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Meiosis matters in psychiatry because errors in this cell-division process directly cause chromosomal conditions like Down syndrome (trisomy 21), which carry recognisable psychiatric and developmental profiles. Meiosis is the type of cell division that produces gametes (sperm and egg cells) with half the normal chromosome number, haploid (23 chromosomes) rather than diploid (46 chromosomes). For example, imagine a chromosome pair that fails to separate properly during this halving process, called non-disjunction: an egg ends up with an extra copy of chromosome 21, and the resulting embryo has trisomy 21 (three copies instead of two). In practice, the risk of this error rises with maternal age, which is why older maternal age is a recognised risk factor discussed in genetic counselling.

Meiosis differs from mitosis, the type of division that makes identical diploid daughter cells for normal tissue growth, because meiosis specifically produces haploid gametes with half the chromosome number. It happens across two sequential divisions. Meiosis I is the reductional division, where homologous chromosome pairs separate, halving the number. Meiosis II is the equational division, where sister chromatids separate, similar to mitosis. During meiosis I, crossing over (exchange of genetic material between homologous chromosomes) and independent assortment (random distribution of chromosome pairs) both increase genetic diversity between resulting gametes. Non-disjunction, the failure of chromosomes to separate properly during either division, produces gametes with an abnormal chromosome number. This leads to aneuploidy (an abnormal chromosome count), such as trisomy 21 (Down syndrome, three copies of chromosome 21 instead of two). Take two sperm samples from the same man: crossing over and independent assortment mean no two sperm cells carry an identical genetic combination. The risk of non-disjunction, and so the risk of conditions like Down syndrome, rises measurably with increasing maternal age, which is why this is discussed in antenatal genetic counselling.

• A student compares the products of the two kinds of cell division. How many daughter cells does one parent cell yield through meiosis, and how many through mitosis?
• A genetic counsellor explains that the chance of a chromosomally abnormal pregnancy rises steeply with one parental factor in particular. Which factor carries this effect most strongly?
• A baby has Down syndrome, and testing shows 46 chromosomes with extra chromosome 21 material fused to chromosome 14. Which mechanism does this describe?
• A child has Down syndrome features that are unusually mild, and testing shows some cells with 47 chromosomes and others with 46. Which explanation fits this finding?

• Meiosis, produces haploid gametes with half the chromosome number, unlike mitosis which produces identical diploid daughter cells.
• Meiosis I, the reductional division separating homologous chromosome pairs, whereas meiosis II is the equational division separating sister chromatids.
• Crossing over and independent assortment, generate genetic diversity, rather than causing aneuploidy on their own.
• Non-disjunction, chromosomes fail to separate, causing aneuploidy such as trisomy 21, unlike normal segregation which produces a balanced chromosome number.

• Ordinary cell division produces two rather than four daughter cells.
• Paternal age chiefly raises the rate of new single-gene mutations instead.
• This reverses the two figures.
• The gamete-forming process involves two divisions, giving four cells.
• No third division occurs in gamete formation.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.