Punnett Squares — Flashcards & Quiz

Key Points

• Monohybrid cross: single trait, heterozygous parents (Aa × Aa) → 1 AA : 2 Aa : 1 aa genotype ratio; 3:1 phenotype ratio.
• Dihybrid cross: two independent traits, heterozygous parents (AaBb × AaBb) → 9:3:3:1 phenotype ratio.
• Test cross: crossing with a homozygous recessive parent reveals the unknown genotype of the other parent.
• Probability: the chance of a specific genotype or phenotype; combine using product rule (AND) and sum rule (OR).
• Sex-linked traits: males (XY) have only one X chromosome, so they express any X-linked recessive allele they carry.
• Pedigree analysis: look for patterns — appears in every generation (dominant), skips generations (recessive), mostly males (X-linked recessive).

Common Mistakes to Avoid

1. Setting up the Punnett square with the wrong parental gametes — always list all possible gametes from each parent.
2. Mixing up ratios for monohybrid (3:1) and dihybrid (9:3:3:1) crosses.
3. Forgetting that test crosses require a homozygous recessive partner.
4. Misreading pedigrees — square = male, circle = female, filled = affected.
5. Applying autosomal rules to sex-linked traits — always check which chromosome the gene is on.

Exam Strategy

SCSA Unit 3 genetics questions give you parental genotypes or a pedigree and ask you to predict offspring or identify the inheritance pattern. Method for crosses: list gametes, draw the Punnett square, calculate ratios. Method for pedigrees: check generation pattern, sex ratio, and parents of affected individuals to identify the mode of inheritance.

Revision Tip

Punnett squares are drillable — build a Revizi deck with 10+ crosses covering monohybrid, dihybrid, sex-linked and test crosses for fluency under exam conditions.