A gene occupies a fixed position, called a locus, on a particular strand of DNA. Genes are sections of DNA that contain coded information as a specific sequence of bases. Genes code for polypeptides that determine the nature and development of organisms. The base sequence of a gene can change as a result of a mutation, producing one or more alleles of the same gene. A sequence of three bases, called a triplet, codes for a specific amino acid. The base sequence of a gene determines the amino acid sequence in a polypeptide.

Three base pairs code for a amino acid. This is called the triplet code.

E.g. a thymine followed by a guanine followed by a thymine is the code for the amino acid cysteine.

A sequence of base pairs can, therefore, make a polypeptide chain (chain of amino acids that makes up a protein.)

A section of DNA that codes for a specific protein is called a gene.

Organisms are made of and controlled by proteins, so genes determine what an organism is like.

The base sequence of a gene can change as a result of a mutation,  producing one or more alleles of the same gene

An allele of a gene is a different protein produced for the same purpose.

E.g the proteins in the iris can be brown or blue (or many other colours.) So brown and blue are different alleles for the same gene.

Different alleles occur when there is a change (mutation) in the base pairs, so the amino acids are made differently resulting in a different protein.

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The locus is the place on the DNA where a gene is.