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Why do siblings from the same parents have different traits? Why do some disorders skip generations? The answers are found in AP Biology Unit 5: Heredity.
Unit 5 is a key AP Biology topic covering meiosis, Mendel’s laws, pedigrees, inheritance patterns, and chi-square analysis. This guide explains every major concept with AP-style practice questions, model answers, and exam-focused vocabulary to help you score highe
AP Biology Unit 5: Heredity explains how traits pass from parents to offspring through genetic and cellular processes. It makes up 8–11% of the AP Biology exam and is divided into five key topics in the College Board CED.
| CED Topic | Title | Exam Relevance |
| 5.1 | Meiosis | High – meiosis stages show up in both MCQ and FRQ, and questions that ask for diagrams are common. |
| 5.2 | Meiosis and Genetic Diversity | High – crossing over, independent assortment, and nondisjunction are all very important to know. |
| 5.3 | Mendelian Genetics | Very High: Every year, you see Punnett squares, probability, pedigrees, and chi-square. |
| 5.4 | Non-Mendelian Genetics | High – linked genes, incomplete dominance, codominance, and sex-linkage are all things that are tested a lot. |
| 5.5 | Environmental Effects on Phenotype | Medium: conceptual; links genotype to phenotype in real-life situations |
The following table maps every Unit 5 CED topic to its key concepts, essential vocabulary, and the most common question formats in which they appear on the AP Biology exam.
| Topic | Key Concepts | Most-Tested On AP Exam |
| 5.1 Meiosis | Meiosis I and II, haploid and diploid cells, how they are different from mitosis, and how they form gametes | Questions about chromosome numbers, diagram identification, and stage-by-stage descriptions |
| 5.2 Genetic Diversity | Crossing over (prophase I), independent assortment (metaphase I), nondisjunction, and recombination | Explaining where variation comes from, predicting what will happen when nondisjunction happens, and genetic diversity multiple-choice questions |
| 5.3 Mendelian Genetics | Mendel’s laws, Punnett squares, test crosses, dihybrid crosses, probability rules, and chi-square | Calculating with the Punnett square, probability math, chi-square analysis, and pedigree interpretation |
| 5.4 Non-Mendelian | Incomplete/codominance, sex-linkage, linked genes, recombination frequency, pleiotropy, and epistasis | Figuring out non-Mendelian ratios, sex-linked pedigrees, and how to get map units from recombination data |
| 5.5 Environment & Phenotype | Norm of reaction; polygenic traits; fundamentals of epigenetics; impact of temperature; phenotypic plasticity. | Explaining why the same genotype can lead to different phenotypes; linking the environment to gene expression |
Meiosis produces gametes (sperm and eggs). It reduces chromosome number from diploid (2n) to haploid (n) so fertilization restores the correct diploid number.
| Stage | Key Event |
| Prophase I | Homologous chromosomes pair; crossing over occurs |
| Metaphase I | Homologous pairs line up randomly |
| Anaphase I | Homologous chromosomes separate |
| Telophase I | Two haploid cells form |
| Prophase II | No DNA replication before this stage |
| Metaphase II | Chromosomes line up individually |
| Anaphase II | Sister chromatids separate |
| Telophase II | Four haploid cells form |
| Feature | Mitosis | Meiosis |
| Divisions | 1 | 2 |
| Daughter Cells | 2 | 4 |
| Chromosome Number | Diploid | Haploid |
| Genetic Identity | Identical | Unique |
| Crossing Over | No | Yes |
A diploid organism (2n = 6) finishes Meiosis I. What cells are produced?
a. 2 diploid cells with 6 chromosomes
b. 4 haploid cells with 3 chromosomes
c. 2 haploid cells with 3 chromosomes, each still duplicated
d. 2 haploid cells with 6 chromosomes
✅ Answer: C
After Meiosis I, cells are haploid (n = 3), but sister chromatids are still joined.
Meiosis creates genetic diversity, which is essential for natural selection and evolution.
| Mechanism | What Happens | Result |
| Crossing Over | During Prophase I, homologous chromosomes swap DNA. | Combinations of new alleles |
| Independent Assortment | In Metaphase I, chromosome pairs line up at random. | Different combinations of chromosomes |
| Random Fertilization | Any sperm can join with any egg. | A lot of genetic differences in the children |
In humans, 2²³ = 8.4 million possible gametes from independent assortment alone.
Failure of chromosomes to separate properly causes aneuploidy.
| When It Happens | Result |
| Meiosis I | All gametes abnormal |
| Meiosis II | Some normal, some abnormal |
Mendel’s laws explain how traits are inherited and are heavily tested on AP Biology.
| Law | Meaning |
| Segregation | Alleles separate during gamete formation |
| Independent Assortment | Genes on different chromosomes assort independently |
| Dominance | Dominant allele masks recessive in heterozygotes |
| Cross | Result |
| Aa × Aa | 3:1 phenotype, 1:2:1 genotype |
| AaBb × AaBb | 9:3:3:1 phenotype |
| Test Cross | Unknown × recessive parent |
A 1:1:1:1 offspring ratio usually means a test cross with a heterozygote.
Used to compare observed vs expected results.
Not all traits follow simple dominant-recessive inheritance. These exceptions are common on AP Biology exams.
| Pattern | Meaning | Example |
| Incomplete Dominance | Heterozygote shows blended trait | Red × White flowers = Pink |
| Codominance | Both alleles fully expressed | AB blood type |
| X-Linked Inheritance | Gene on X chromosome; males affected more often | Color blindness, hemophilia |
| Linked Genes | Genes on same chromosome inherited together | Low recombination frequency |
| Pleiotropy | One gene affects many traits | Sickle cell disease |
| Polygenic Inheritance | Many genes control one trait | Height, skin color |
| Epistasis | One gene masks another | Labrador coat color |
Phenotype is influenced by both genes and environment. The same genotype can produce different traits under different conditions.
| Concept | Meaning | Example |
| Norm of Reaction | Range of phenotypes from one genotype in different environments | Height affected by nutrition |
| Phenotypic Plasticity | Organism changes phenotype with environment | Daphnia develop helmets near predators |
| Temperature-Sensitive Genes | Gene products work differently at certain temperatures | Himalayan rabbit fur color |
| Epigenetics | Environment changes gene expression without changing DNA | Stress, diet, toxins |
| Polygenic + Environment | Many genes + environment shape trait | Skin color + sun exposure |
Identical twins can look different because of:
Q1. Crossing over increases genetic diversity by:
a. Increasing chromosome number
b. Creating new allele combinations between non-sister chromatids
c. Making sister chromatids identical
d. Doubling DNA before meiosis
Answer: B
Crossing over in Prophase I exchanges DNA between homologous chromosomes.
Q2. A carrier mother (XᴬXᵃ) and unaffected father (XᴬY). Chance an affected son?
a. 0%
b. 25% of all children, 50% of sons
c. 100% of sons
d. 50% of daughters
Answer: B
Each son has a 50% chance of receiving the recessive allele.
Q3. Red flowers (RR) × White flowers (rr) produce all pink offspring. This is:
a. Codominance
b. Incomplete dominance
c. Complete dominance
d. Epistasis
Answer: B
Pink is an intermediate phenotype, showing incomplete dominance.
FRQ 1: Meiosis & Diversity (4 pts)
How does meiosis create genetic diversity?
Answer:
FRQ 2: Pedigree Analysis (4 pts)
Two unaffected parents have an affected child. What inheritance pattern is likely?
Answer:
FRQ 3: Genetics Cross (10 pts)
Female heterozygous cross with recessive male. Find offspring ratios.
Answer:
Chi-square (χ²) analysis appears on AP Biology exams as both an MCQ and FRQ topic. You must be able to calculate χ², determine degrees of freedom, compare to the critical value, and interpret the result. Here is the complete procedure.
| Step | Action | Example (Monohybrid cross, 200 offspring) |
| 1. State hypothesis | H₀: observed results fit the expected Mendelian ratio | H₀: 3:1 ratio is expected (150 dominant : 50 recessive) |
| 2. Determine expected | E = expected ratio × total N | Dominant: (3/4)×200 = 150; Recessive: (1/4)×200 = 50 |
| 3. Calculate χ² | χ² = Σ(O-E)²/E for each class | O=160, E=150: (160-150)²/150 = 0.67; O=40, E=50: (40-50)²/50 = 2.0; χ² = 2.67 |
| 4. Degrees of freedom | df = number of phenotypic classes − 1 | df = 2 − 1 = 1 |
| 5. Compare to critical value | Critical value (p=0.05, df=1) = 3.84 | 2.67 < 3.84 |
| 6. Interpret | If χ² < critical value → fail to reject H₀ (data fit the expected ratio) | The observed data ARE consistent with a 3:1 Mendelian ratio |
Q: How much of the AP Biology exam is Unit 5?
A: Unit 5 (Heredity) makes up 8–11% of the exam. It often appears in MCQs and short FRQs.
Q: What are the hardest Unit 5 topics?
A: Linked genes, recombination frequency, pedigrees, nondisjunction, and chi-square analysis are commonly missed topics.
Q: Difference between incomplete dominance and codominance?
A: Incomplete dominance shows a blended trait (red + white = pink). Codominance shows both traits fully (AB blood type).
Q: How do you know if genes are linked?
A: If parental traits appear more often than recombinant traits and recombination is below 50%, the genes are linked.
Q: Should I use Punnett squares on FRQs?
A: Yes. Use clear Punnett squares for inheritance ratios, label parents, and state genotype/phenotype ratios clearly.
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