AP Syllabus focus:
‘Pedigrees and probability equations, such as P(A or B) and P(A and B), help predict genotypes and phenotypes.’
Pedigrees translate family history into genetic predictions. By combining careful symbol-based interpretation with core probability rules, you can infer likely genotypes, identify carriers, and estimate the chance of specific offspring phenotypes.
Pedigrees as genetic models
A pedigree is a simplified model of inheritance across generations, where individuals are connected by mating lines and descent lines.
Practice Questions
FAQ
Treat “carrier” as event $A$ and “unaffected child” as event $B$, then update using conditional probability: compare $P(B|A)$ to $P(B|\text{not }A)$. Renormalise so updated probabilities sum to 1.
They are independent only after conditioning on fixed parental genotypes. If parental genotypes are uncertain, siblings’ outcomes provide shared information, so probabilities become dependent until you condition on each genotype case.
Introduce events for each unknown (e.g., “individual is affected”) and carry multiple genotype scenarios forward. Use $P(A \text{ or } B)$ to combine mutually exclusive scenarios, weighting by how consistent each is with observed relatives.
Adding two pathways that are not mutually exclusive (both can happen) without subtracting $P(A \cap B)$. This inflates the final probability beyond the true value.
Define events as biological checkpoints:
genotype state (e.g., “is a carrier”)
transmission (e.g., “passes the allele”)
resulting phenotype
Then combine checkpoints with “and,” and combine alternative genotype pathways with “or.”
