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Naturally Occurring Polymers—Animals 365

heart disease. But when they change diet so that it is similar to many of ours, they become more
susceptible to heart disease.
Our blood type is determined by a gene that is present on chromosome 9, near the end of the
long arm. There are four general blood types, A, AB, B, and O. Some of these are “inter-mixable”
while others are not. For instance, A blood from a person is compatible with A and AB; B with B
and AB; and AB with only AB; and O blood is compatible with all of the blood types—a person
with type O is then an universal donator. These compatibility scenarios are not race related. For
all but the native Americans that have almost totally type O the rest of us have about 40% type O;
another 40% type A; 15% type B; and 5% with type AB. (Some of the Eskimos are type AB or B
and some Canadian tribe are type A.) A and B are codominant versions of the same gene and O is
the “recessive” form of this gene.
The active codons of the blood-type gene are about one thousand base-pairs long and are divided
into six short and one longer sequence of exons. The difference between the type A and type B
gene is seven letters of which three do not make any difference in the amino acid coded for. The
four truly different bases are positioned at sites 523, 700, 793, and 800 and are C, G, C, G for type
A and G, A, A, C for type B blood. Type O people have just a single change from the type A peo-
ple with a deletion in the type A base at base pair 258, omitting the G base. While this appears to

be very minor, it is significant in that it causes a reading or frame-shift mutation. These seemingly

minor changes are sufficient to cause the body to have an immune response to different types of
blood. Even so, while this causes a different type of blood, it appears to have little or nothing to do
with other parts of the overall human genome so that tendencies toward cancer, ageing, and so forth

are not influenced by this change but it appears to have something to do with some general tenden-
cies towards some diseases. For instance, those with AB blood are the most resistant toward chol-
era while those with type O blood are most susceptible. Those with two copies for the sickle-cell
mutation generally contract sickle-cell anemia while those with one copy of the mutation are more
susceptible to contracting sickle-cell anemia than the general public but they are more resistant to
malaria. Some of these connections can be found in tracing the ancestry of individuals with the par-
ticular connections between blood type and susceptibility.
Recently, we have been working on Parkinson’s disease. Parkinson’s disease, and other simi-
lar diseases, are due to a depletion of dopamine in the corpus striatum. Direct addition of dopa-
mine is not effective in the treatment presumably because it does not cross the blood–brain barrier.
However, levodopa, the metabolic precursor of dopamine, does cross the blood–brain barrier and is
believed to then be converted to dopamine in the basal ganglia.
On the short arm of chromosome 11 is a gene known as D4DR that manufactures the protein
dopamine receptor. It is active in some parts of the brain and inactive in other parts. Dopamine is
a neurotransmitter released from the end of neutrons by an electrical signal. When the dopamine
receptor is exposed to dopamine, it also releases an electrical signal. In general, much of the brain
activity is related to such stimulation of chemical reactions by electrical changes, and counter, elec-
trical current calling for chemical reactions. Many of these exchanges occur essentially at the same
time. Digressing for a moment, our brain is often compared to the operation of a computer. In some
sense it is but in other senses it is not. In our brain “switches” are activated and closed, opened and
shut, by not simply electrical charge, but rather by an electrical switch associated to a very sensitive,
selective chemical site.
Brain sites that have an active D4DR are then part of the brain’s dopamine-mediated system. A
lower amount of dopamine causes this part of the brain’s system to either shutdown or be less than
fully active and in extreme cases resulting in Parkinson’s and related diseases. Excess of dopamine
may led to schizophrenia. Some of the hallucinogenic drugs act to increase the amount of dopa-
mine. Thus, there is a tight balance between good health and health problems.
D4DR has a variable repeat sequence in it about 48 base pairs in length. Most of us have between
4 and 7 such sequences. The larger the number of repeat units the more ineffective is the dopamine
receptor at capturing dopamine while a low number of such repeat sequences means the D4DR gene

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