6 - Monohybrid Crosses 2025 copy

Introduction to Genetics

  • Genetics: Study of heredity, which is the passing of traits from parents to offspring.

Gregor Mendel

  • Gregor Johann Mendel (1822-1884): Austrian monk, recognized as the "Father of Genetics."

    • Conducted experiments on pea plants to understand inheritance of traits.

    • Worked unnoticed until the early 20th century.

Mendel's Experiments

  • Mendel's Garden: Location in Czech Republic where he did experiments.

  • Conducted over 28,000 pea plant experiments (1856-1863).

    • Found that offspring retain traits of parents.

    • Stated traits are inherited as "particles" (now known to be chromosomes & DNA).

Key Genetic Terminology

  • Trait: Any characteristic passed from parent to offspring.

  • Heredity: The process of passing traits from parents to offspring.

  • Genetics: The scientific study of heredity.

  • Alleles: Two forms of a gene (dominant & recessive).

Types of Genetic Crosses

  • Monohybrid Cross: Cross involving a single trait (e.g., flower color).

  • Dihybrid Cross: Cross involving two traits (e.g., flower color & plant height).

Punnett Squares

  • Tools for predicting genotypes and phenotypes of offspring.

    • Steps to Create a Punnett Square:

      1. Establish a grid with parental alleles.

      2. Fill in the grid with combinations of alleles.

      3. Determine phenotypes using the Law of Dominance.

  • Example: For Yy and Yy parents, possible outcomes show 1 YY, 2 Yy, 1 yy (genotypic ratio of 1:2:1, phenotypic ratio of 3:1).

Understanding Genotypes and Phenotypes

  • Genotype: Genetic makeup for a trait (e.g., RR, Rr, rr).

  • Phenotype: Physical appearance resulting from genotype (e.g., red, white flowers).

  • Genotypes and their corresponding phenotypes in flowers:

    • R = red flower, r = yellow flower

    • Combinations: RR (red), Rr (red), rr (yellow).

Mendel's Genetic Framework

  • Homozygous Genotype: Two identical alleles (e.g., RR or rr).

  • Heterozygous Genotype: One dominant and one recessive allele (e.g., Rr).

Mendel’s Pea Plant Traits Studied

  • Traits observed:

    • Seed Shape: Round (R) or Wrinkled (r)

    • Seed Color: Yellow (Y) or Green (y)

    • Pod Shape: Smooth (S) or Wrinkled (s)

    • Pod Color: Green (G) or Yellow (g)

    • Flower Color: Purple (P) or White (p)

    • Plant Height: Tall (T) or Short (t).

Ratios of Traits in Mendel's Results

  • Ratio of dominant to recessive traits in F2 generation:

    • Smooth/Wrinkled seeds: 2.96:1

    • Yellow/Green seeds: 3.01:1

    • Inflated/Wrinkled pods: 2.95:1

    • Average ratio for all traits is approximately 3:1.

  • Observed ratios often differ slightly from theoretical due to statistical error.

Generational Distinctions

  • P1 Generation: Parental generation.

  • F1 Generation: First-generation offspring.

  • F2 Generation: Second-generation offspring derived from F1.

Monohybrid Cross Examples

  • P1 Monohybrid Cross: Round seeds (RR) crossed with wrinkled seeds (rr) produces all F1 offspring as heterozygous (Rr, round).

  • F1 Monohybrid Cross: Round seeds (Rr) crossed with round seeds (Rr) yields ratios of genotypes and phenotypes (1:2:1 for genotypes, 3:1 for phenotypes).

Test Crosses

  • Purpose: To determine the genotype of a phenotypically dominant individual.

  • Case Examples:

    • If all offspring show the dominant trait, the unknown parent is homozygous (BB).

    • If one offspring shows the recessive trait, the unknown parent is heterozygous (Bb).

Conclusion

  • Next Steps: Review sections on Dihybrid crosses and complete assigned worksheets on Monohybrid Punnett squares.