GENETICSMULTIPLE ALLELES
About 30% of the genes in humans are di-allelic, that is they exist in two forms.
About 70% are mono-allelic, they only exist in one form and they show no variation.
A very few are poly-allelic having more than two forms.
Poly-allelic genes are usually associated with tissue types. They are so varied that they provide us with our genetic finger print.
Di-allelic genes can generate 3 genotypes.
Genes with 3 alleles can generate 6 genotypes.
Genes with 4 alleles can generate 10 genotypes.
Genes with 8 alleles can generate 36 genotypes
This is very important to our immune system which must tell the difference between our own cells and invading disease causing microbes.
The ABO blood system
This is a controlled by a tri-allelic gene. It can generate 6 genotypes.
The alleles control the production of antigens on the surface of the red blood cells. Two of the alleles are co-dominant to one another and both are dominant over the third.
| Allele IA produces antigen A |
| Allele IB produces antigen B |
| Allele i produces no antigen. |
| |
| Genotypes |
Phenotypes
(blood types) |
| IA IA |
A |
| IA IB |
AB |
| IAi |
A |
| IB IB |
B |
| IBi |
B |
| ii |
O |
Note:
Blood types A and B have two possible genotypes – homozygous and heterozygous.
Blood types AB and O only have one genotype each.
Blood types and transfusions
Blood types vary and your immune system recognises your own blood type as being self. Other blood types are recognised as non-self.
If a blood which is incompatible with your body is transfused it will result in the agglutination of the foreign red blood cells. This could be harmful to the patient so it is essential that the blood types are determined before a transfusion.
People who are Type A blood produce antibodies to agglutinate cells which carry Type B antigens. They recognise them as non-self.
The opposite is true for people who are Type B.
Neither of these people will agglutinate blood cells which are Type O. Type O cells do not carry any antigens for the ABO system. Type O cells pass incognito.
The table below summarises the compatibilities between the blood types at transfusion.
Note:
Type O blood may be transfused into all the other types. It is called the universal donor.
Type AB blood can receive blood from all the other blood types. It is called the universal recipient.
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