AP Biology Unit 5: Heredity — Mastering Mendelian and Non-Mendelian Genetics

TLDR (Quick Summary)

• Mendel’s Laws: Segregation and Independent Assortment form the foundation of inheritance.

• Use Punnett squares + probability rules (product & sum) to predict outcomes.

• Non-Mendelian patterns (incomplete dominance, codominance, polygenic traits, sex-linkage) often appear on the exam.

• Pedigrees and chi-square tests test your ability to analyze data and justify claims.

• Download our AP Biology Unit 5 Cheat Sheet above for a one-page exam-ready summary.

Why Unit 5 Matters

Unit 5 takes you from “genes are inherited” to predicting how traits will actually show up in populations.
This unit is exam-heavy with Punnett squares, chi-square calculations, and pedigree analysis. If you master these skills, you’ll be ready for both multiple-choice and free-response genetics questions.

Mendelian Genetics: The Foundation

• Law of Segregation: Each gamete gets only one allele.

• Law of Independent Assortment: Alleles for different genes sort independently.

• Ratios to memorize:

  • Monohybrid → 3:1 phenotype.

  • Dihybrid → 9:3:3:1 phenotype.

Mnemonic: “Segregation separates, Assortment assigns.”

Probability & Punnett Squares

Punnett squares are fast, but probability shortcuts can save time:

• Product Rule = chance of A AND B → multiply.

• Sum Rule = chance of A OR B → add.

👉 Tutor Tip: On FRQs, always state the expected ratio first before crunching numbers. It shows you understand the biology, not just the math.

Beyond Mendel: Non-Mendelian Inheritance

• Incomplete dominance: blended phenotype (red × white = pink).

• Codominance: both alleles fully visible (spotted cow, AB blood type).

• Polygenic traits: multiple genes → continuous variation (height, skin color).

• Epistasis: one gene masks another (e.g. coat color in labs).

• Sex-linked traits: usually X-linked, seen more in males (color blindness, hemophilia).

Mnemonic: “Incomplete = in-between. Codominant = co-appear.”

Pedigrees Made Simple

• Square = male, Circle = female, Shaded = affected.

• Look for skipping generations (often recessive) or traits more common in males (often sex-linked).

Chi-Square: Data Meets Genetics

Use chi-square to test if observed results match expected ratios.

Formula: χ² = Σ((observed – expected)² / expected).

• Degrees of freedom = categories – 1.

• p > 0.05 → fits expected; p < 0.05 → reject null.

Think of it as: Observed → Expected → Compare.

Common Pitfalls

• Mixing up incomplete dominance vs codominance.

• Forgetting to apply probability rules.

• Mislabeling pedigrees (esp. autosomal vs sex-linked).

• Not stating null hypothesis in chi-square problems.

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