AP Biology — Unit 5: Heredity

What AP Biology Unit 5 covers

The College Board Course and Exam Description breaks Unit 5 into 5 topics:

1. 5.1 Meiosis
2. 5.2 Meiosis and Genetic Diversity
3. 5.3 Mendelian Genetics
4. 5.4 Non-Mendelian Genetics
5. 5.5 Environmental Effects on Phenotype

AP Biology Unit 5 practice questions

• Meiosis produces four cells, but mitosis produces two — your friend says they're basically the same process. What are they missing?

  Meiosis: 4 haploid cells; mitosis: 2 diploid. Meiosis runs two divisions with no replication between, so it also reshuffles genes.

• In meiosis I anaphase, homologous chromosomes separate — but what holds sister chromatids together at that same moment?

  Cohesins at the centromere hold sister chromatids together during meiosis I. They are not cleaved until meiosis II anaphase.

• A spermatocyte and an oocyte both enter meiosis with the same DNA — so why does spermatogenesis yield four sperm while oogenesis yields one egg and three polar bodies?

  Oogenesis splits cytoplasm unequally — one big egg, three polar bodies. Spermatogenesis divides it equally, yielding four functional sperm.

• A drug prevents spindle fiber formation during meiosis I. What is the most likely outcome for the resulting gametes?

  Nondisjunction of homologs results, so gametes end up n+1 or n−1. Neither has the correct haploid chromosome number.

• Nondisjunction occurs in meiosis I. How many of the four resulting gametes are chromosomally abnormal?

  All four gametes are abnormal. When homologs fail to separate in meiosis I, each secondary oocyte/spermatocyte already has the wrong number — meiosis II then splits those incorrectly numbered cells, so every resulting gamete is aneuploid.

• Two genes sit on the same chromosome, far apart. Your friend says they won't assort independently. What are they missing?

  Distant genes still assort independently, via frequent crossing over. That swap reshuffles their alleles between homologs in prophase I.

• Crossing over looks like DNA repair — so what actually makes it a source of genetic diversity rather than just fixing a break?

  Crossing over swaps DNA segments between the non-sister chromatids of homologs. That mixes maternal and paternal alleles on one chromatid — unlike repair.

• Two parents are genetically identical clones. Their offspring will be genetically identical to them — right? What actually happens in sexual reproduction?

  Sexual reproduction remixes alleles three ways, so identical parents vary offspring. The three: crossing over, independent assortment, and random fertilization.

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Key terms in Unit 5

These 12 terms show up in the Unit 5 cards. Each one links to its definition in the AP Biology key-term reference.

• Chi-square test
• Dihybrid cross
• Gene expression
• Genetic diversity
• Homologous chromosomes
• Incomplete dominance
• Independent assortment
• Law of independent assortment
• Law of segregation
• Null hypothesis
• Recessive allele
• Sister chromatids