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:
- Meiosis and genetic variation
- Inheritance patterns and Mendelian genetics
- 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
- Master Punnett Squares: Practice mono-, di-, and test crosses.
- Understand Chromosome Behavior: Relate meiosis stages to Mendel’s laws.
- Do Probability Problems: Practice with both product and sum rules.
- 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.
