Lecture Notes on Patterns of Inheritance

Introduction

• Topic: Patterns of Inheritance
• Today we'll focus on sections related to inheritance, particularly monohybrid crosses.
• Overview of inheritance in genetics, starting with Gregor Mendel's foundational work.

Historical Context of Inheritance

• Before the 20th century, two concepts around heredity existed:
  • Heredity occurs within species.
  • Traits are transmitted directly from parent to offspring.
• Early belief in "bloodlines" suggested traits blended like fluids.
  • This led to paradox as not all individuals look alike.

Early Experiments in Genetics

• Joseph Kölreuter (1700s): Crossed tobacco strains to produce hybrids with differing traits.
• T.A. Knight: Worked with peas and noted that offspring traits did not blend but resembled one parent or the other.

Gregor Mendel's Experiments

• Model Organism: Pea plants (Pisum sativum)
• Reasons peas were ideal:
  • Existing research on hybrid production
  • Variety in traits (flower color, seed color/shape, etc.)
  • Easy to grow, can self or cross fertilize

Mendel's Experiment Methodology

1. Produce true-breeding strains (self-fertilization).
2. Cross-fertilize these strains (reciprocal crosses).
3. Allow hybrid offspring to self-fertilize, counting offspring traits.

Key Observations

• F1 Generation: Resulted in all purple flowers (dominant trait).
• F2 Generation: Recessive traits reappeared (3:1 ratio of purple to white flowers).
  • This revealed a deeper genotypic ratio of 1:2:1.

Mendelian Genetics

• Monohybrid Crosses: Examining two variations of a single trait.
• Dominance: Dominant traits mask recessive ones.
• Genotype vs. Phenotype:
  • Genotype: Genetic makeup (e.g., homozygous dominant, heterozygous, homozygous recessive).
  • Phenotype: Observable traits (e.g., flower color).
• Mendel’s Five-Element Model:
  1. Parents transmit discrete factors.
  2. Individuals receive one copy from each parent.
  3. Not all gene copies are identical (alleles: alternative forms).
  4. Alleles remain discrete.
  5. Presence of an allele doesn’t guarantee expression.

Principle of Segregation

• Alleles segregate during gamete formation and rejoin at random during fertilization.
• Physical basis is chromosome movement during meiosis.

Punnett Squares

• Visual representation of allele combinations in offspring.
• Female genotype on the side, male across the top.
• Example: Big P (purple) is dominant, Little P (white) is recessive.
  • Heterozygous (Big P Little P) results in purple phenotype.
  • Genotypic ratio: 1 (true breeding purple): 2 (heterozygous purple): 1 (true breeding white).
  • Phenotypic ratio: 3 purple to 1 white.

Conclusion

• Mendel's work laid the foundation for understanding inheritance, showing that traits are discrete and not a simple blend.
• Next session will cover dihybrid crosses and more complex patterns of inheritance.