The genetics unit. How meiosis creates the genetic variation that fuels evolution, how Mendel's laws predict offspring ratios, and how real-world inheritance often gets messier — incomplete dominance, codominance, sex linkage, and beyond.
A specialized cell division that produces haploid gametes. Two rounds of division, four resulting cells.
Topic 5.2
Meiosis & Genetic Diversity
Crossing over, independent assortment, and random fertilization — the three sources of variation in sexually reproducing organisms.
Topic 5.3
Mendelian Genetics
Mendel's laws of segregation and independent assortment, alleles, dominance, Punnett squares, and predictable inheritance.
Topic 5.4
Non-Mendelian Genetics
When inheritance gets messier — incomplete dominance, codominance, multiple alleles, polygenic traits, and linked genes.
Topic 5.5
Environmental Effects on Phenotype
Same genotype, different environments → different phenotypes. Temperature, diet, and other factors shape how genes are expressed.
Topic 5.6
Chromosomal Inheritance
Sex-linked inheritance and what happens when chromosomes don't separate properly (nondisjunction → trisomy and monosomy).
About Unit 5
Unit 5 is the genetics unit. You'll learn how meiosis halves the chromosome number to produce gametes — and how three mechanisms during meiosis (crossing over, independent assortment, and random fertilization) create the genetic variation that fuels evolution. You'll meet Mendel's laws of inheritance and the Punnett square — and then see how real-world genetics is often more complicated (incomplete dominance, codominance, sex linkage, polygenic traits).
This unit is 8–11% of the AP Bio exam and takes about 10–12 class periods. The most-tested topics are predicting offspring ratios from crosses, recognizing inheritance patterns (especially X-linked recessive), and understanding why meiosis is the foundation of genetic diversity. Expect plenty of Punnett squares and pedigree-style questions.
The College Board ties Unit 5 to three of the four Big Ideas:
Big Idea 1
Evolution — meiosis generates the variation natural selection acts on
Big Idea 3
Information — genes are passed predictably from parent to offspring
Big Idea 4
Systems — genotype + environment together determine phenotype