Solving Pedigree Questions: An Effective Algorithm

Introduction

• Purpose: Share a method to solve pedigree questions, especially useful for USMLE exams.
• Focus: Determine the mode of inheritance using a simple algorithm.

Key Steps in the Algorithm

Step 1: Evaluate the Parents

• Question: Are the parents affected?
  • If parents are free:
    • The condition is recessive.
  • If one or both parents are affected:
    • The condition is dominant.

Step 2: Determine Recessive Conditions

• If Recessive:
  • Examine the offspring.
  • Autosomal Recessive: Affects both males and females.
  • X-linked Recessive: Predominantly affects males.

Step 3: Determine Dominant Conditions

• If Dominant:
  • Check for male-to-male transmission.
  • Autosomal Dominant: Male-to-male transmission exists (father to son).
  • X-linked Dominant: No male-to-male transmission.

Applying the Algorithm with Examples

• Example 1:

  • Parents Free: Recessive condition.
  • Offspring: Only males affected.
  • Conclusion: X-linked recessive.

• Example 2:

  • Parents Free: Recessive condition.
  • Offspring: Both males and females affected.
  • Conclusion: Autosomal recessive.
  • Genetic Disorders Insight:
    • Structural defects: Autosomal dominant.
    • Functional/enzymatic defects: Autosomal recessive.
  • Example Conditions:
    • Classic Galactosemia: Autosomal recessive.
    • Hemophilia B: X-linked.
    • Huntington Disease: Autosomal dominant.
    • Leber Hereditary Optic Neuropathy: Mitochondrial.

Conclusion

• The algorithm simplifies the process of determining the mode of inheritance.
• Helps to cut down time when solving pedigree questions.
• Encourages focus on the relationship between parents and offspring.

Additional Tips

• Remember structural vs. functional defects in genetic disorders.
• Use real questions to practice and reinforce the algorithm.