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Genotyping
is a word that describes a series of procedures to help determine the
unique set of genes that make up an individual's DNA. After DNA is
collected (from buccal cells), we use a technique called the Polymerase
Chain Reaction (PCR) that is based on Nature's own method of copying
genes. However, whereas all
genes are duplicated when a cell divides, PCR specifically makes copies of
a particular gene of interest.
An example
of how this works will be helpful. We are interested in the role of
the neurotransmitter serotonin in human brain function. Serotonin
has been implicated in a variety of disorders of mood and thought. One of
the genes involved in serotonin signaling codes for the serotonin
transporter, a protein that regulates the 'strength' of the signal at
serotonin synapses. Although all humans have the gene for the
serotonin transporter (we actually have 2 copies of the gene - one from each
parent), humans vary in the genetic coding of the serotonin transporter gene,
which may lead to stronger vs. weaker signaling at serotonin synapses.
The
variable coding region of the serotonin transporter gene that we are
interested in is called an 'insertion/deletion' site. That is, most
humans either have an additional 44 'letters' (called base-pairs, bp) of
genetic code in this region, or they do not. The addition of the
44-bp region results in the 'long' form of the gene, whereas the absence
of the 44-bp region results in the 'short' form. Since we all have
two copies of every gene, three different genotypes are possible. An
individual may be l/l (two copies of the long form), l/s (one copy of the
long, one copy of the short), or s/s (two copies of the short).
Therefore, after
obtaining a small quantity of DNA from cheek cells, PCR is used to specifically
make copies of the region of the serotonin transporter gene that contains the
44-bp insertion/deletion site. Millions of copies of just this region will
be made, the point being to make enough so that we can see it! That is,
depending on the presence or absence of the 44-bp, the PCR will generate two
different-sized fragments of DNA, which can then be visualized. The different-sized fragments of DNA appear as
'bands' on a gel, separated by size. Below is a picture of a gel showing
the banding pattern associated with each of the three serotonin transporter
genotypes.
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