MULTIPLE
ALLELES
Genes which have more than two
alleles
© 2007 Paul Billiet ODWS
Genes and their alleles
About 30% of the genes in humans
are di-allelic, that is they exist in two
forms, (they have two alleles)
 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
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© 2007 Paul Billiet ODWS
Combinations
Di-allelic genes can generate 3 genotypes
 Genes with 3 alleles can generate 6
genotypes (3+2+1)
 Genes with 4 alleles can generate 10
genotypes
 Genes with 8 alleles can generate 36
genotypes
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© 2007 Paul Billiet ODWS
Genes and the immune
system
Poly-allelic alleles are usually associated
with tissue types
 These genes are so varied that they
provide us with our genetic finger print
 This is very important to our immune
system which must tell the difference
between our own cells (self) and invading
disease causing microbes (non-self)
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© 2007 Paul Billiet ODWS
The ABO blood system
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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 codominant 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
© 2007 Paul Billiet ODWS
The ABO blood system
Genotypes
Phenotypes (Blood types)
IA IA
IA IB
IAi
A
AB
A
IB IB
IBi
ii
B
B
O
Note:
 Blood types A and B have two possible genotypes –
homozygous and heterozygous.
 Blood types AB and O only have one genotype each.
© 2007 Paul Billiet ODWS
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
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© 2007 Paul Billiet ODWS
Antigens
© Bioformatica
© Biology Labs Online
Agglutination
© Dr Delphine Grézel, Ecole Nationale Vétérinaire de Lyon
Blood types and transfusions
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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
What about type AB people?
© 2007 Paul Billiet ODWS
Donor-recipient compatibility
Recipient
Type
A
B
AB
O
A
Donor
B
AB
O
Note:
 Type O blood may be transfused into all
= Agglutination
the other types = the universal donor.
 Type AB blood can receive blood from
= Safe transfusion
all the other blood types = the universal
recipient.
© 2007 Paul Billiet ODWS