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Understanding Monohybrid Crosses and Punnett Squares

Apr 25, 2025

Lecture Notes: Monohybrid Crosses and Punnett Squares

Introduction

  • Topic: Monohybrid crosses
  • Focus: Layout of a monohybrid cross, working out a Punnett square, and examples

Key Terminology

  • Monohybrid Cross: Involves crossing one trait at a time (e.g., color or height)
  • Gene: Section of DNA coding for a trait
  • Allele: Variation of a gene (e.g., variations in eye color: brown, blue, green)
  • Symbols in Genetics:
    • Capital letter for dominant allele
    • Lowercase letter for recessive allele

Genotype and Phenotype

  • Genotype: Combination of alleles (e.g., TT, Tt, tt)
  • Phenotype: Physical appearance resulting from the genotype
  • Homozygous: Two identical alleles (TT or tt)
    • "Homozygous Dominant": Two capital letters
    • "Homozygous Recessive": Two lowercase letters
  • Heterozygous: One dominant and one recessive allele (Tt)

Mendel's Laws

  • Law of Dominance:
    • In purebred crosses, only the dominant trait appears
    • Recessive alleles are masked by dominant alleles
  • Law of Segregation:
    • Characteristics controlled by allele pairs
    • During meiosis, alleles separate into different gametes

Constructing a Genetic Cross

  1. P1 Generation: Parent generation
    • Write physical characteristics and genotypes
    • Example: Black mouse (BB) and white mouse (bb)
  2. Gamete Formation:
    • Separate alleles (e.g., B, b)
  3. Fertilization:
    • Use Punnett square to determine F1 generation outcomes
  4. F1 Generation: Resulting offspring
    • Analyze genotypes and phenotypes

Punnett Square Examples

  • True Breeding Crosses:
    • Example with yellow and green pods
    • Dominant trait (green) uses capital letter G
    • Recessive trait (yellow) uses lowercase g
    • Result: 100% heterozygous (Gg), phenotype 100% green
  • Heterozygous Cross:
    • Example with purple (dominant) and white (recessive) flowers
    • Ratio: 3 purple (dominant) to 1 white (recessive)
    • Genotype breakdown: 25% homozygous dominant, 50% heterozygous, 25% homozygous recessive

Recap

  • Monohybrid: Cross one trait at a time
  • Genes and Alleles: Code and variations
  • Mendel’s Laws: Dominance and segregation
  • Homozygous vs. Heterozygous: Allele combinations
  • Punnett Squares: Tool for genetic crosses
  • True Breeding: Individuals with identical alleles (purebred)

Conclusion

  • Summary: Importance of understanding genetic crosses and Punnett squares for analyzing genetic traits

Note: These notes cover the key points discussed in the lecture on monohybrid crosses and the use of Punnett squares for genetic analysis.

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