Genetics: Probability Rules and Pedigree Analysis

Probability Rules in Genetics

• Product Rule (AND Rule):

  • Used for independent events where both Event 1 AND Event 2 occur.

  • Probability of (Event 1 AND Event 2) = P(Event 1) * P(Event 2).

  • Example: Rolling a 4 on a red die (1/6) AND a 3 on a gray die (1/6) = (1/6) * (1/6) = 1/36.

• Sum Rule (OR Rule):

  • Used for mutually exclusive events where Event 1 OR Event 2 occurs.

  • Probability of (Event 1 OR Event 2) = P(Event 1) + P(Event 2).

  • Example: Rolling (4 on red AND 3 on gray) OR (3 on red AND 4 on gray) = (1/36) + (1/36) = 2/36 = 1/18.

Branch Diagrams for Complex Crosses

• Simplifies analysis of multiple traits without large Punnett squares.

• Break down complex crosses into single-gene Punnett squares.

• Example: Predicting gender and eye color in F1 generation.

  • Gender: 1/2 female, 1/2 male.

  • Eye Color (heterozygous x heterozygous): 3/4 wild type, 1/4 vermillion.

  • Combine probabilities using the Product Rule (e.g., probability of male AND wild type eyes = (1/2) * (3/4) = 3/8).

• Total probabilities of all outcomes should sum to 1.

Pedigree Analysis

• Examines family histories to infer inheritance patterns and genotypes.

• Symbols:

  • Square: Male

  • Circle: Female

  • Filled symbol: Affected individual

  • Unfilled symbol: Unaffected individual

  • Horizontal line: Mating

  • Vertical line: Descent

• Key Assumption for Rare Traits:

  • Individuals marrying into the family (outside the pedigree) are assumed to be homozygous dominant for the trait, unless empirical data (their children) proves otherwise.

  • This initial assumption can be adjusted based on offspring phenotypes.

• Inferring Inheritance Patterns:

  • Recessive Traits: Often