AP Biology: Chromosomal Basis – Nondisjunction, Aneuploidy, Chromosomal Mutations

Chromosomal Basis – Nondisjunction, Aneuploidy, Chromosomal Mutations

Concept Summary

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate during meiosis or mitosis, leading to aneuploidy.
• Aneuploidy: Abnormal chromosome number (e.g., trisomy, monosomy) caused by nondisjunction, often resulting in developmental disorders.
• Chromosomal mutations: Large-scale changes in chromosome structure (deletions, duplications, inversions, translocations) altering gene dosage or expression.
• Trisomy: Presence of three copies of a chromosome (e.g., Down syndrome, trisomy 21), typically due to nondisjunction in meiosis I or II.
• Polyploidy: Condition of having entire extra sets of chromosomes (e.g., triploidy), common in plants but usually lethal in animals.

Core Questions

WHAT (definitional)

Q: What is nondisjunction? A: The failure of chromosome pairs (homologs or sisters) to separate properly during cell division, causing gametes with abnormal chromosome numbers. Trap/Clarification: Nondisjunction can occur in meiosis I (homologs fail to separate) or meiosis II (sister chromatids fail to separate)—both produce aneuploid gametes.

Q: What is a chromosomal deletion? A: A mutation where a segment of a chromosome is lost, often leading to missing genes and phenotypic abnormalities. Trap/Clarification: Deletions are not the same as gene mutations (point mutations); they involve large-scale DNA loss.

WHY (causal/explanatory)

Q: Why does aneuploidy often cause severe developmental disorders? A: Imbalanced gene dosage disrupts cellular processes, as extra/missing chromosomes alter protein production critical for development. Trap/Clarification: Not all aneuploidies are lethal—some (e.g., trisomy 21) allow survival but with phenotypic consequences.

Q: Why are translocations sometimes harmful even if no genetic material is lost? A: Translocations can disrupt gene regulation (e.g., by separating genes from their promoters) or create fusion genes (e.g., Philadelphia chromosome in leukemia). Trap/Clarification: Balanced translocations (no net DNA loss) may have no phenotype but can produce unbalanced gametes.

HOW (process/application)

Q: How does nondisjunction in meiosis I differ from meiosis II? A: Meiosis I nondisjunction: All 4 gametes are abnormal (2 n+1, 2 n-1). Meiosis II nondisjunction: 2 normal gametes (n), 1 n+1, 1 n-1. Trap/Clarification: Meiosis I errors affect homologous pairs; meiosis II errors affect sister chromatids.

Q: How is aneuploidy detected in karyotyping? A: Count chromosomes in a metaphase spread: trisomy = 47 chromosomes (e.g., 47,XX,+21); monosomy = 45 chromosomes (e.g., 45,X). Trap/Clarification: Karyotypes show chromosome number, not gene mutations (e.g., sickle cell anemia is undetectable this way).

CAN (conditions/possibilities)

Q: Can polyploidy occur in humans? A: Yes, but it is almost always lethal (e.g., triploidy causes miscarriage); rare mosaicism may allow survival. Trap/Clarification: Polyploidy is common in plants (e.g., bananas are triploid) but typically fatal in animals.

Q: Under what conditions can a chromosomal inversion have no phenotypic effect? A: If the inversion is pericentric (includes centromere) or paracentric (excludes centromere) but does not disrupt gene function or regulatory regions. Trap/Clarification: Inversions can still cause problems during crossing over (e.g., dicentric/acentric chromosomes).

Quick Facts & Traps

• Fact: Down syndrome (trisomy 21) is the most common viable aneuploidy, with risk increasing with maternal age.
• Trap: "Nondisjunction only happens in meiosis"-Reality: It can also occur in mitosis, causing mosaicism (e.g., some cells 46,XX; others 47,XX,+21).
• Fact: Turner syndrome (45,X) is the only viable monosomy in humans; most other monosomies are lethal.
• Trap: "All chromosomal mutations are harmful"-Reality: Some (e.g., duplications) can drive evolution by providing extra gene copies for divergence.
• Fact: Robertsonian translocations (fusion of acrocentric chromosomes) can cause familial Down syndrome (parent is a carrier with 45 chromosomes).
• Trap: "Aneuploidy always causes disease"-Reality: Some aneuploidies (e.g., XYY syndrome) have mild or no symptoms.

Rapid-Fire True/False

• Statement: Nondisjunction in meiosis II produces two normal gametes and two abnormal gametes. Answer: TRUE Why the common mistake happens: Students confuse meiosis I (all gametes abnormal) with meiosis II (half normal).

• Statement: A deletion and a duplication are functionally identical because they both change gene dosage. Answer: FALSE Why the common mistake happens: While both alter dosage, deletions remove genes (often more severe), while duplications add genes (may be less harmful).

• Statement: Polyploidy is a type of aneuploidy. Answer: FALSE Why the common mistake happens: Both involve abnormal chromosome numbers, but aneuploidy = extra/missing single chromosomes; polyploidy = extra sets of chromosomes.