{title2=$n,2n,\ldots$}
{wiki}

<Ploidy> counts complete sets of <chromosomes> in a <cell nucleus>. Write $n$ for the number of <chromosomes> in one set: <haploid> means one set, <diploid> two. <DNA replication> produces <sister chromatids> without changing <ploidy>.<Biology 2e by Mary Ann Clark and others/Cell Division>[Biology 2e, §10.1, Genomic DNA; Eukaryotic Chromosomal Structure and Compaction, final paragraph]{ref}

Consider an illustrative <species> with $2n=6$, hence $n=3$. One set contains three distinguishable <chromosome> types, $A,B,D$; the <diploid> <cell nucleus> contains two <homologous chromosomes> of each type. Labels describe <chromosome> identity, not particular <alleles>. Let $C$ be the <Nuclear DNA content> of one unreplicated set. All entries below refer to one completed product of division, or the starting <cell nucleus>, rather than the sum over products. Here “<chromatid> count” includes the single <DNA> copy in each unreplicated <chromosome>.

|| State
|| <Ploidy>
|| <Chromosomes>
|| <Chromatid> copies
|| <Nuclear DNA content>

| <G1 phase>, before <DNA replication>
| <Diploid>, $2n$
| 6
| 6
| $2C$

| After <S phase>, in <G2 phase>
| <Diploid>, $2n$
| 6
| 12
| $4C$

| Each product of <meiosis I>
| <Haploid>, $n$
| 3
| 6
| $2C$

| Each product of <meiosis II>
| <Haploid>, $n$
| 3
| 3
| $1C$

\Image[/chromosome-centromere-counting.png]
{title=Illustrative <chromosome> and <chromatid> bookkeeping for a stipulated $2n=6$ <species>}
{description=Original counting model, not a microscopy image or measured dataset. Colors distinguish parental <homologous chromosomes>; lengths distinguish three <chromosome> types. Paired dots on <sister chromatids> represent their two copied <centromere> regions, counted together as one replicated <chromosome> until separation. One representative product is shown after each division; the model omits <chromosomal crossover>.}

The arithmetic follows the allocation rules. <DNA replication> replaces each of six single copies with two associated <sister chromatids>: $6\times2=12$ copies still grouped as six <chromosomes>. <Meiosis I> allocates one <homologous chromosome> of every type to each product: three replicated <chromosomes>, with $3\times2=6$ copies. <Meiosis II> allocates one copy from each pair of <sister chromatids> to each final product: three single-copy <chromosomes>. Compare <meiotic DNA content> for the general symbolic bookkeeping. <Chromosomal crossover> can change the <alleles> on a <chromatid> without changing these counts when exchanges preserve material.

Keeping <ploidy>, <chromosome> count and <Nuclear DNA content> separate makes the beauty of <meiosis> explicit: copying supplies material for two divisions, while sorting determines which complete sets each product inherits. It also prevents interpreting a doubled <DNA> signal as evidence of an extra <homologous chromosome> set.


 Ploidy (source code)