Understanding Dihybrid Crosses

Introduction

• Dihybrid crosses allow tracking of inheritance of two traits simultaneously.
• Key concepts to review: genotype, phenotype, dominance vs recessiveness, Punnett Squares.

Key Concepts

Genotype vs Phenotype

• Genotype: Genetic makeup of an organism.
• Phenotype: Physical expression of genes.
• Example: Pea plant height
  • Big T: Tall (dominant)
  • Little t: Short (recessive)

Dominance vs Recessiveness

• Dominant Allele: Trait expressed even if only one allele is present (e.g., Big T).
• Recessive Allele: Trait expressed only when two recessive alleles are present (e.g., Little t).
• Genotype possibilities:
  • Big T Big T: Homozygous dominant
  • Big T Little t: Heterozygous
  • Little t Little t: Homozygous recessive

Punnett Squares

• Tool for predicting genetic outcomes of a cross.
• Each square represents a possible genotype for offspring.

Dihybrid Cross Explained

Definition

• A breeding experiment tracking inheritance of two traits at once.

Example Traits

• Height (Tall or Short)
• Flower Color (Purple or White)
  • Big P: Purple (dominant)
  • Little p: White (recessive)

Initial Cross

• Parent 1: Tall with purple flowers (Big T Big T, Big P Big P)
• Parent 2: Short with white flowers (little t little t, little p little p)
• F1 Generation: Heterozygous (Big T Little t, Big P Little p)
  • All offspring tall with purple flowers

Using a Punnett Square

• Dihybrid cross involves a 4x4 Punnett Square (16 combinations).
• Parent gamete combinations:
  • Big T Big P, Big T Little p, Little t Big P, Little t Little p
• Offspring genotypes determined by combining parental alleles.
• Phenotypic Ratio: 9 Tall Purple : 3 Tall White : 3 Short Purple : 1 Short White

Conclusion

• Starting with homozygous dominant and recessive parents:
  • F1 Generation: Heterozygous offspring.
  • Offspring Phenotypic Ratio: 9:3:3:1 in F2 generation.
• Use Punnett Squares to determine genotypes and phenotypes.

Next Steps

• Next video covers sex-linked traits and genetic disorders.

Interactive Biology with Leslie Samuel aims to make biology engaging and fun.