AP Biology, AP exams, 8–11%

Unit 5: Heredity

Unit 5 covers meiosis as the source of genetic variation and the inheritance patterns that follow from it. Mendelian ratios, sex linkage, linkage and recombination, and the ways environment modifies phenotype all appear. Chi-square analysis of observed versus expected offspring counts is a recurring quantitative task.

Position: 5 of 8 units
Published weighting: 8–11% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Biology. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Meiosis: The two divisions, the reduction from diploid to haploid, and how meiosis differs from mitosis at each stage.
Meiosis and genetic diversity: Crossing over, independent assortment, and random fertilization as the three sources of variation.
Mendelian genetics: Segregation and independent assortment, monohybrid and dihybrid crosses, and pedigree interpretation.
Non-Mendelian genetics: Incomplete dominance, codominance, multiple alleles, polygenic traits, sex linkage, and linked genes.
Environmental effects on phenotype: How the same genotype can produce different phenotypes depending on environmental conditions.
Chromosomal inheritance: Nondisjunction and other chromosomal errors, and how they connect to observed inheritance patterns.

Study tips

How to practice Heredity.

Section-specific guidance that follows from how this material is usually tested.

Run chi-square problems with the provided table until degrees of freedom and the reject-or-fail-to-reject conclusion take under a minute.
Diagram meiosis and mitosis side by side once a week; the comparison is a frequent short free-response prompt.
Translate every cross into a testable prediction before touching a Punnett square; setup errors cost more than arithmetic errors.

Where this fits

Unit 5: Heredity is 5 of 8 units in the AP Biology guide, and it carries a published weighting of 8–11% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Biology guide Official exam information

Sample Heredity questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
A human somatic cell contains 46 chromosomes. How many chromosomes does a normal human gamete contain?
1. A22
2. B23
3. C46
4. D92
Show answer
Answer: B. 23
Meiosis reduces the chromosome number by half, so a diploid cell with 46 chromosomes produces haploid gametes with 23. Answering 46 confuses a gamete with a body cell, and 92 is the number of chromatids present after replication rather than a number of chromosomes.
Key insight: Fertilization restores the diploid number, so gametes must carry exactly half of it.
Question 2Foundational
During which stage of meiosis do homologous chromosomes pair up and exchange corresponding segments in crossing over?
1. AMetaphase I
2. BAnaphase II
3. CTelophase II
4. DProphase I
Show answer
Answer: D. Prophase I
Homologs synapse in prophase I and form chiasmata where segments are exchanged, producing recombinant chromatids. Metaphase I is the tempting distractor because homologs are still paired then, but they are lining up on the metaphase plate rather than exchanging DNA. By meiosis II the homologs have already separated, so crossing over is no longer possible.
Key insight: Crossing over in prophase I is one of three sources of variation, along with independent assortment and random fertilization.
Question 3Intermediate
Two organisms heterozygous for the same completely dominant gene are crossed. What fraction of their offspring is expected to show the recessive phenotype?
1. A1/16
2. B1/4
3. C1/2
4. D3/4
Show answer
Answer: B. 1/4
A cross between two heterozygotes produces a 1:2:1 genotype ratio, and only the homozygous recessive quarter shows the recessive phenotype. The fraction 3/4 is the tempting distractor because it is the share showing the dominant phenotype. The fraction 1/16 would apply to a dihybrid cross in which both traits are recessive.
Key insight: A monohybrid cross of two heterozygotes gives a 3:1 phenotype ratio, so recessives are one in four.
Question 4Intermediate
A woman who carries one allele for X-linked red-green color blindness has children with a man who has normal color vision. What fraction of their sons is expected to be color blind?
1. A1/2
2. B1/4
3. C0
4. DAll of them
Show answer
Answer: A. 1/2
Sons receive their only X chromosome from their mother, and half of her X chromosomes carry the recessive allele, so half of the sons are expected to be affected. The fraction 1/4 is the tempting distractor because it is the share of all children who are affected sons, not the share of sons. No daughters are affected, since each receives a normal allele on the father's X.
Key insight: For X-linked recessive traits, look only at the mother's X chromosomes when predicting sons.
Question 5Advanced
A cross predicted to give a 3 to 1 phenotype ratio produces 80 offspring: 52 with the dominant phenotype and 28 with the recessive phenotype. Using the sum of the squared difference between observed and expected divided by expected, what is the chi-square value?
1. AAbout 1.07
2. BAbout 3.20
3. CAbout 4.27
4. DAbout 6.40
Show answer
Answer: C. About 4.27
Expected counts are 60 dominant and 20 recessive. The dominant term is 64 divided by 60, which is 1.07, and the recessive term is 64 divided by 20, which is 3.20, so the total is 4.27. Answering 3.20 uses only the category with the larger contribution and ignores the other half of the sum; with one degree of freedom the critical value at the 0.05 level is 3.84, so this result rejects the null hypothesis.
Key insight: Chi-square adds a term for every phenotype category, and degrees of freedom equal the number of categories minus one.

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