AP Biology / Unit 5
AP Biology Unit 5: Heredity
Unit 5 is where cell division becomes inheritance. It covers meiosis, Mendelian and non-Mendelian genetics, pedigrees, and chi-square. It's 8–11% of the exam and reliably produces FRQs that demand quantitative work.
Aligned to the 2025–26 College Board CED (Topics 5.1–5.5).
Meiosis: creating genetic variation
One diploid (2N) parent cell produces four haploid (N) genetically unique gametes through two rounds of division.
| Phase | Key events |
|---|---|
| Prophase I | Homologs pair (synapsis); crossing over forms chiasmata |
| Metaphase I | Homologous pairs align; independent assortment of which homolog goes where |
| Anaphase I | Homologous chromosomes separate (NOT sister chromatids) |
| Telophase I / Cytokinesis | Two haploid cells; chromatids still joined |
| Meiosis II (PMAT) | Sister chromatids separate → 4 haploid cells |
Three sources of variation
- Crossing over (Prophase I) — recombines alleles between homologs.
- Independent assortment (Metaphase I) — ~8.4 million human gamete combos.
- Random fertilization — any sperm × any egg.
Mendelian genetics
| Cross | Setup | Expected ratio |
|---|---|---|
| Monohybrid | Aa × Aa | 3 dominant : 1 recessive (phenotype) |
| Dihybrid | AaBb × AaBb | 9 : 3 : 3 : 1 (phenotype) |
| Test cross | Unknown dominant × aa | 1:1 = heterozygous; all dominant = homozygous |
Non-Mendelian patterns
| Pattern | Definition | Example |
|---|---|---|
| Incomplete dominance | Heterozygote shows blended phenotype | Red × White → Pink |
| Codominance | Both alleles fully expressed | ABO blood type (AB) |
| Sex-linked | Gene on X or Y | Color blindness, hemophilia |
| Polygenic | Multiple genes → continuous variation | Skin color, height |
| Gene linkage | Genes on same chromosome inherited together | Fewer recombinants than independent |
| Organelle / maternal | Mitochondrial or chloroplast DNA | Leber hereditary optic neuropathy |
X-linked recessive in pedigrees
- More affected males than females.
- Affected males often have unaffected parents.
- Trait skips through carrier females.
- Fathers cannot pass to sons (sons get Y).
Chi-square analysis
χ² = Σ(O − E)² / E. Compare to the critical value at p = 0.05 with df = categories − 1. For a 4-class dihybrid (df = 3), the critical value is 7.815.
- χ² < critical → fail to reject the null; data are consistent with expected ratio.
- χ² > critical → reject the null; deviation is significant.
Gene linkage and recombination frequency
Recombination frequency = (recombinants / total) × 100. 1% = 1 map unit (centimorgan). Genes >50 map units apart look like they assort independently.
Environmental effects on phenotype
Genotype sets potential; environment shifts which traits express. Himalayan rabbits get dark fur where it's cold; PKU causes disease only with dietary phenylalanine; nutrition determines where in a genetic range height lands. The key phrase: “environment alters gene expression” — not the genotype itself.
Why students lose points here
- Mixing up Meiosis I (homologs) vs. Meiosis II (sister chromatids).
- Using “dominant” to mean “more common.” Huntington's is dominant and rare.
- Forgetting sex-linked asymmetry — males need only one recessive allele.
- Confusing genotypic (1:2:1) and phenotypic (3:1) ratios.
- Skipping the null hypothesis statement on chi-square FRQs.
FRQ patterns
- Pedigree: identify the pattern, determine carrier genotypes, calculate probabilities.
- Dihybrid prediction: Punnett square, expected ratios, explain deviations.
- Chi-square: calculate χ², state df, compare to critical value, state conclusion using the null hypothesis.
- Variation in meiosis: name all three mechanisms with mechanisms — not just a list.
How Unit 5 connects
- Unit 4: meiosis uses the same PMAT logic as mitosis with different outcomes.
- Unit 6: mutations alter alleles that then segregate according to Unit 5 rules.
- Unit 7: Hardy-Weinberg tracks the allele frequencies meiosis produces.
FAQ
What is the difference between crossing over and independent assortment?
Crossing over recombines alleles within a chromosome — it shuffles which alleles sit on the same chromatid. Independent assortment determines which whole chromosomes go to which gamete. Crossing over happens in Prophase I; independent assortment in Metaphase I.
What is nondisjunction?
Nondisjunction is the failure of chromosomes (Meiosis I) or sister chromatids (Meiosis II) to separate properly. Gametes end up with extra or missing chromosomes; fertilization produces aneuploidy (e.g., trisomy 21).
What does it mean for genes to be 'linked'?
Genes on the same chromosome are linked and tend to be inherited together. The closer they sit, the less likely crossing over separates them. Recombination frequency reflects physical distance — 1% recombination = 1 map unit.
How do I identify X-linked recessive inheritance in a pedigree?
Look for more affected males than females, affected males with unaffected parents, the trait skipping through carrier females, and no father-to-son transmission (sons get Y from father).
What is polygenic inheritance?
A trait controlled by two or more genes, each contributing additively. The more genes involved, the more the phenotype distribution looks like a bell curve rather than discrete Mendelian categories. Human skin color is polygenic.
Keep going
- AP Biology guide
- Unit 4: Cell Communication and Cell Cycle
- Unit 6: Gene Expression and Regulation
- Unit 7: Natural Selection
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