In body cells (all your cells apart from sex cells), chromosomes come in pairs because we have two parents). One chromosome in every pair has come from each parent.
The sex cells (the sperm and egg) are different from ordinary body cells because they contain just 23 single chromosomes - that's one chromosome from each pair.
When the sperm fertilises the egg (during sexual reproduction), the 23 chromosomes in the sperm combine with the 23 chromosomes in the egg.
The fertilised egg then has 23 pairs of chromosomes, just like an ordinary body cell.
The two chromosomes in a pair always carry the same genes and each gene is always found in the same place on the two chromosomes. Because the two chromosomes in a pair came from different parents, they might have different alleles of these genes. Alleles are different versions of the same gene.