Genetics, Monohybrids and Dihybrids

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1. Genetics Basics

  • Gene: A DNA segment encoding a trait (e.g., hair color or dimples).
  • Alleles: Different versions of a gene.
    • Example: "B" for black hair (dominant) and "b" for blonde hair (recessive).
  • Genotype: The genetic composition (e.g., BB, Bb, or bb).
  • Phenotype: The observable trait resulting from the genotype (e.g., black or blonde hair).

2. Monohybrid Cross

  • A cross focusing on one trait (e.g., hair color).
    • Homozygous Dominant: BB (black hair).
    • Heterozygous: Bb (black hair).
    • Homozygous Recessive: bb (blonde hair).

Punnett Square Example:

  1. Cross between Bb (heterozygous black hair) x Bb (heterozygous black hair):

    • Genotype Ratios: 1:2:1 (BB:Bb:bb).
    • Phenotype Ratios: 3:1 (3 black hair:1 blonde hair).

  2. Probability of Offspring with Specific Genotype:

    • Example: Probability of BB = 14\frac{1}{4} = 25%.
    • For 2 children both having BB: 14×14=116\frac{1}{4} \times \frac{1}{4} = \frac{1}{16} or 6.25%.


3. Dihybrid Cross

  • A dihybrid cross studies two traits simultaneously (e.g., hair color and dimples).
    • Example Alleles:
      • B (black hair, dominant) vs. b (blonde hair, recessive).
      • D (dimples, dominant) vs. d (no dimples, recessive).

Punnett Square Example:

  1. Cross BbDd x BbDd (heterozygous for both traits):

    • Phenotype Ratios (Typical Dihybrid Ratio):
      • 9:3:3:1
        • 9 black hair with dimples.
        • 3 black hair with no dimples.
        • 3 blonde hair with dimples.
        • 1 blonde hair with no dimples.

  2. Modified Cross Example: BbDd x bbdd

    • Use a Punnett square to calculate specific genotype and phenotype ratios.
    • Example Phenotype Ratio:
      • 1:1:1:1 (black hair with dimples, black hair without dimples, blonde hair with dimples, blonde hair without dimples).

4. Key Concepts in Probability

  • Independent Assortment: Traits segregate independently during gamete formation (e.g., hair color does not influence dimples).
  • Use multiplication for compound probabilities:
    • Example: Probability of bb and dd offspring = 1/2×1/2=1/4 = or 25%.

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