Unit 5: Heredity explores how traits are passed from one generation to the next. It’s a high-yield section of the AP Biology Exam, making up about 8–11% of test content. Understanding heredity means mastering both classical Mendelian genetics and the molecular mechanisms behind inheritance.

In this RevisionDojo Unit 5 Review, we’ll cover:

• Meiosis and genetic variation
• Mendel’s laws and probability rules
• Types of inheritance patterns
• Chromosomal basis of inheritance
• Common AP exam question types
• Study strategies for maximum score gain

Unit 5 Overview

The College Board organizes Unit 5 into three core ideas:

1. Meiosis and genetic variation
2. Inheritance patterns and Mendelian genetics
3. Chromosomal inheritance and linkage

1. Meiosis: The Foundation of Genetic Variation

Purpose: To produce gametes (sperm and eggs) with half the chromosome number of the parent cell.

Stages:

• Meiosis I:
  • Prophase I – crossing over between homologous chromosomes
  • Metaphase I – homologous pairs line up at equator
  • Anaphase I – separation of homologs
  • Telophase I – two haploid cells form
• Meiosis II:
  • Similar to mitosis, separates sister chromatids

Sources of Variation:

• Crossing over (recombination)
• Independent assortment
• Random fertilization

2. Mendelian Genetics

Mendel’s Laws:

• Law of Segregation: Each individual has two alleles for each gene; alleles separate during gamete formation.
• Law of Independent Assortment: Alleles of different genes are distributed independently during gamete formation (except linked genes).

Vocabulary:

• Genotype: Genetic makeup (e.g., Aa)
• Phenotype: Physical trait (e.g., tall)
• Homozygous: Same alleles (AA or aa)
• Heterozygous: Different alleles (Aa)
• Dominant: Expressed when at least one allele is present
• Recessive: Only expressed when homozygous

3. Probability in Genetics

• Product rule: Probability of two independent events = multiply probabilities
• Sum rule: Probability of either event happening = add probabilities
• Use Punnett squares to predict offspring genotypes/phenotypes

4. Inheritance Patterns Beyond Mendel

• Incomplete dominance: Heterozygote is intermediate (red + white = pink flowers)
• Codominance: Both alleles expressed equally (AB blood type)
• Multiple alleles: More than two possible alleles in a population (e.g., ABO blood group)
• Polygenic inheritance: Multiple genes affect a trait (skin color, height)
• Sex-linked traits: Genes on sex chromosomes (e.g., hemophilia on X chromosome)

5. Chromosomal Basis of Inheritance

• Linked genes: Found close together on same chromosome, often inherited together
• Crossing over frequency: Used to create genetic maps
• Nondisjunction: Failure of chromosomes to separate; can cause Down syndrome, Turner syndrome

6. Connections to Other Units

• Unit 4: Cell cycle regulation during meiosis
• Unit 6: Molecular basis of gene expression after inheritance
• Unit 7: Natural selection acting on inherited traits

Common AP Biology Exam Questions for Unit 5

Multiple Choice Examples:

• Calculate genotype ratios from a dihybrid cross
• Predict effects of nondisjunction in gametes

FRQ Examples:

• Explain how crossing over increases genetic diversity
• Use data from a test cross to determine recombination frequency

Study Tips for Unit 5

1. Master Punnett Squares: Practice mono-, di-, and test crosses.
2. Understand Chromosome Behavior: Relate meiosis stages to Mendel’s laws.
3. Do Probability Problems: Practice with both product and sum rules.
4. Review Pedigrees: Be able to identify inheritance patterns from family trees.

Frequently Asked Questions

1. How much of the AP Bio exam is Unit 5?
Around 8–11%.

2. Do I need to memorize all chromosome disorders?
No—know examples and the process causing them (e.g., nondisjunction).

3. Are dihybrid crosses tested every year?
Yes, often with a twist like incomplete dominance or linkage.

4. Is meiosis tested more in Unit 5 or Unit 4?
Unit 5 focuses on meiosis for variation, Unit 4 for cell cycle basics.

Call to Action

Unit 5 builds your genetics foundation—mastering it unlocks high-value points in MCQs and FRQs.

👉 Use RevisionDojo’s AP Bio problem sets and practice FRQs to strengthen your genetics skills and move closer to a 5.