How are DNA, chromosomes, genes, and alleles related?
Alleles are different forms of same gene.
Genes are linearly arranged on chromosomes.
Chromosomes contain genetic material of cell i.e. DNA. So chemically alleles, genes, chromosomes are all DNA !
- Let us start the story at the level of an eukaryotic diploid cell. There is a nucleus, inside which you see chromosomes in pairs. These are called homologous pairs: they are in duplicate.
- Each chromosome contains a linear DNA molecule, closely associated with histone proteins. DNA is genetic material, made of only four different nucleotides arranged in a double helix. It contains chemical codes which guide life processes, written by using those four letters.
- Along the length of DNA molecule, only some stretches are 'meaningful' and are called genes. Hence, several genes are located on same chromosome. Homologous chromosomes carry same genes, generally in same order.
- Now if we consider one gene, the sequence of genetic letters in one chromosome may slightly differ from the sequence in another. This is why one gene may have different alleles. For example, at the genetic locus where gene for our eye colour is located, so much variations are present that you see various different colours of iris in different people.