Comparisons of amino acid sequences in specific proteins can be used to elucidate relationships between organisms. Immunological comparisons may be used to compare variations in specific proteins. Candidates should be able to interpret data relating to similarities and differences in base sequences in DNA and in amino acid sequences in proteins to suggest relationships between different organisms.

Genes code for amino acids to make proteins. So if we look at a protein in two different species and examine its amino acids, we can see how similar or different their genes are.

For example, (arbitrary data used) if we took heamaglobin from a gorilla and a human and looked at the amino acids and they looked like this:
Human: Ser; Val; Ser; Glu; Ile; Gln; Leu; Met; His; Asn
Gorilla: Ser; Val; Val; Ser; Ile; Gln Leu; Met; His; Asn
then we could see they are relatively genetically similar with  7/10 amino acids are the same.

One way to compare proteins is by comparing antigens on the body cells of a specie. This is immunological comparison:

• Extract blood serum from a species
• Put it in a second species
• This species will produce antibodies that are complimentary to the antigens of the first species
• Extract the antibodies
• Mix them with the blood serum of a third species
• A precipitate will form if the antibodies respond to the antigens of that specie

If the antibodies that were created for the antigens of the first specie can respond to antigens of the third then that means it must be complimentary to both. That means that both antigens had a similar tertiary structure, which means they had a similar primary structure, which means they had a similar sequence of amino acids which means that had a similar base sequence which means they are genetically similar. Therefore antibody can respond to both= more precipitate= more genetically similar.

If there is not very much precipitate its because the antibody can respond to the antigens in the third species because it is not sufficiently similar to the first, this shows us that they are genetically dissimilar.