Unit 5: Heredity
AP Biology: 46 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 5 explains how traits pass between generations and where genetic variation comes from: meiosis, Mendel's laws, and chromosome behavior.
Meiosis
Meiosis I separates homologous chromosomes (reductional); meiosis II separates sister chromatids. One diploid cell yields four haploid, genetically unique gametes. Crossing over (prophase I) exchanges segments between homologs; independent assortment shuffles chromosomes; random fertilization adds more combinations. Nondisjunction — failure to separate — causes aneuploidy (e.g., trisomy 21).
Mendelian genetics
Alleles segregate during gamete formation (law of segregation); genes on different chromosomes assort independently (law of independent assortment). Monohybrid crosses give 3:1 phenotypes; dihybrids give 9:3:3:1. Test crosses reveal unknown genotypes.
Non-Mendelian patterns
Incomplete dominance blends (red × white → pink); codominance shows both (AB blood type); multiple alleles exist (ABO); pleiotropy lets one gene affect many traits; polygenic traits (height, skin color) show continuous variation; epistasis has one gene masking another; linked genes violate independent assortment unless crossing over separates them (recombination frequency maps distance).
Environment and phenotype
Phenotype = genotype + environment: nutrition changes height; temperature affects pigment; pH changes flower color in hydrangeas.
Chromosomal inheritance
Sex-linked genes ride the X or Y chromosome. X-linked recessive traits (color blindness, hemophilia) appear far more often in males, who carry a single X. Pedigrees reveal inheritance mode: autosomal vs sex-linked, dominant vs recessive.