Haploid and diploid chromosome sets

A chromosome set contains one chromosome of each type. A nucleus may contain one set or two sets:

• A haploid nucleus contains one set, with one chromosome of each type.
• A diploid nucleus contains two sets, with a pair of each type of chromosome.

In a diploid human cell, the nucleus contains 23 pairs of chromosomes: 46 chromosomes in total. The 23 pairs represent two sets of 23 chromosomes.

The distinction between haploid and diploid describes the number of chromosome sets in a nucleus. It does not describe whether the chromosomes contain genes: chromosomes in both haploid and diploid nuclei carry genes.

Genes in chromosomal DNA

Chromosomes contain DNA, which carries genetic information. A gene is a length of DNA that codes for a protein. Genes are therefore parts of the DNA in chromosomes, rather than separate structures attached to them.

An allele is an alternative form of a gene. Different alleles of the same gene have differences in their DNA sequence.