AP Biology Unit 5 ReviewHeredity

AP Biology Unit 5, Heredity, is about how genetic information passes from one generation to the next and how that process creates variation. The single biggest idea is that meiosis shuffles and separates chromosomes to produce genetically diverse haploid gametes, and that this variation, plus Mendel's rules of probability, explains the traits offspring inherit. It makes up 8-11% of the AP exam.

What this unit covers

Meiosis and how chromosomes get passed on

• Meiosis takes one diploid cell (2n) and produces four haploid (1n) gametes, cutting the chromosome number in half so fertilization can restore it. Two identical gametes never form here, unlike mitosis.
• Meiosis I separates homologous chromosomes (one maternal, one paternal). In prophase I, homologs pair up in synapsis and can swap segments at chiasmata. Metaphase I lines up the homolog pairs; anaphase I pulls whole chromosomes apart.
• Meiosis II separates sister chromatids, working like mitosis but on haploid cells. No DNA replication happens between meiosis I and II.
• Mitosis and meiosis both use a spindle apparatus to move chromosomes, but mitosis makes two genetically identical diploid cells while meiosis makes four genetically different haploid cells.

How meiosis builds genetic diversity

• Crossing over (recombination) during prophase I exchanges genetic material between non-sister chromatids of homologous chromosomes, creating brand-new allele combinations on a single chromosome.
• Independent assortment means each homolog pair lines up at metaphase I independently of the others, so maternal and paternal chromosomes get shuffled into gametes in many possible combinations.
• Random fertilization combines two already-unique gametes, multiplying the variation even further.
• Nondisjunction is what happens when separation fails. Homologs (meiosis I) or sister chromatids (meiosis II) don't separate correctly, and gametes end up with too many or too few chromosomes (aneuploidy).

Mendelian genetics and probability

• Mendel's law of segregation: the two alleles for a gene separate during gamete formation, so each gamete carries only one allele.
• Mendel's law of independent assortment: genes on different chromosomes are inherited independently of one another.
• Monohybrid crosses track one trait, dihybrid crosses track two. A heterozygous monohybrid cross gives the classic 3:1 phenotypic ratio; a dihybrid cross of two heterozygotes gives 9:3:3:1.
• Probability rules do the heavy lifting. Use the multiplication rule for "and" events and the addition rule for "or" events to predict offspring ratios without drawing a giant Punnett square.
• Test crosses pair an unknown dominant phenotype with a homozygous recessive individual to figure out whether the unknown is homozygous or heterozygous.

Non-Mendelian inheritance

• Linked genes sit on the same chromosome and tend to be inherited together, breaking the 9:3:3:1 expectation. The frequency of recombination between them gives the map distance in map units.
• Incomplete dominance produces a blended intermediate phenotype (red x white flowers gives pink).
• Codominance shows both alleles fully and separately (ABO blood types, where A and B are both expressed).
• Polygenic traits result from many genes adding small effects, producing continuous variation like human height or skin color.
• Sex-linked genes (often on the X chromosome) show different inheritance patterns in males and females.
• You confirm a non-Mendelian pattern with a chi-square test, checking whether observed ratios differ significantly from what Mendel's laws predict.

Environment and phenotype

• The same genotype can produce different phenotypes depending on conditions. This is phenotypic plasticity.
• Named examples: flower color shifting with soil pH, arctic animals changing fur color by season, temperature-dependent sex determination in reptiles, and increased UV boosting melanin production.
• Genotype sets the possibilities; the environment helps decide which phenotype actually shows up.

Unit 5, Heredity at a glance

Why Unit 5, Heredity matters in AP Bio

This unit is where the course explains how information gets passed down and why no two offspring (except identical twins) are alike. It links the physical event of cell division to the math of inheritance, and it sets up the raw genetic variation that natural selection later acts on.

• It supports the big idea of information storage and transmission, showing exactly how genetic instructions move from parent to offspring.
• It explains the source of heritable variation, which is the fuel for evolution.
• It builds a recurring exam skill of using probability and statistics (Punnett squares, chi-square) to make and test biological predictions.
• It shows that phenotype is not just genotype, connecting genes to the environment that shapes their expression.

How this unit connects across the course

• Builds on the cell cycle and mitosis from cell division (Unit 4). Meiosis is the variation-making counterpart to the identical-copy process you learned there.
• Feeds directly into gene expression and regulation (Unit 6). The alleles you track here are sequences that get transcribed and translated, and environmental effects on phenotype preview gene regulation.
• Sets up natural selection (Unit 7). The genetic diversity from crossing over, independent assortment, and random fertilization is the variation that selection and Hardy-Weinberg analysis depend on.
• Connects forward to ecology (Unit 8), where phenotypic plasticity helps explain how organisms respond and adapt to changing environmental conditions.

Key equations and processes

• Meiosis: one diploid cell to four haploid gametes through meiosis I (separates homologs) and meiosis II (separates sister chromatids). Use it whenever the question is about gamete formation or genetic diversity.
• Multiplication rule: P(A and B) = P(A) x P(B). Use for the chance of independent events happening together, like two specific alleles meeting.
• Addition rule: P(A or B) = P(A) + P(B). Use when an offspring can have one outcome or another.
• Monohybrid and dihybrid ratios: expect 3:1 for a heterozygous one-trait cross and 9:3:3:1 for a two-trait cross of double heterozygotes.
• Map distance: recombination frequency between linked genes equals map units (1% recombinant offspring = 1 map unit). Use to figure out how far apart linked genes sit.
• Chi-square test: x² = Σ (observed − expected)² / expected. Use to test whether observed ratios match Mendelian predictions or signal a non-Mendelian pattern.

Unit 5, Heredity on the AP exam

Heredity is 8-11% of the exam and leans heavily on the quantitative skills the test loves. On the multiple-choice section, expect to read pedigrees, interpret cross data, and predict offspring ratios. On the free-response section, heredity shows up in questions where you justify a claim with genetics, often analyzing data from a cross or experiment.

• Calculate offspring probabilities and phenotype ratios from crosses, using Punnett squares or the probability rules.
• Run or interpret a chi-square test to decide whether data fit a predicted ratio, then explain what the result means.
• Trace inheritance through a pedigree to determine genotypes or a mode of inheritance (dominant, recessive, sex-linked).
• Explain how a process like crossing over or nondisjunction affects gametes, and connect that variation to evolution.
• Predict how a given genotype's phenotype could change under different environmental conditions.

Essential questions

• How does meiosis cut the chromosome number in half and still pass on a complete, functional set of genes?
• What are the three main sources of genetic variation, and how does each one work?
• How do probability rules let you predict inheritance without modeling every individual offspring?
• Why can two organisms with the same genotype end up looking different?

Key terms to know

• Haploid (1n): a cell with one set of chromosomes, like a gamete, half the diploid number.
• Homologous chromosomes: a matching pair of chromosomes carrying the same genes, one from each parent.
• Synapsis: the pairing of homologous chromosomes during prophase I that allows crossing over.
• Crossing over: the exchange of segments between non-sister chromatids, creating new allele combinations.
• Independent assortment: the random alignment of homolog pairs at metaphase I, shuffling chromosomes into gametes.
• Nondisjunction: the failure of chromosomes or chromatids to separate, producing gametes with the wrong chromosome number.
• Allele: one version of a gene that can produce variation in a trait.
• Genotype: an organism's genetic makeup; phenotype is the observable trait it produces.
• Homozygous / heterozygous: having two identical alleles vs. two different alleles for a gene.
• Linked genes: genes on the same chromosome that tend to be inherited together.
• Codominance: a pattern where both alleles are fully and separately expressed in a heterozygote.
• Polygenic trait: a trait controlled by many genes that produces continuous variation.
• Phenotypic plasticity: the ability of one genotype to produce different phenotypes in different environments.
• Chi-square test: a statistical test comparing observed data to expected ratios.

Common mix-ups

• Meiosis I vs. meiosis II: meiosis I separates homologous chromosomes (reducing the count to haploid); meiosis II separates sister chromatids. The reduction division is the first one, not the second.
• Incomplete dominance vs. codominance: incomplete dominance blends the two traits into an intermediate (pink); codominance shows both traits fully and side by side (AB blood).
• Independent assortment vs. crossing over: independent assortment shuffles whole chromosomes at metaphase I; crossing over swaps pieces between homologs in prophase I. Both add variation, but at different steps and levels.
• Genotype vs. phenotype: the genotype is the alleles you carry; the phenotype is what's actually expressed, which the environment can change.