Lecture Notes: Genetics - Black and White Mice Example

Introduction

• Context: Using black and white mice to explain genetic crosses.
• Alleles:
  • b1/b1 - Black (dominant)
  • b2/b2 - White (recessive)

Generations and Crosses

Parental Generation (P Generation)

• Characteristics: Pure breeding, homozygous
  • b1/b1 (Black)
  • b2/b2 (White)
• Gametes:
  • b1 from black individuals
  • b2 from white individuals
• Result: All progeny are b1/b2 (heterozygous)

First Filial Generation (F1 Generation)

• Genotype: All individuals b1/b2
• Phenotype: All black (since b1 is dominant)

Second Filial Generation (F2 Generation)

• Cross: Between two b1/b2 individuals from F1
• Genotypic Ratio:
  • 1 b1/b1
  • 2 b1/b2
  • 1 b2/b2
• Phenotypic Ratio:
  • 3 Black : 1 White
• Significance: Shows recessive traits (white) can skip generations.

Law of Equal Segregation (Mendel)

• Observation of F2 generation supports Mendel’s law.
• Alleles separate equally into gametes, ensuring 3:1 phenotypic ratio.

Test Cross

• Purpose: Determine if a black phenotype (b1/b1 or b1/b2) is homozygous dominant or heterozygous.
• Method:
  • Cross black individual with white individual (b2/b2)
• Scenarios:
  • If black is b1/b1: All progeny are b1/b2 (all black)
  • If black is b1/b2: Progeny are 1 b1/b2 (black) : 1 b2/b2 (white)
• Conclusion:
  • All black progeny suggest b1/b1
  • 1:1 black to white ratio suggests b1/b2

Tester

• Definition: The homozygous recessive individual (b2/b2) used in test crosses.

Summary

• Understanding genetic crosses through mice example.
• Importance of test crosses in determining genotypes.
• Observations align with Mendelian genetics principles.