A biologist observes a human egg cell that contains two X chromosomes. During which of the following meiotic stages did nondisjunction most likely occur to produce this specific gamete?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Nondisjunction arises from the failure of the chromosome segregation machinery during meiosis, specifically involving defects in cohesin complexes, kinetochore-microtubule attachments, and the anaphase-promoting complex/cyclosome (APC/C) pathway. During normal meiosis I, homologous chromosomes must separate, while during meiosis II, sister chromatids must separate. The cohesin protein complex, which includes the meiosis-specific subunit REC8, holds sister chromatids together along their arms and at centromeres. At anaphase I, the enzyme separase cleaves REC8 along chromosome arms—releasing chiasmata formed during prophase I crossover events—while centromeric REC8 remains protected by shugoshin protein, keeping sister chromatids attached until meiosis II. Nondisjunction in meiosis I occurs when both homologous chromosomes migrate to the same pole rather than segregating to opposite poles, often due to improper monopolar attachment of sister kinetochores or premature loss of arm cohesin. The spindle assembly checkpoint (SAC), which monitors proper amphitelic attachment at kinetochores via MAD2 and BUBR1 checkpoint proteins, may fail to detect merotelic or syntelic attachments, allowing anaphase onset despite erroneous tension patterns.

Why Other Options Are Wrong

PILLAR 2 — STEP-BY-STEP LOGIC

A human egg cell containing two X chromosomes must have received both homologous X chromosomes (one maternally derived, one paternally derived from the individual's parents) rather than just one. Tracing the cell lineage: the primary oocyte is diploid (46, XX) with two distinct homologous X chromosomes. During normal anaphase I, these homologs should segregate to opposite poles, yielding a secondary oocyte with 23 chromosomes including one X. If nondisjunction occurs at anaphase I, both homologous X chromosomes move to the same daughter cell—the secondary oocyte—while the first polar body receives zero X chromosomes. When this secondary oocyte then completes meiosis II normally, the resulting ovum inherits both X chromosomes. Had nondisjunction instead occurred at anaphase II, the secondary oocyte would already possess only a single X chromosome with two identical sister chromatids; failure of sister chromatid separation would produce an ovum with two copies of the same X (identical DNA sequences), not two distinct homologous X chromosomes. The question specifies an egg with two X chromosomes, indicating retention of both homologs from the original diploid complement, pinpointing anaphase I as the stage of failure.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A (Prophase I) traps students who conflate the timing of recombination with the timing of segregation failure. While SPO11-mediated double-strand breaks and crossing-over during prophase I establish the physical linkages (chiasmata) necessary for proper homolog alignment, the actual nondisjunction event—the failure to separate—occurs during anaphase I when separase should cleave cohesin and homologs should migrate to opposite poles. Option B (Metaphase I) appeals to students who recognize that improper kinetochore-microtubule attachments form during chromosome alignment, but metaphase I is the checkpoint stage where the SAC evaluates attachment correctness; the mechanical failure of segregation itself manifests at anaphase I when the APC/C triggers securin degradation and separase activation. Option D (Anaphase II) is the most seductive distractor because students recognize that anaphase stages involve chromosome separation. However, anaphase II nondisjunction would produce an egg with two identical sister chromatids of a single X chromosome, not two distinct homologous X chromosomes inherited from the individual's two parents. The distinction between homologous chromosomes (meiosis I) versus sister chromatids (meiosis II) is the critical discriminating concept. Option E (Metaphase II) reflects the same confusion as metaphase I—it represents a monitoring stage rather than the mechanical execution of chromosome movement, and it would not produce the specific genotype of two distinct X homologs observed in this gamete.