Chromosomal errors change how many chromosomes or chromosome sets a cell has. In AP Biology, the key focus is aneuploidy caused by nondisjunction during cell division and how these errors alter phenotype and can cause developmental disorders.

Core idea: chromosome-number errors

Chromosome-number changes typically result from mistakes in chromosome movement during mitosis or meiosis.

Aneuploidy matters because gene expression depends on gene dosage: extra or missing chromosomes change the amounts of many gene products at once.

Polyploidy vs. aneuploidy (what to recognise)

• Aneuploidy: one chromosome too many or too few (e.g., $2n+1$, $2n-1$)

• Polyploidy: extra whole sets of chromosomes (e.g., $3n$, $4n$); less emphasised here than aneuploidy

Nondisjunction: the main cause

Most aneuploidy arises from nondisjunction, when chromosomes fail to separate properly.

This diagram compares nondisjunction in meiosis I versus meiosis II and traces how the segregation error changes the chromosome number of the resulting gametes. It emphasizes the key AP Biology pattern: meiosis I nondisjunction makes all gametes abnormal, while meiosis II nondisjunction produces a mix of normal gametes and aneuploid gametes (with $n+1$ or $n-1$). Source

Where nondisjunction happens

Meiosis I nondisjunction (homologs fail to separate)

• Produces gametes that are all abnormal:

  • two gametes with an extra chromosome ($n+1$)

  • two gametes missing that chromosome ($n-1$)

• After fertilisation with a normal gamete ($n$), possible zygotes include:

  • trisomy ($2n+1$)

  • monosomy ($2n-1$)

Meiosis II nondisjunction (sister chromatids fail to separate)

This OpenStax figure shows how nondisjunction at different meiotic stages changes the chromosome content of gametes and explains why the resulting zygotes can be trisomic or monosomic after fertilization. The visual reinforces the mechanistic distinction between homologs failing to separate in meiosis I and sister chromatids failing to separate in meiosis II. Source

• Produces two normal gametes ($n$) and two abnormal gametes:

  • one ($n+1$)

  • one ($n-1$)

• Leads to a mix of normal and aneuploid zygotes after fertilisation.

Aneuploidy from meiosis affects every cell of the resulting organism because the error is present at fertilisation (unless corrected very early).

Mitotic errors and mosaicism

Nondisjunction can also occur in mitosis after fertilisation.

• If nondisjunction happens in an early embryo, it can produce mosaicism: different cell lineages within the same individual have different chromosome numbers.

• Mosaicism can make phenotypes less severe or variable, depending on:

  • which tissues contain aneuploid cells

  • the proportion of affected cells

  • which chromosome is involved

Why aneuploidy changes phenotype

Aneuploidy often has large effects because it changes expression for hundreds to thousands of genes simultaneously.

Key biological consequences include:

• Gene dosage imbalance: too much or too little of many proteins disrupts pathways and development

• Developmental timing disruption: embryos rely on tightly regulated gene expression sequences

• Cell stress and viability issues: aneuploid cells may divide poorly or trigger cell-cycle checkpoints

Developmental disorders and viability patterns

The syllabus highlights that chromosome-number changes can lead to altered phenotypes and developmental disorders. In general:

• Many autosomal monosomies are not viable and may result in early embryonic loss.

• Some trisomies can be viable to birth and are associated with characteristic developmental effects.

• Sex chromosome aneuploidies are often more compatible with life than autosomal aneuploidies because:

  • organisms can tolerate variation in sex chromosome dosage more than most autosomes

  • dosage compensation mechanisms reduce, but do not eliminate, imbalance

Recognising that nondisjunction can occur in either mitosis or meiosis is essential, because it predicts whether the entire organism or only certain tissues will carry the chromosomal error.