Unit 5: Heredity
AP Biology — 108 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 5: Heredity explores how genetic information is transmitted from one generation to the next, anchoring the mechanisms of reproduction in molecular reality and quantitative prediction. The unit begins with meiosis, a reductional division that halves chromosome number to produce haploid gametes, and proceeds through the ways in which this process—along with fertilization—generates the genetic variation upon which evolution acts. Students then examine Mendel's laws of segregation and independent assortment, learning to model inheritance patterns using Punnett squares and probability logic. The curriculum extends beyond classical Mendelian ratios to encompass non-Mendelian phenomena: incomplete dominance, codominance, multiple alleles, epistasis, pleiotropy, polygenic inheritance, and sex-linked traits. Environmental influences on phenotype and the principles of chromosomal inheritance—including linkage, crossing over, and chromosomal abnormalities—round out the unit, equipping students to reason about heredity in both mechanistic and statistical terms.
Molecular Deep-Dive
Meiosis is the cornerstone of sexual reproduction and a central focus of this unit. Unlike mitosis, which produces genetically identical somatic cells through a single division, meiosis involves two successive nuclear divisions—meiosis I and meiosis II—that reduce the chromosome number from diploid (2n) to haploid (n). During prophase I, homologous chromosomes pair up in a process called synapsis, forming tetrads (bivalents) held together by the synaptonemal complex. It is within this intimate alignment that crossing over occurs: non-sister chromatids exchange corresponding segments of DNA at chiasmata. This recombination shuffles alleles between maternal and paternal homologs, producing recombinant chromatids that carry novel combinations of genes. The molecular machinery of crossing over involves programmed double-strand breaks catalyzed by the Spo11 enzyme, followed by repair through homologous recombination pathways that physically link non-sister chromatids.
Meiosis I is termed the reductional division because homologous chromosomes—each consisting of two sister chromatids—separate to opposite poles. The orientation of each homologous pair at the metaphase plate is random, a phenomenon known as independent assortment. For an organism with n pairs of chromosomes, the number of possible combinations of maternal and paternal chromosomes distributed to gametes is 2^n. In humans (n = 23), this yields over 8 million possible gamete configurations from assortment alone, even before accounting for crossing over. The combined effect of recombination and independent assortment is the mechanistic basis for the vast genetic diversity observed in sexually reproducing populations.
Mendelian genetics builds on the behavior of chromosomes during meiosis. Mendel's law of segregation states that the two alleles of a gene separate (segregate) during gamete formation, with each gamete receiving only one allele. This directly mirrors the separation of homologous chromosomes during anaphase I. Mendel's law of independent assortment holds that alleles of different genes assort independently of one another, a principle that holds true only when genes are located on different chromosomes or are sufficiently far apart on the same chromosome. When genes are closely linked on the same chromosome, they tend to be inherited together, violating independent assortment. Recombination frequency—the proportion of recombinant offspring—can be used to map the relative distances between linked genes, with a recombination frequency of 1% corresponding approximately to one map unit (centimorgan).
Non-Mendelian genetics introduces patterns that deviate from simple dominant/recessive relationships. In incomplete dominance, heterozygotes exhibit an intermediate phenotype (e.g., pink flowers from red and white parents), while in codominance, both alleles are fully expressed (e.g., AB blood type). Epistasis occurs when the expression of one gene is modified or masked by a second gene, as seen in Labrador retriever coat color, where the E gene determines whether pigment is deposited and the B gene determines whether that pigment is black or brown. Polygenic inheritance involves multiple genes contributing additively to a single phenotypic trait, producing continuous variation (e.g., human height). Pleiotropy describes a single gene affecting multiple phenotypic traits simultaneously.
Environmental effects on phenotype highlight that genotype alone does not determine phenotype. The Himalayan rabbit phenotype, for example, results from a temperature-sensitive mutation in the tyrosinase gene: the enzyme is inactive at core body temperature but active in cooler extremities, producing dark pigment only on ears, nose, feet, and tail. This illustrates how environmental conditions can modulate enzyme activity and gene expression, resulting in phenotypic plasticity.
Chromosomal inheritance encompasses the transmission of whole chromosomes and the consequences of their abnormal segregation. Nondisjunction—the failure of homologous chromosomes or sister chromatids to separate properly during meiosis—produces gametes with abnormal chromosome numbers (aneuploidy). Examples include trisomy 21 (Down syndrome), Turner syndrome (XO), and Klinefelter syndrome (XXY). Sex-linked inheritance patterns, particularly X-linked traits such as color blindness, arise because males have only one X chromosome and thus express whatever allele is present on it, making recessive X-linked traits more frequently expressed in males.
AP Exam Trap (FRQ)
Interactive Glossary
| Term | Definition |
|---|---|
| ------ | ------------ |
| Meiosis | A specialized form of cell division in sexually reproducing organisms that reduces the chromosome number from diploid to haploid, producing four genetically distinct gametes. It involves one round of DNA replication followed by two successive nuclear divisions called meiosis I and meiosis II. |
| Homologous Chromosomes | A pair of chromosomes in a diploid organism, one inherited from each parent, that carry the same genes in the same order but may carry different alleles. They pair up during prophase I of meiosis and undergo crossing over. |
| Crossing Over | The reciprocal exchange of DNA segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis. This process produces recombinant chromatids with new combinations of alleles, increasing genetic diversity. |
| Synapsis | The precise alignment and pairing of homologous chromosomes during prophase I of meiosis, facilitated by the synaptonemal complex. This intimate pairing is required for crossing over to occur. |
| Chiasma | The physical point of contact between two non-sister chromatids where crossing over has occurred, visible as an X-shaped structure under microscopy. Chiasmata are essential for proper segregation of homologous chromosomes during meiosis I. |
| Law of Segregation | Mendel's first law, stating that the two alleles for a gene separate during gamete formation so that each gamete carries only one allele. The fusion of two gametes during fertilization restores the diploid condition with two alleles per gene. |
| Law of Independent Assortment | Mendel's second law, stating that alleles of different genes assort independently during gamete formation when those genes are on different chromosomes or far apart on the same chromosome. This principle generates new combinations of alleles in offspring. |
| Punnett Square | A grid-like diagram used to predict the genotypes and phenotypes of offspring from a genetic cross by systematically combining all possible gamete types from each parent. Each cell in the grid represents an equally likely zygotic genotype. |
| Incomplete Dominance | An inheritance pattern in which heterozygous individuals exhibit a phenotype intermediate between the two homozygous phenotypes, such as pink flowers from red and white parents. Neither allele is fully dominant over the other. |
| Codominance | An inheritance pattern in which both alleles in a heterozygote are fully expressed simultaneously in the phenotype, resulting in a phenotype showing both traits. The classic example is the AB blood type in humans. |
| Epistasis | A gene interaction in which the phenotypic expression of one gene is suppressed or modified by a second gene located at a different locus. The modifying gene is called the epistatic gene, and the gene being modified is the hypostatic gene. |
| Pleiotropy | A phenomenon in which a single gene influences multiple, seemingly unrelated phenotypic traits. An example is the PKU gene, which affects both skin pigmentation and neurological development when mutated. |
| Polygenic Inheritance | An inheritance pattern in which multiple genes contribute additively to a single phenotypic trait, producing a continuous range of variation. Human height and skin color are classic examples of polygenic traits. |
| Linked Genes | Genes located on the same chromosome that tend to be inherited together because they do not assort independently during meiosis. The closer two genes are on a chromosome, the lower the probability of crossing over occurring between them. |
| Recombination Frequency | The proportion of recombinant offspring produced in a genetic cross, expressed as a percentage, which reflects the likelihood of crossing over between two loci. A recombination frequency of 1% corresponds to approximately one map unit on a genetic map. |
| Nondisjunction | The failure of homologous chromosomes to separate properly during meiosis I or of sister chromatids to separate during meiosis II, resulting in gametes with abnormal chromosome numbers. Nondisjunction can lead to aneuploid conditions such as trisomy and monosomy. |
| Aneuploidy | A chromosomal condition in which an organism has an abnormal number of chromosomes, typically resulting from nondisjunction during meiosis. Down syndrome (trisomy 21) is one of the most common aneuploid conditions in humans. |
| Sex-linked Inheritance | A pattern of inheritance in which a gene is located on a sex chromosome, most commonly the X chromosome in mammals. X-linked recessive traits are expressed more frequently in males because they have only one X chromosome. |
| Carrier | An individual who is heterozygous for a recessive allele and does not display the phenotype but can pass the allele to offspring. Carriers are particularly significant in the inheritance of autosomal recessive and X-linked recessive disorders. |
| Phenotypic Plasticity | The ability of a single genotype to produce different phenotypes under different environmental conditions. This concept highlights that phenotype is the product of both genotype and environment interacting during development. |
| Test Cross | A genetic cross between an individual expressing a dominant phenotype (unknown genotype) and a homozygous recessive individual to determine whether the dominant individual is homozygous or heterozygous. The phenotypic ratios of the offspring reveal the unknown genotype. |
| Dihybrid Cross | A genetic cross between two individuals that are heterozygous for two different genes, typically producing a 9:3:3:1 phenotypic ratio in the offspring when the genes assort independently. This ratio is modified when linkage or epistasis is involved. |
Quantitative Skill-Set
The chi-square (χ²) goodness-of-fit test is the central quantitative tool for Unit 5, allowing you to determine whether observed genetic data are consistent with expected Mendelian ratios.
The formula:
χ² = Σ((O − E)² / E)
Where O = observed frequency for each category, E = expected frequency for each category, and Σ indicates summation across all categories.
Degrees of freedom: df = (number of categories) − 1. For a monohybrid cross with two phenotypic classes (dominant and recessive), df = 1. For a dihybrid cross with four phenotypic classes, df = 3.
Interpreting results: Compare your calculated χ² value to the critical value from the chi-square distribution table at the appropriate degrees of freedom. On the AP Exam, the standard significance level is α = 0.05 (sometimes written as p = 0.05).
Example: In a monohybrid cross, you expect a 3:1 ratio among 100 offspring (75 dominant, 25 recessive). You observe 68 dominant and 32 recessive.
χ² = ((68−75)²/75) + ((32−25)²/25) = (49/75) + (49/25) = 0.653 + 1.96 = 2.613
With df = 1, the critical value at p = 0.05 is 3.841. Since 2.613 < 3.841, you fail to reject the null hypothesis. The observed data are consistent with the expected 3:1 ratio.
Common error: Students sometimes calculate expected values incorrectly. The expected number for each category is the total number of offspring multiplied by the expected proportion for that category, NOT the total divided by the number of categories.
Study Moves
Exam Linkage
AP Biology FRQs frequently use the task verbs explain, predict, calculate, justify, and identify in the context of heredity questions. When asked to explain an inheritance pattern, graders expect a mechanistic chain connecting meiotic events to phenotypic ratios—do not simply state the ratio without describing the underlying chromosomal behavior. For predict questions, generate specific genotype and phenotype ratios using Punnett squares and state them as fractions or percentages. When asked to justify a conclusion about whether data fit a particular inheritance pattern, use the chi-square test explicitly: state the null hypothesis, show the calculation, compare χ² to the critical value, and interpret the result. Graders reward mechanistic precision—connecting crossing over in prophase I to recombination frequency, or connecting nondisjunction to aneuploid karyotypes—rather than vague statements about "random chance" or "genetic variation." Always define your symbols (O, E, df, p) and never simply write a number without units or context.