In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example, introns within genes and multiple repeats between genes.

Introns are sections of DNA that do not code for anything.

Sometimes they are in the middle of genes and sometimes they come between different genes.

Due to these introns very little of DNA has a code that actually makes proteins.

Differences in base sequences of alleles of a single gene may result in non-functional proteins, including non-functional enzymes.

Bases in DNA can change (mutate) during replication.

The amino acid that the base pair coded will change, this will result in a different protein being made.

This different protein is called an allele, because it is a different form of a gene. (E.g blue or brown eyes.)

Sometimes the changes in the base pairs can make a polypeptide chain that makes a non-functional protein.

If a base is changed, one amino acid will be different which will change the protein a bit.

If a base is added in or taken away, it will change every amino acid in the chain- because it is read as in triplets so the whole sequence will move along one- which is likely to make a non-functional protein.

Enzymes are proteins, they can be made non-functional by changes in bases.

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.