One set or two sets

1. A set of chromosomes contains one of each chromosome type for that species.
2. A haploid nucleus contains one set, represented by $n$, while a diploid nucleus contains two sets, represented by $2n$.
3. The symbols $n$ and $2n$ describe the number of chromosome sets rather than the size of a cell.
4. Human gametes, sperm and eggs, are haploid and normally contain $23$ chromosomes.
5. Most human nucleated body cells are diploid and normally contain $46$ chromosomes.
6. The human diploid total can be organised as $23$ pairs, with one chromosome of each pair inherited from each parent.

Chromosome pairs

1. The chromosomes in an ordinary homologous pair have the same genes at the same positions.
2. They need not have identical DNA sequences because they can carry different alleles of those genes.
3. A haploid gamete has one member from each ordinary homologous pair and therefore one copy of each of those genes.
4. The matching pair in a diploid nucleus contains two gene copies, even when their alleles differ.

Fertilisation combines two sets

1. At fertilisation, the nuclei of two haploid gametes fuse and combine their chromosome sets.
2. The resulting zygote has two sets of chromosomes and is therefore diploid.
3. The relationship is $n+n\to 2n$.
4. In humans, this is $23+23\to 46$, with $23$ chromosomes contributed by each parent.
5. Subsequent normal mitotic divisions copy and distribute the chromosomes so that the developing organism's daughter cells retain the diploid number.

Applying chromosome numbers

1. Different species have different chromosome numbers, so use the number supplied rather than automatically using the human total.
2. If the diploid number is known, calculate the haploid number using $n=\frac{diploid\; chromosome\; number}{2}$.
3. If the haploid number is known, double it to find the diploid number: $2n=2\times haploid\; chromosome\; number$.
4. Count sets rather than assuming that any even chromosome number must be diploid, since a haploid number can also be even.

Reading chromosome diagrams

1. Identify matching chromosome types using their length, shape and gene positions when these are shown.
2. Two copies of each ordinary chromosome type indicate two sets, whereas one copy of each type indicates one set.
3. The two joined arms of an X-shaped replicated chromosome are sister chromatids, not a homologous pair.
4. Copying DNA does not by itself change a diploid nucleus into a nucleus with four chromosome sets.
5. Mature human red blood cells have no nucleus, so the usual diploid body-cell chromosome count does not apply to them.