Chromosomes

A chromosome is a structure made of one long DNA molecule associated with proteins. Along the DNA are many genes: sections of DNA at particular positions on the chromosome.

Homologous chromosome pairs

In a cell with two sets of chromosomes, the chromosomes form pairs. One chromosome in each pair is inherited from each parent. The two chromosomes in a pair are called homologous chromosomes.

Homologous chromosomes carry the same genes at corresponding positions. However, they do not necessarily carry identical versions of those genes. Different versions of a gene are called alleles, so the two chromosomes may carry different alleles of the same gene.