Electrophoresis is used extensively in revealing
genetic variation in
enzymes and other
proteins. In the case where all individuals in the samples reveal an enzyme with the same
electrophoretic mobility it is called
monomorphism. A hypothetical
gel showing the stained bands can be seen below.
Monomorphism
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all five bands display identical electrophoretic mobility (they are the same enzyme)
That shows how electrophoresis works in a more simple scenario in which the individuals are
homozygous for an
allele represented by the migrating enzyme, however, often you must take into account the presence of
heterozygous individuals.
Allozymes are enzymes that have differing eletrophoretic mobilites resulting from allelic differences at a single
locus. For example, consider a hypothetical allele which exists in two forms; one associated with a rapidly migrating enzyme and the other associated with a more slowly migrating enzyme. When tested using gel electrophoresis, the results yielded will show three possible outcomes; homozygous for the rapidly migrating enzyme, homozygous for the slowly migrating enzyme and heterozygous for both rapid and slow enzymes. This means that in homozygous individuals only one band will appear in the gel, representing either rapid or slow enzymes, and in heterozygous individuals two bands will appear, once stained, to represent both rapid and slow enzymes.
Monomeric Polymorphism
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R= rapid
R/S R/R S/S R/S S/S S= slow
In the above example the enzyme itself is
monomeric, meaning it consists of only one
polypeptide chain, so the heterozygous individual will show the two possibilities of a rapid enzyme and slow enzyme; each enzyme representing a single allele. However, if the enzyme were a
dimer, molecularly consisting of two polypeptide chains, three possibilities arise; enzymes containing two rapidly migrating polypeptides, two slowly migrating polypeptides, and both rapidly and slowly migrating polypeptides. The rapidly migrating (rapid + rapid) and slowly migrating (slow + slow)
homodimers behave in a similar manner as in the case of a
monodimer, but the
heterodimer (rapid + slow) will typically have an intermediate electrophoretic mobility. In the case of
multimeric (consisting of multiple polypeptide chains) enzymes the outcome becomes increasingly complex.
Dimeric Polymorphism
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__ __ RR= rapidly migrating homodimer
__ __ SS= slowly migrating homodimer
RS= heterodimer (rapidly + slowly migrating)
RR/RS RS/SS RR/RR SS/SS