Lecture on Dihybrid Crosses

Introduction

• Dihybrid Crosses: Process to track inheritance of two traits at once.
• Objective: Understand dihybrid crosses through the lecture.

Key Concepts

Genotype and Phenotype

• Genotype: Genetic makeup of an organism.
• Phenotype: Physical expression of genes.
• Example: In pea plants:
  • Big T (Tall) and Little t (Short) are genotypes for height.

Dominant vs Recessive Traits

• Dominant: Trait that is expressed when present (e.g., Big T).
• Recessive: Trait masked by the dominant trait (e.g., Little t).
• Genotypes:
  • Big T Big T: Homozygous dominant
  • Big T Little T: Heterozygous
  • Little T Little T: Homozygous recessive

Punnett Square

• A tool to predict possible genetic outcomes of a cross.
• Each square represents a possible genotype for offspring.

Dihybrid Crosses

Definition

• Dihybrid Cross: Breeding experiment tracking inheritance of two traits.
• Example: Height and flower color in pea plants.
  • Traits: Tall/Short and Purple/White.
  • Purple is dominant, White is recessive.
  • Alleles: Big P (Purple), Little p (White).

Parental Generation 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)
• Result: F1 generation is heterozygous for both traits (Big T Little T, Big P Little P)
• Phenotype of F1: Tall with purple flowers

Punnett Square for Dihybrid Cross

• 4x4 grid, 16 combinations.
• Possible gametes for each parent:
  • Big T Big P, Big T Little P, Little T Big P, Little T Little P
• Fill Punnett Square with combinations to find genotypes of offspring.
• Phenotypic ratio: 9 Tall Purple : 3 Tall White : 3 Short Purple : 1 Short White

Summary

• Homozygous Parents Cross: One dominant, one recessive for both traits leads to heterozygous F1.
• F1 Generation Cross: Results in 9:3:3:1 phenotypic ratio.
• Tool: Punnett Square to determine genotypes and phenotypes.

Next Steps

• Upcoming topic: Sex-linked traits and genetic disorders.
• Focus on understanding trait and disorder inheritance.

Leslie Samuel from Interactive Biology presented this lecture. The aim is to make biology fun and accessible.