The basics
What it covers: Meiosis and how it generates variation; Mendelian genetics; non-Mendelian patterns (incomplete dominance, codominance, sex linkage, polygenic); environmental effects on phenotype; and chromosomal inheritance.
Exam weight: 8–11% of the AP Biology exam.
The big question: How is genetic information passed from parents to offspring, and what produces the variation we see between individuals?
Big Ideas covered: Evolution (BI 1), Information Storage & Transmission (BI 3), Systems Interactions (BI 4).
Key topics at a glance
Meiosis vs. Mitosis
Mitosis: 1 division, 2 identical diploid cells. Meiosis: 2 divisions, 4 unique haploid gametes. Only meiosis has crossing over and independent assortment.
The Two Divisions of Meiosis
Meiosis I: homologous chromosomes pair up, cross over, and separate. Reduces 2n to n. Meiosis II: sister chromatids separate (just like mitosis).
Three Sources of Variation
Crossing over (prophase I), independent assortment (metaphase I), random fertilization. Together they make every offspring genetically unique.
Mendel's Two Laws
Segregation: alleles for a gene separate during gamete formation. Independent assortment: alleles of different genes are distributed independently into gametes.
Key Cross Ratios
Heterozygous × heterozygous (Aa × Aa): 3:1 phenotype, 1:2:1 genotype. Test cross (Aa × aa): 1:1. Dihybrid (AaBb × AaBb): 9:3:3:1.
Incomplete Dominance vs. Codominance
Incomplete: heterozygote shows intermediate phenotype (red × white = pink). Codominance: heterozygote shows BOTH phenotypes (IA IB = AB blood).
Sex-Linked Inheritance
Genes on the X chromosome. X-linked recessive traits (colorblindness, hemophilia) show up far more often in males (XY) than females (XX), since males only need one copy.
Polygenic & Pleiotropy
Polygenic: one trait controlled by many genes (height, skin color → continuous variation). Pleiotropy: one gene affects many traits (sickle cell allele).
Environment + Genotype = Phenotype
Same genotype can produce different phenotypes depending on environment. Himalayan rabbits' fur color depends on temperature; hydrangea flower color depends on soil pH.
Nondisjunction
Failure of chromosomes to separate properly in meiosis. Produces gametes with extra or missing chromosomes. Trisomy 21 = three copies of chromosome 21 = Down syndrome.
The key terms you must know
- Meiosis — two-round cell division that produces four haploid gametes from a diploid parent.
- Homologous chromosomes — matching pair (one from each parent) with the same genes but possibly different alleles.
- Crossing over — exchange of DNA between homologous chromosomes during prophase I; produces new allele combinations.
- Independent assortment — random orientation of homologous pairs at metaphase I; each gamete gets a random mix of maternal and paternal chromosomes.
- Genotype / phenotype — the alleles you have vs. the trait you show.
- Homozygous / heterozygous — two identical alleles vs. two different alleles.
- Dominant / recessive — expressed in heterozygotes vs. only expressed in homozygotes.
- Law of segregation — Mendel's first law; alleles separate during gamete formation.
- Law of independent assortment — Mendel's second law; alleles of different genes are distributed independently.
- Incomplete dominance — heterozygotes have intermediate phenotype (pink from red × white).
- Codominance — both alleles fully expressed in the heterozygote (AB blood).
- Multiple alleles / Polygenic / Pleiotropy — three patterns of complex inheritance beyond Mendel's simple cases.
- Sex-linked / X-linked inheritance — genes on the sex chromosomes; X-linked recessive traits more often appear in males.
- Nondisjunction — chromosomes fail to separate in meiosis, producing gametes with too many or too few chromosomes.
Key themes to remember
- Meiosis = variation. Almost every Unit 5 concept comes back to how meiosis generates genetic diversity — that's why sex evolved.
- One gene, many alleles, two per individual. A population may have dozens of alleles for one gene, but each individual still has only two.
- Crosses are math. Punnett squares are just a way of multiplying probabilities. Once you internalize this, dihybrid and complex crosses become routine.
- Real inheritance is often messier than Mendel. But Mendel's logic (alleles segregate; genes assort independently) still works underneath.
- Genotype isn't destiny. Environment matters too. Same genes can produce different phenotypes in different conditions.
- X-linked traits affect males disproportionately. When you see a cross or pedigree where the trait skips generations and shows up only in males, suspect X-linked recessive.
Common exam traps
- Sister chromatids ≠ homologous chromosomes. Sister chromatids are IDENTICAL copies of the same chromosome. Homologous chromosomes are MATCHING (same genes) but may have different alleles.
- Crossing over happens in PROPHASE I — not anaphase, not mitosis. Easy to misplace on a timing question.
- Independent assortment is a meiosis I event. It happens at metaphase I, when homologous pairs (not chromatids) line up randomly.
- Incomplete dominance and codominance aren't the same. Incomplete = blend (pink). Codominance = both at once (red AND white speckles, or AB blood).
- "Heterozygous" doesn't mean the trait is intermediate. In most cases the heterozygote shows the dominant phenotype. Only in incomplete dominance is the heterozygote intermediate.
- X-linked recessive males can't be "carriers." They either have the allele (and show the trait) or they don't. Only XX females can be carriers without showing the trait.
- A 9:3:3:1 ratio is for INDEPENDENT GENES. If two genes are linked (close together on the same chromosome), the ratio shifts toward the parental combinations.
- The Punnett square shows PROBABILITIES, not guarantees. A 3:1 ratio is an expectation across many offspring — not a rule for any single family.