What you'll learn
Heredity is the passing of traits from parents to offspring, and it is one of the central topics of AP Biology (Unit 5). For AP Biology you need to understand meiosis and how it generates variation, the laws of inheritance, how to use Punnett squares and probability, the patterns of non-Mendelian inheritance, and how environment and chromosomal errors affect phenotype. This guide covers meiosis, Mendel's laws, inheritance patterns, and the sources of genetic variation. By the end you should be able to predict the outcomes of genetic crosses, explain how variation arises, and apply the science practices of probability and analysis expected on the exam.
Key terms and definitions
Heredity — The passing of genetic information from parents to offspring.
Meiosis — Cell division that produces gametes with half the number of chromosomes.
Allele — A version of a gene.
Genotype — The alleles an organism has for a trait.
Phenotype — The observable characteristics resulting from the genotype and environment.
Homozygous / heterozygous — Having two identical / two different alleles for a gene.
Law of segregation — The two alleles for a trait separate during gamete formation.
Law of independent assortment — Alleles of different genes assort independently of one another.
Core concepts
Meiosis and the source of variation
Meiosis produces gametes (sex cells) with half the number of chromosomes, so that fertilization restores the full number. Meiosis is important not only for reducing the chromosome number but for generating genetic variation in three ways:
- Crossing over — homologous chromosomes exchange segments during prophase I, creating new combinations of alleles.
- Independent assortment — homologous pairs line up randomly at the metaphase plate, so gametes get different mixtures of maternal and paternal chromosomes.
- Random fertilization — any sperm can fertilize any egg, multiplying the variation.
This variation is the raw material for evolution, which is why heredity connects directly to natural selection.
Mendel's laws
Gregor Mendel's work established the basic rules of inheritance:
- The law of segregation states that the two alleles for a trait separate during gamete formation, so each gamete carries only one allele.
- The law of independent assortment states that the alleles of different genes are distributed to gametes independently of one another (for genes on different chromosomes).
These laws are the basis for predicting the outcomes of genetic crosses.
Genotype, phenotype and Punnett squares
An organism's genotype (its alleles) determines its phenotype (observable traits), together with the environment. A Punnett square predicts the probability of each genotype and phenotype in the offspring of a cross. For a simple monohybrid cross between two heterozygotes (Aa × Aa), the expected genotype ratio is 1 AA : 2 Aa : 1 aa, and the phenotype ratio is 3 dominant : 1 recessive. AP Biology expects you to use probability rules — such as multiplying probabilities of independent events — for dihybrid and more complex crosses, rather than always drawing large Punnett squares.
Non-Mendelian inheritance
Many traits do not follow simple dominant/recessive patterns:
- Incomplete dominance — the heterozygote shows an intermediate phenotype (for example, red × white flowers giving pink).
- Codominance — both alleles are fully expressed in the heterozygote (for example, the AB blood type).
- Multiple alleles — a gene has more than two possible alleles in the population (for example, the ABO blood group).
- Polygenic traits — several genes contribute to one trait (such as height or skin colour), producing continuous variation.
- Sex-linked traits — genes on the sex chromosomes (usually X) show different inheritance patterns in males and females.
Environmental effects and linkage
The environment also affects phenotype: two organisms with the same genotype can look different because of factors such as temperature, nutrition or sunlight. In addition, genes located close together on the same chromosome are linked and tend to be inherited together, which is an exception to independent assortment; the frequency of recombination between them can be used to map their positions.
Chromosomal inheritance and errors
Errors in meiosis can change the number of chromosomes. Nondisjunction occurs when chromosomes fail to separate properly, producing gametes with too many or too few chromosomes. This can lead to conditions such as trisomy (an extra chromosome). These errors show how the mechanics of meiosis directly affect heredity and the health of offspring.
The chi-square test for genetics
AP Biology expects you to use a chi-square (χ²) goodness-of-fit test to decide whether observed genetic results match the ratio predicted by a hypothesis. You calculate χ² = Σ (observed − expected)² ÷ expected for each category, then compare the result with a critical value from a table, using the correct degrees of freedom (number of categories minus one) and usually a significance level of 0.05. If the calculated χ² is greater than the critical value, you reject the hypothesis (the observed data differ significantly from the expected ratio); if it is smaller, the data are consistent with the hypothesis. Being able to set up and interpret this test is a frequently assessed quantitative skill in the heredity unit.
Pedigrees and tracing inheritance
A pedigree is a chart that shows how a trait is inherited through the generations of a family, using squares for males and circles for females, shaded to show who has the trait. Pedigrees let you work out the likely pattern of inheritance — for example, whether a trait is dominant or recessive, and whether it is autosomal or sex-linked. If two unaffected parents have an affected child, the trait is likely recessive; if the trait appears in every generation, it is more likely dominant. Reading pedigrees and using them to determine genotypes and probabilities is a common exam task, and it draws together the ideas of alleles, dominance and sex linkage.
Worked examples
Example 1: A monohybrid cross
In pea plants, tall (T) is dominant to short (t). Two heterozygous tall plants (Tt) are crossed. What is the expected phenotype ratio of the offspring? The Punnett square gives 1 TT : 2 Tt : 1 tt, so the phenotype ratio is 3 tall : 1 short.
Example 2: Using probability in a dihybrid cross
For the cross AaBb × AaBb, what is the probability of an offspring showing both recessive traits (aabb)? The probability of aa is 1/4 and of bb is 1/4. Because the genes assort independently, multiply: 1/4 × 1/4 = 1/16.
Example 3: Incomplete dominance
A red-flowered plant (RR) is crossed with a white one (WW), and all offspring are pink. What kind of inheritance is this, and what will the F2 ratio be? This is incomplete dominance, because the heterozygote (RW) is intermediate (pink). Crossing two pink plants gives a phenotype ratio of 1 red : 2 pink : 1 white.
Example 4: Explaining variation from meiosis
Explain two ways meiosis increases genetic variation. Crossing over exchanges segments between homologous chromosomes, creating new allele combinations. Independent assortment randomly distributes maternal and paternal chromosomes into gametes, producing many possible combinations. (Random fertilization then adds further variation.)
Common mistakes and how to avoid them
A common error is confusing genotype and phenotype. Genotype is the alleles (e.g. Aa); phenotype is the observable trait (e.g. tall). Keep them distinct.
Students often mix up incomplete dominance and codominance. In incomplete dominance the heterozygote is an intermediate (pink); in codominance both alleles are fully expressed (AB blood type shows both). Learn the difference.
Another mistake is drawing huge Punnett squares for dihybrid crosses when probability rules are faster: find the probability of each trait separately and multiply for independent genes.
When discussing meiosis, do not forget that variation comes from crossing over, independent assortment and random fertilization — students often name only one.
Finally, remember that environment as well as genotype shapes phenotype, and that linked genes are an exception to independent assortment. Ignoring these leads to incomplete answers.
Exam technique for "Heredity"
AP Biology emphasizes applying concepts and using quantitative reasoning, so be ready to calculate probabilities for crosses and interpret data rather than just recall definitions. Multiply probabilities of independent events for dihybrid crosses.
Be prepared to analyze non-Mendelian patterns from given data — identify incomplete dominance, codominance or sex linkage from ratios and phenotypes. On free-response questions, explain your reasoning and connect heredity to variation and evolution.
Use precise terminology — segregation, independent assortment, nondisjunction, phenotype — and support answers with the mechanism (meiosis) that produces the pattern. Practice reading pedigree and cross data, as interpretation is heavily tested.
Quick revision summary
- Meiosis halves the chromosome number and generates variation via crossing over, independent assortment and random fertilization.
- Law of segregation: alleles separate into gametes; law of independent assortment: different genes assort independently.
- Use Punnett squares and probability rules (multiply independent events) to predict genotype and phenotype ratios (e.g. 3:1 monohybrid, 9:3:3:1 dihybrid).
- Non-Mendelian patterns: incomplete dominance (intermediate), codominance (both expressed), multiple alleles, polygenic, and sex-linked traits.
- Environment affects phenotype, and linked genes are inherited together (an exception to independent assortment).
- Nondisjunction in meiosis produces gametes with abnormal chromosome numbers.