If Unit 4 was about how cells divide, Unit 5 is about what gets passed on when they do. This unit covers meiosis, Mendelian genetics, and all the ways that traits can be inherited. You'll learn why you look like your parents (but not exactly), how traits skip generations, and why the rules Mendel discovered over 150 years ago still predict inheritance today. Plus, you'll discover the exceptions to those rules.
🎯 What You Need to Know for the Exam
Unit 5 makes up about 8-11% of the AP Biology exam. Focus your energy on these priorities:
- Meiosis I and II: all phases and how they differ from mitosis
- How meiosis generates genetic diversity through crossing over, random assortment, and fertilization
- Nondisjunction and what happens when chromosomes don't separate properly
- Mendel's law of segregation and independent assortment
- Monohybrid, dihybrid, and test crosses, and how to interpret Punnett squares
- Genotype versus phenotype, homozygous versus heterozygous
- Linked genes, codominance, incomplete dominance, and sex-linked traits
- Pleiotropy and non-nuclear inheritance
- How the environment can influence phenotype even when genotype stays the same
What's in this review:
- Meiosis, Part 1: The Overview
- Meiosis, Part 2: The Phases
- Meiosis and Genetic Diversity
- Mendelian Genetics and Probability
- Pedigrees and Punnett Squares
- Non-Mendelian Genetics, Part 1: Linked Genes and Codominance
- Non-Mendelian Genetics, Part 2: Incomplete Dominance and Other Exceptions
- Environmental Effects on Phenotype
- Study Tips for Unit 5
- Summary, Review Questions & Practice
Topic 5.1: Meiosis, Part 1: The Overview
Meiosis is the process that creates gametes (sperm and egg cells). It's like mitosis, but with a twist: instead of creating two identical daughter cells, it creates four genetically unique haploid cells (cells with half the number of chromosomes). This is why sexual reproduction creates genetic diversity.
The big difference between mitosis and meiosis is that meiosis involves two rounds of division. The first round, called Meiosis I, separates the homologous chromosomes. The second round, called Meiosis II, separates the sister chromatids. The result is four haploid cells, each with a unique set of chromosomes.
Before we dive into the phases, understand this: mitosis and meiosis are similar in that they both use a spindle apparatus to move chromosomes. But they differ in the number of cells produced (two versus four) and the genetic content of the daughter cells (identical versus unique, diploid versus haploid).
Key concepts to know:
- Haploid and diploid: A diploid cell (2n) has two sets of chromosomes, one from each parent. A haploid cell (n) has one set. Meiosis starts with a diploid cell and produces four haploid cells.
- Homologous chromosomes: A diploid cell has pairs of chromosomes, called homologous pairs. One chromosome in each pair came from your mother, one from your father. In meiosis, these pairs separate.
- Sister chromatids: Remember from Unit 4 that DNA replication creates sister chromatids, two identical copies of the same chromosome connected at a centromere. Sister chromatids separate during Meiosis II.
⚠ Watch out for:
Students often confuse homologous chromosomes with sister chromatids. Sister chromatids are two identical copies of the same chromosome, created during S phase. Homologous chromosomes are two different copies of the same chromosome (one from mom, one from dad). They separate in Meiosis I. Sister chromatids separate in Meiosis II. Also, remember: meiosis produces gametes. You have diploid cells everywhere in your body, but your sperm and eggs are haploid.
Topic 5.2: Meiosis, Part 2: The Phases
Meiosis I is the big separation. Homologous chromosomes line up, pair with each other (a process called synapsis), and then move to opposite poles of the cell.
During Prophase I, homologous chromosomes pair up and condense. This pairing is called synapsis. While paired, the non-sister chromatids can exchange genetic material (we'll come back to this). This exchange is marked by visible structures called chiasmata. The meiotic spindle begins to form, centrosomes move to opposite poles, and the nuclear envelope breaks down.
During Metaphase I, the meiotic spindle fibers align the homologous pairs along the equator of the cell at the metaphase plate. The orientation is random: one mom chromosome could be on the left and the dad chromosome on the right, or vice versa. This randomness matters (we'll see why later).
During Anaphase I, the homologous chromosomes separate. The spindle fibers pull them toward opposite poles. Here's the key difference from mitosis: sister chromatids stay attached at the centromere. So each pole gets chromosomes that consist of two sister chromatids still stuck together.
During Telophase I, the spindle breaks down, new nuclear envelopes form around each set of chromosomes, cleavage furrows form, and cytokinesis occurs. You now have two haploid cells, each with half the chromosomes of the original cell. But each chromosome still consists of two sister chromatids.
Now Meiosis II begins. It's essentially like mitosis, but starting with a haploid cell instead of a diploid cell.
Prophase II: The meiotic spindle forms again. Sister chromatids (still connected at the centromere) attach to meiotic spindle fibers.
Metaphase II: Chromosomes align along the metaphase plate.
Anaphase II: The proteins holding sister chromatids together break down. Sister chromatids are pulled apart and move toward opposite poles.
Telophase II: Spindle breaks down, nuclear envelopes form, cleavage furrows or cell plates form, chromosomes decondense, and cytokinesis occurs. Four haploid daughter cells are formed, each with an unduplicated single chromatid.
Key concepts to know:
- Meiosis I events: Prophase I (synapsis, crossing over, chiasmata), Metaphase I (homologous pairs align), Anaphase I (homologous chromosomes separate), Telophase I (new nuclei form)
- Meiosis II events: Prophase II (spindle forms), Metaphase II (chromosomes align), Anaphase II (sister chromatids separate), Telophase II (four haploid cells result)
- The key difference: Meiosis I separates homologous chromosomes. Meiosis II separates sister chromatids.
⚠ Watch out for:
The phases have similar names in mitosis and meiosis, which is confusing. In mitosis, you have prophase, metaphase, anaphase, and telophase. In meiosis, you have the same four phases, but they happen twice (Prophase I, Metaphase I, etc., then Prophase II, Metaphase II, etc.). The big difference is what separates: in mitosis, sister chromatids separate. In Meiosis I, homologous chromosomes separate. In Meiosis II, sister chromatids separate.
Topic 5.3: Meiosis and Genetic Diversity
Meiosis creates genetic diversity in three ways: crossing over, random assortment, and fertilization.
Crossing over (also called recombination) happens during Prophase I when homologous chromosomes are paired up. The non-sister chromatids exchange segments of DNA. This swaps genetic material between the maternal and paternal chromosomes, creating new combinations of alleles. It's like shuffling two decks of cards together.
Random assortment happens because the orientation of homologous pairs at the metaphase plate is random. Imagine you have one pair of chromosomes, one from mom and one from dad. At the metaphase plate, mom's could be on the left or the right. When they separate, half the gametes will get mom's chromosome, and half will get dad's. With 23 pairs in humans, there are 2^23 possible combinations, which is over 8 million. That's a lot of genetic diversity from just random assortment.
Fertilization adds another layer. A sperm with one random assortment of chromosomes fertilizes an egg with a different random assortment. Each gamete is unique, and the zygote created from their fusion is unique too.
Sometimes something goes wrong: the chromosomes don't separate properly. This is called nondisjunction. If two homologous chromosomes fail to separate during Meiosis I, both chromosomes go to the same pole, and one gamete gets both while the other gets neither. When these gametes fuse with normal gametes at fertilization, the resulting zygote has the wrong number of chromosomes (aneuploidy), which often results in developmental disorders.
Key concepts to know:
- Crossing over: During Prophase I, non-sister chromatids exchange genetic material. This increases genetic diversity by creating new combinations of alleles on the same chromosome.
- Random assortment: The random orientation of homologous pairs at the metaphase plate creates 2^n possible combinations in the gametes, where n is the number of chromosome pairs.
- Fertilization: The fusion of two haploid gametes (each with its own random assortment) creates a unique diploid zygote.
- Sexual reproduction: Increases genetic variation through all three mechanisms: crossing over, random assortment, and fertilization.
- Nondisjunction: When chromosomes fail to separate properly during meiosis, gametes receive the wrong number of chromosomes. When these gametes fuse with normal gametes, the zygote has an abnormal chromosome number.
⚠ Watch out for:
Crossing over doesn't create new alleles (mutations do that). It creates new combinations of existing alleles. Also, nondisjunction is different from aneuploidy (which is the result of nondisjunction). Nondisjunction is the event that causes unequal separation. Aneuploidy is the condition of having an abnormal number of chromosomes. The exam tests whether you understand both.
Topic 5.4: Mendelian Genetics and the Laws of Inheritance
Gregor Mendel did experiments with pea plants over 150 years ago, and his findings still predict how traits are inherited today. His laws are powerful because they're simple and they usually work.
Mendel's law of segregation states that the two alleles (versions) of a gene separate during meiosis, so each gamete gets only one allele. When gametes fuse at fertilization, the offspring receives two alleles again, one from each parent. This segregation explains why heterozygous individuals (Aa) can pass on either allele to their offspring.
Mendel's law of independent assortment states that genes on different chromosomes segregate independently. One gene's inheritance doesn't affect another gene's inheritance. This is why you might inherit your mom's eye color gene and your dad's hair color gene, with no interference between them.
In Mendelian genetics, you need to know a few key terms. Alleles are different versions of the same gene. An individual can be homozygous (two identical alleles) or heterozygous (two different alleles). The genotype is the genetic makeup (what alleles you have). The phenotype is the observable trait (what you look like). In simple dominant-recessive inheritance, a dominant allele is usually represented by a capital letter (A), and a recessive allele by a lowercase letter (a). An AA or Aa individual shows the dominant phenotype. An aa individual shows the recessive phenotype.
The key tool in Mendelian genetics is the Punnett square. It's a simple grid that shows all possible combinations of alleles when two parents produce offspring. If you're looking at one gene (monohybrid cross), you have a 2x2 Punnett square. If you're looking at two genes (dihybrid cross), you have a 4x4 Punnett square.
Test crosses are another important tool. If you want to know whether an individual with the dominant phenotype is homozygous or heterozygous, you cross it with a homozygous recessive individual (aa). If the homozygous dominant parent (AA) produces all dominant offspring. If the heterozygous parent (Aa) produces about half dominant and half recessive offspring. This tells you the genotype of the dominant-phenotype parent.
Key concepts to know:
- Law of segregation: Alleles separate during meiosis. Each gamete gets one allele. Fertilization restores the diploid number.
- Law of independent assortment: Genes on different chromosomes segregate independently. One gene doesn't affect the segregation of another.
- Genotype and phenotype: Genotype is the genetic makeup. Phenotype is the observable trait. The same phenotype can result from different genotypes (AA and Aa both show the dominant phenotype).
- Probability in inheritance: Rules of probability apply. If A and B are independent, then P(A and B) = P(A) × P(B). If A and B are mutually exclusive, then P(A or B) = P(A) + P(B).
- Monohybrid cross: A cross involving one gene. A cross between Aa and Aa produces a 3:1 ratio of dominant to recessive phenotypes.
- Dihybrid cross: A cross involving two genes. A cross between AaBb and AaBb produces a 9:3:3:1 ratio of phenotypes (assuming independent assortment).
- Test cross: A cross between an individual with the dominant phenotype and a homozygous recessive individual (aa). Used to determine whether the dominant-phenotype parent is homozygous or heterozygous.
- Pedigrees: Charts showing family relationships and inheritance patterns. Used to predict genotypes and phenotypes and to identify patterns of inheritance (autosomal, sex-linked, etc.).
⚠ Watch out for:
Mendel's laws assume independent assortment. If genes are on the same chromosome (linked), the law of independent assortment doesn't apply. Also, Mendel's laws assume complete dominance, where one allele completely masks the other. There are exceptions (we'll get to those). Also, don't confuse probability rules. "Or" means you add probabilities (for mutually exclusive events). "And" means you multiply probabilities (for independent events).
Topic 5.5: Pedigrees and Punnett Squares
Pedigrees are family trees that show inheritance patterns. They use symbols: circles for females, squares for males, filled symbols for individuals with the trait, empty symbols for individuals without the trait. Horizontal lines connect mates, and vertical lines show parent-child relationships.
Pedigrees are powerful because they can reveal patterns of inheritance. If a trait appears in every generation, it's likely dominant. If a trait skips generations, it's likely recessive. If a trait appears only in males, it's likely X-linked. If a trait affects both males and females equally, it's likely autosomal.
Punnett squares are simpler. For a monohybrid cross, you create a 2x2 grid. The alleles of one parent go across the top, the alleles of the other parent go down the side, and you fill in the grid with the combinations. The result shows the genotypic and phenotypic ratios of the offspring.
For a dihybrid cross, it's a 4x4 grid, and the process is the same, just more combinations to fill in.
Key concepts to know:
- Reading pedigrees: Patterns of inheritance can be predicted from pedigree data, showing the genotypes and phenotypes of parents and offspring.
- Using Punnett squares: Can be used to predict genotypes and phenotypes of parents and offspring in both monohybrid and dihybrid crosses.
⚠ Watch out for:
Pedigrees and Punnett squares are tools. The exam tests whether you can use them correctly. Don't just memorize the 3:1 or 9:3:3:1 ratios. Understand why they occur (segregation and independent assortment). Also, pedigrees are drawn from family data, so not every family will show the expected ratios, especially in small families. A 3:1 ratio is theoretical; actual families might have 2:2 or 4:0 just by chance.
Topic 5.6: Non-Mendelian Genetics, Part 1: Linked Genes and Codominance
Mendel's laws work great for genes on different chromosomes, but genes on the same chromosome are linked. They tend to be inherited together because the physical DNA doesn't split up.
When genes are linked, they don't assort independently. If your mom has alleles A and B on one chromosome and a and b on another, she's likely to pass on either AB or ab, not Ab or aB. The probability of the "new" combinations (Ab and aB) increases only if crossing over occurs between the two genes. The distance between the genes determines how often crossing over happens between them. Genes far apart cross over more often. Genes close together cross over less often. The map distance (or map units) between two genes is based on how often recombinants appear. If 1% of gametes are recombinant, the genes are 1 map unit apart. This principle is called genetic mapping.
Codominance is when both alleles are fully expressed, so the heterozygote has a different phenotype than either homozygote. The classic example is human ABO blood types. A and B alleles are codominant, so individuals with AB genotype have both A and B antigens and type AB blood (different from type A or type B blood).
Key concepts to know:
- Linked genes: Genes on the same chromosome. They segregate together more often than by chance, unless crossing over occurs between them.
- Genetic mapping: Map distance is based on the frequency of recombinant gametes. Genes that are farther apart cross over more often and are farther apart on the map.
- Codominance: Both alleles are fully expressed. The heterozygote has a different phenotype than either homozygote.
⚠ Watch out for:
Linked genes violate the law of independent assortment. If the exam gives you a cross and the results don't match the expected Mendelian ratios, think about whether the genes might be linked. Also, codominance is different from incomplete dominance (which we'll cover next). In codominance, both alleles are expressed fully. In incomplete dominance, neither allele is completely dominant, so the heterozygote shows a blended phenotype.
Topic 5.7: Non-Mendelian Genetics, Part 2: Incomplete Dominance and Other Exceptions
Incomplete dominance occurs when neither allele of a gene completely masks the other. The phenotype of the heterozygote is a blended version of the two homozygous phenotypes. A classic example is snapdragons: the homozygous red (RR) parent is red, the homozygous white (WW) parent is white, and the heterozygous (RW) offspring are pink. Neither red nor white completely dominates; they blend.
Sex-linked traits are determined by genes on sex chromosomes. In mammals and many other organisms, males are XY and females are XX. Genes on the X chromosome can be recessive and hidden in heterozygous females but expressed in hemizygous males (who have only one X chromosome). Color blindness and hemophilia are examples of X-linked recessive traits. Affected individuals are usually male. Carrier females are heterozygous and unaffected but can pass the allele to their offspring.
Pleiotropy is when a single gene affects multiple traits. The gene for sickle cell hemoglobin, for example, codes for a single change in a hemoglobin protein, but this single mutation causes misshapen red blood cells, pain, organ damage, and other problems. These traits don't segregate independently because they're all controlled by the same gene.
Non-nuclear inheritance involves genes in organelles: mitochondria and chloroplasts. These organelles are inherited differently from nuclear DNA. In animals, mitochondria are usually inherited maternally, passed through the egg and not the sperm. In plants, both mitochondria and chloroplasts are inherited through the ovule, not the pollen. Chloroplasts and mitochondria are randomly assorted to daughter cells and gametes, so traits determined by their DNA don't follow simple Mendelian rules.
Key concepts to know:
- Incomplete dominance: Neither allele completely masks the other. The heterozygote shows a blended phenotype.
- Sex-linked traits: Determined by genes on sex chromosomes. X-linked traits are more common in males because they have only one X. Females can be carriers.
- Pleiotropy: A single gene affects multiple traits. These traits are controlled by the same gene and don't segregate independently.
- Non-nuclear inheritance: Genes in mitochondria and chloroplasts are inherited maternally in animals and through the ovule in plants. These traits don't follow Mendelian rules because organelles are randomly assorted.
⚠ Watch out for:
Incomplete dominance and codominance are different. Know the difference and be able to recognize each from examples. Also, sex-linked inheritance changes the expected ratios. A monohybrid cross involving an X-linked trait will not produce a 3:1 ratio; the ratios are different for males and females. Non-nuclear inheritance is less commonly tested, but the exam might ask about maternal inheritance or about why a trait doesn't follow Mendelian rules.
Topic 5.8: Environmental Effects on Phenotype
Your phenotype isn't just your genotype. The environment also matters. The same genotype can produce different phenotypes under different environmental conditions. This is called phenotypic plasticity.
Consider height. Your genes set a range, but nutrition, exercise, sleep, and other environmental factors determine where you fall within that range. Two genetically identical twins raised in different environments might have different heights, different muscle mass, and different body compositions.
Or consider skin color. Your genotype determines the range of possible skin colors, but sun exposure affects how much melanin you produce, changing your phenotype.
Some examples are more dramatic. Certain plants have different leaf shapes depending on whether they grow in water or on land. Some butterflies change color depending on the temperature during development. These are extreme examples of phenotypic plasticity.
The takeaway is this: genotype is not destiny. Environment matters. Two individuals with the same genotype might have different phenotypes, and two individuals with different genotypes might have the same phenotype.
Key concepts to know:
- Phenotypic plasticity: The ability of a single genotype to produce different phenotypes under different environmental conditions.
- Environmental influence: Environmental conditions influence gene expression and can lead to variation in phenotype.
⚠ Watch out for:
The exam might show you data on phenotypic variation and ask whether it's due to genetic or environmental differences. You can't tell just by looking. You need experimental evidence. If genetically identical individuals in different environments have different phenotypes, it's environmental. If genetically different individuals in the same environment have different phenotypes, it's genetic. If both vary, it's both.
Study Tips for Unit 5
Understand mechanisms, not just ratios. Don't just memorize that a 3:1 cross happens. Understand segregation and why it produces that ratio. Same with 9:3:3:1 for dihybrid crosses.
Draw it out. Sketch a Punnett square. Draw meiosis I and II. Visual learning locks in concepts better than reading alone.
Practice with scenarios. The exam doesn't ask "What is the law of segregation?" It asks, "In this pedigree, why does the trait skip generations?" Practice applying concepts to new situations.
Quiz yourself on exceptions. Mendelian inheritance assumes complete dominance and independent assortment. Linked genes, codominance, and incomplete dominance break those assumptions. Know when and why they apply.
Use StarSpark's flashcards and practice quizzes to lock in these concepts. The StarSpark prompts mentioned in the Study Tips section are excellent for generating scenario-based questions that test your understanding.
Summary: What Actually Matters for the Exam
You've covered all the topics in Unit 5. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're in great shape for this section of the exam.
Review Questions: Test Yourself
- During meiosis, crossing over occurs and then chromosomes are randomly assorted into gametes. Explain how these two processes together increase genetic diversity in offspring, and compare your explanation to what happens in mitosis.
- A researcher performs a test cross on a pea plant with purple flowers and gets some white-flowered offspring. What does this tell you about the genotype of the purple-flowered parent? (Assume purple is dominant to white.)
- A woman is a carrier for an X-linked recessive disorder. Her husband is not affected. What is the probability that their sons will be affected? What is the probability that their daughters will be carriers?
- In a dihybrid cross between two heterozygous parents (AaBb x AaBb), the offspring show a 9:3:3:1 ratio. Explain where each ratio class comes from and what would happen if the two genes were linked (on the same chromosome).
- Two plants with the same genotype are grown in different environments: one with abundant water and nutrients, one with limited water and nutrients. The plants grown in the rich environment are taller and have more leaves. Explain this difference using the concept of phenotypic plasticity.
Want more practice? Paste these questions into StarSpark to generate a full quiz with explanations.
Explore the Full AP Biology Study Guide
Unit 5 is foundational for understanding genetics, evolution, and life itself. Many of the concepts here come back in Units 6 and 7, so mastering this material now will make those units easier.
Check out the full AP Biology study plan to see how this unit connects to the rest of the course.
Other Unit Reviews:
- AP Biology Unit 1: Chemistry of Life
- AP Biology Unit 2: Cells
- AP Biology Unit 3: Cellular Energetics
- AP Biology Unit 4: Cell Communication and Cell Cycle
- AP Biology Unit 6: Gene Expression and Regulation
- AP Biology Unit 7: Natural Selection
- AP Biology Unit 8: Ecology
- AP Biology Exam Prep Study Guide
For official AP Biology resources, visit apcentral.collegeboard.org.
This review is aligned with the AP Biology Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.
