by Margret Casal, med. vet, PhD Section of Medical Genetics Veterinary Hospital of the University of Pennsylvania  What happens
to the blood samples that you have been collecting and sending to the
Section of Medical Genetics at the Veterinary Hospital of the University
of Pennsylvania? In Part 1 we discussed
the processes that are used to extract the DNA from the blood samples
that you have been sending us. This article describes DNA and its function,
which provides the basis for Part 3.
Remember that
there are about 40,000 genes that are contained in every individual’s
DNA and each gene codes for a specific protein. To put this in perspective:
Imagine your DNA as a chain of glass beads using beads of four different
colors. Each sequence of three beads codes for a specific amino acid (there
are 21 different amino acids) and a chain of amino acids make up a specific
protein. For example, a red bead followed by another red bead followed
by a green bead codes for lysine. Then another set of three colors (beads)
codes for tryptophan, and so forth. In the end you have a chain of amino
acids that make up one protein that is part of your hair, for example.
This sequence of colored beads that code for this whole protein is called
a gene. (Figure 1) Figure
1. This schematic diagram is example a gene made up of single
building blocks. A triplet of building blocks code for one amino acid
(AA). DNA essentially directs the cell to assemble amino acids into proteins.
Now to make
matters more complicated, imagine again your DNA as a string of glass
beads. Your whole DNA would extend from New York City to San Francisco.
You would have, for example, a sequence of glass beads from New Brunswick,
NJ to Lambertville, NJ that codes for one protein. This would be one gene.
If one single glass bead is mixed up in this sequence, the result will
usually be evident. It is called a mutation, which may lead to disease.
Thus, in healthy individuals, all of these genes are almost exactly the
same, and we can’t tell the difference between these individuals just
by looking at their genes. Following a
sequence of beads coding for a gene, there might be a sequence of beads
from Lambertville, NJ to Allentown, PA that codes for absolutely nothing,
as far as we know. If glass beads get mixed up in this so-called junk
DNA, it doesn’t matter for the health of the individual because it wouldn’t
be a mutation within a gene. However, this junk DNA is extremely useful
for our purposes: there are numerous repeated sequences of DNA (glass
beads) that we can find. (Figure 2) And because, mutations
in these sequences don’t affect the health or wellbeing of the individual,
differences can be passed on without consequences. This cartoon
shows a portion of DNA containing a marker (triplet repeats) in between
two genes. 
We call this
string of repeated sequences ‘markers’ because specific patterns of bead
repeats (with minor but unique differences) are found next to genes. For
example, we know that the junk DNA between Lambertville, NJ and Allentown,
PA is an approximately twenty-fold repeat of red-green-yellow, an eighteen-fold
repeat of blue-red-green, and so forth. In one dog breed, for example,
it may be 17 repeats of the first color combination and in another breed
19 repeats. These differences allow us not only to find the gene next
to these repeats but also to distinguish dog breeds on a molecular level.
Another stretch of junk DNA may have 16 repeats of red-green-green in
a son while its mother may have 17, the father 16 and an unrelated male
21 repeats. Thus, this marker allows us to tell who the father is. Since
we know some of the markers, we can use them to do paternity testing among
other things. Unfortunately, we don’t know all the markers yet and we
often don’t know what genes they are associated with. However, many of
us are trying to resolve these unknowns. Two examples are the Human Genome
Project and the Dog Genome Project. The details will be explained in
Part 3. For inquiries regarding submission of blood samples, contact: Anne Janis PO Box 928, Fayetteville, GA 30214  |
