AP Biology Unit 5: Heredity
Study meiosis, Mendelian genetics, non-Mendelian inheritance, chromosomal basis with exam-format practice questions and rubric-based scoring.
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Inside This Unit: The Full Breakdown
Heredity covers how traits are passed from parents to offspring through meiosis, Mendelian genetics, and patterns of inheritance. Meiosis produces genetically unique gametes, and Mendel's laws explain how alleles segregate and assort independently.
Why it matters
Genetics problems are a guaranteed part of every AP Bio exam. You need to set up and solve crosses, interpret pedigrees, and explain non-Mendelian patterns. Mastering meiosis is also essential for understanding genetic variation and evolution.
Key concepts
- Meiosis produces four genetically unique haploid cells through two rounds of division, introducing variation via crossing over and independent assortment.
- Mendel's Law of Segregation states that two alleles for a gene separate during gamete formation; the Law of Independent Assortment states that genes on different chromosomes sort independently.
- Non-Mendelian inheritance includes incomplete dominance, codominance, multiple alleles, polygenic traits, epistasis, and sex-linked traits.
- Chromosomal errors like nondisjunction (failure of chromosomes to separate) cause aneuploidy conditions such as Down syndrome.
Meiosis and Genetic Variation
Meiosis is a specialized cell division that reduces the chromosome number by half, producing haploid gametes (sperm and eggs) from diploid parent cells. It consists of two divisions: meiosis I separates homologous chromosome pairs, and meiosis II separates sister chromatids (similar to mitosis). Three mechanisms generate genetic diversity during meiosis. Crossing over during prophase I exchanges segments between homologous chromosomes, creating new allele combinations. Independent assortment during metaphase I randomly orients homologous pairs, producing 2²³ (about 8 million) possible chromosome combinations in humans. Random fertilization further multiplies variation. Together, these mechanisms ensure that each offspring is genetically unique.
Mendelian Genetics
Gregor Mendel discovered the fundamental rules of inheritance by studying pea plants. His Law of Segregation states that each organism carries two alleles for each trait and these alleles separate during gamete formation so each gamete carries only one. His Law of Independent Assortment states that alleles of different genes sort independently during meiosis, provided the genes are on different chromosomes. A monohybrid cross (Aa × Aa) yields a 3:1 phenotypic ratio in the F₂ generation. A dihybrid cross (AaBb × AaBb) yields a 9:3:3:1 ratio. Test crosses (crossing an unknown genotype with a homozygous recessive) reveal whether an organism is homozygous dominant or heterozygous. Punnett squares are the standard tool for predicting offspring ratios.
Non-Mendelian Inheritance
Many traits do not follow simple dominant-recessive patterns. In incomplete dominance, heterozygotes show a blended phenotype (red × white = pink flowers). In codominance, both alleles are fully expressed (AB blood type shows both A and B antigens). Multiple alleles means more than two allele versions exist in a population (like the three alleles for ABO blood type). Polygenic traits like skin color and height are controlled by many genes, producing a continuous range of phenotypes. Epistasis occurs when one gene masks or modifies the expression of another gene. Sex-linked traits (genes on the X chromosome) show different inheritance patterns in males and females, explaining why conditions like hemophilia and color blindness are more common in males.
AP exam tip
On genetics free-response questions, always show your work with a Punnett square or chi-square analysis. Define your allele symbols, state genotypes and phenotypes clearly, and give ratios. Partial credit depends on showing your reasoning step by step.
Connections to other units
- Unit 3 (Cell Communication and Cell Cycle): Meiosis shares spindle mechanics with mitosis but has unique features like synapsis and crossing over.
- Unit 6 (Gene Expression): The alleles studied in heredity produce their effects through transcription and translation.
- Unit 7 (Natural Selection): Genetic variation from meiosis and mutation provides the raw material for evolution.
