Beginner's Guide to Punnett Squares

Introduction

• Speaker: Mr. Andersen
• Topic: Punnett Squares and common mistakes
• Objective: Understand Punnett squares and correct common misconceptions

Reginald Punnett

• Not directly involved with Punnett squares
• Worked with genetics and butterfly mimicry
• Associated with genetics like Mendel

Understanding Punnett Squares

• Often overused without understanding genetics
• Key concept: Two sides of a Punnett square represent gametes after meiosis
• Represents possible gene combinations post fertilization

Monohybrid Cross

• Definition: Cross looking at one trait
• Example: Crossing homozygous purple (dominant) and white (recessive) flowers
  • Purple (PP) vs White (pp)
  • Outcomes: All Pp (Purple), 100% identical

Heterozygous Cross

• Cross: Heterozygous flowers (Pp)
• Gametes: Big P or little p
• Outcome:
  • Genotypic ratio: 1 PP : 2 Pp : 1 pp
  • Phenotypic ratio: 3 Purple : 1 White

Incomplete Dominance

• Example: Snapdragon flowers
  • Red (RR), White (WW), Pink (RW)
• Phenotype & Genotype Ratios:
  • 1 RR : 2 RW : 1 WW
  • Phenotypic ratio also 1:2:1

Codominance

• Same Punnett square use as incomplete dominance
• Heterozygous expresses both genes

Sex-Linked Genes

• Example: Color blindness
  • Parents: XX (carrier) and XY (normal)
  • Outcomes:
    • Female carrier (XcX)
    • Normal female (XX)
    • Normal male (XY)
    • Color blind male (XcY)

Dihybrid Cross

• Traits: Round (R) vs Wrinkled (r) and Yellow (Y) vs Green (y)
• Common mistake: Incorrectly setting up gametes
• Gamete Formation: One of each allele
• Correct Setup:
  • 4 possible gamete combinations per parent
  • Example: RrYy can give RY, Ry, rY, ry

Dihybrid Ratio

• Phenotypic Ratio: 9:3:3:1
  • Round yellow, round green, wrinkled yellow, wrinkled green

Importance of Understanding

• Most traits are controlled by multiple genes
• Complexity increases with more genes (e.g., height)

Application

• Consider complexity when dealing with multiple gene traits
• Practice setting up correct gametes

Conclusion

• Understanding conceptually rather than just mechanically using Punnett squares is crucial
• Good grasp of monohybrid and dihybrid crosses aids in understanding genetic inheritance