses)

Mendel began by crossing pure lines that differed in only one trait, known as a monohybrid cross.

• Parental (P) generation: The initial pure-breeding parents

• F1 generation: First filial generation, offspring of the P generation

• F2 generation: Offspring of the F1 generation (produced by self-fertilization or crossing F1 individuals)

• Example: Crossing round-seed (dominant) and wrinkled-seed (recessive) plants

• F1 generation: All round seeds (dominant phenotype)

• F2 generation: 3:1 ratio of round to wrinkled seeds

Key Genetic Terms

• Gene: A hereditary factor that influences a particular trait

• Allele: Different versions of a gene

• Genotype: The specific combination of alleles an individual carries (e.g., RR, Rr, rr)

• Phenotype: The observable trait (e.g., round or wrinkled seeds)

• Homozygous: Having two identical alleles for a gene (e.g., RR or rr)

• Heterozygous: Having two different alleles for a gene (e.g., Rr)

• Dominant allele: The allele that determines the phenotype in a heterozygote

• Recessive allele: The allele whose effect is masked in a heterozygote

Mendel’s Laws

• Law of Segregation: The two alleles for a gene segregate (separate) during gamete formation, so each gamete receives only one allele.

  • Represented by the separation of homologous chromosomes during meiosis.

  • Example: A cross between two heterozygotes (Rr x Rr) produces offspring in a 3:1 phenotypic ratio (round:wrinkled).

• Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation.

  • Tested using dihybrid crosses (e.g., seed shape and seed color).

  • Predicted phenotypic ratio in F2 generation: 9:3:3:1 for two traits.

Practice Problems (with Academic Context)

• Monohybrid Cross (Aa x Aa):

  • Genotypic ratio: 1 AA : 2 Aa : 1 aa

  • Phenotypic ratio (if A is dominant): 3 dominant : 1 recessive

• Test Cross (Aa x aa):

  • Genotypic ratio: 1 Aa : 1 aa

  • Phenotypic ratio: 1 dominant : 1 recessive

• Dihybrid Cross (AaBb x AAbb):

  • Predicts phenotypic ratios based on independent assortment.

Chromosome Theory of Inheritance

Mendel’s principles are explained by the behavior of chromosomes during meiosis.

• Principle of Segregation: Homologous chromosomes (and thus alleles) separate during meiosis I.

• Principle of Independent Assortment: Different chromosome pairs align independently during meiosis I, leading to independent assortment of genes located on different chromosomes.

Summary Table: Mendel’s Laws and Their Chromosomal Basis

Key Equations

• Monohybrid Cross (heterozygotes):

• Dihybrid Cross (AaBb x AaBb):

Applications and Importance

• Mendelian genetics forms the basis for understanding inheritance in all sexually reproducing organisms.

• Modern genetics builds on these principles to study complex traits, genetic disorders, and molecular biology.