Chromosome mutations & non-disjunction

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A change in the **number (or structure) of whole chromosomes**, rather than a change to the DNA bases of one gene.

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The **failure of chromosomes (meiosis I) or sister chromatids (meiosis II) to separate** during meiosis, so both copies end up in the same gamete.

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'Disjunction' = separating, so **non-disjunction = not separating**.

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One gamete with an **extra chromosome (n + 1)** and another **missing that chromosome (n − 1)**.

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Having an **abnormal number of chromosomes** — one too many or one too few — rather than a whole extra set.

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Having **three copies** of a particular chromosome instead of the normal two.

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**Chromosome 21** — three copies is called **trisomy 21**.

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Chromosome 21 **fails to separate** in meiosis → a gamete gets an **extra copy** → **fertilisation** adds a third copy → **trisomy 21**.

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The whole body grows from the single zygote by **mitosis**, so **every cell** inherits the extra chromosome.

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A gene mutation changes **a few DNA bases** in one gene; a chromosome mutation adds or loses a **whole chromosome** and can be seen on a karyogram.

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It **increases with age**, slowly at first and then **steeply** at older ages.

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**Yes** — an extra or missing whole chromosome shows up as an extra (or absent) band, unlike a tiny gene mutation.