Genetic Diagrams and Inheritance Patterns

Introduction

• Genetic diagrams help illustrate different combinations of alleles from two parents.
• Example used: A single gene determining muscularity in mice.
  • Dominant Allele: Capital 'A' for normal muscle.
  • Recessive Allele: Lowercase 'a' for muscular mass.

Representation of Alleles

• Alleles are represented with letters:
  • Uppercase letter = Dominant allele
  • Lowercase letter = Recessive allele
• Example Question: Draw a genetic diagram for a cross between a homozygous normal mouse and a homozygous muscular mouse.

Steps to Draw Genetic Diagrams

1. Find Parents' Phenotype and Genotype
   • Often provided in the question.
2. Determine Gametes' Genotypes
3. Find Offspring's Genotypes and Phenotypes

Worked Example

• Parent 1 (Normal): Homozygous dominant (AA)
  • Phenotype: Normal
• Parent 2 (Muscular): Homozygous recessive (aa)
  • Phenotype: Muscular
• Gametes: Split each parent's genotype to represent meiotic division.
• Offspring Genotypes: Combine gametes to find possible offspring genotypes.
  • All offspring in this example are heterozygous (Aa).
  • Phenotype for offspring: Normal (dominant trait expressed).

Punnett Squares

• Alternative diagram that provides a simple visualization.
• Process:
  1. Place one parent's gametes on top and the other's on the left.
  2. Fill in squares to show the combination of gametes.
• Advantages: Simple drawing.
• Disadvantages: Doesn't show phenotypes directly.

Example: Heterozygous Offspring

• If two heterozygous offspring (Aa) mate:
  • Use the Punnett square to predict offspring.
  • Result:
    • 1 Homozygous Dominant (AA)
    • 2 Heterozygous (Aa)
    • 1 Homozygous Recessive (aa)
• Phenotypes: 3 normal, 1 muscular
• Ratio/Probability: 3:1 ratio of normal to muscular mice, or 25% chance of muscular offspring.

Genetic Diagram Revisited

• Crossing two heterozygous mice (Aa):
  • Results in the same genotype and phenotype distribution as the Punnett square.
  • Shows the importance of both diagrams in predicting genetic outcomes.

Important Note

• Typically, traits are determined by multiple genes and environmental factors.
  • Example: Height influenced by nutrition and sleep, despite genetic potential.

Conclusion

• Understanding genetic diagrams and Punnett squares are crucial for predicting inheritance patterns.

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