|
More is known about
the genetics of the blood than about any other human tissue. 
One reason for this is that blood samples can be easily secured
and subjected to biochemical analysis without harm or major discomfort
to the person being tested. Perhaps a more cogent reason is that
many chemical properties of human blood display relatively simple
patterns of inheritance.
Certain
chemical substances within the red blood cells (such as the ABO
and MN substances noted above) may serve as antigens. When cells
that contain specific antigens are introduced into the body of
an experimental animal such as a rabbit, the animal responds by
producing antibodies in its own blood.
In addition
to the ABO and MN systems, geneticists have identified about 14
blood-type gene systems associated with other chromosomal locations.
The best known of these is the Rh system. The Rh antigens are
of particular importance in human medicine. 
Curiously,
however, their existence was discovered in monkeys. When blood
from the rhesus monkey (hence the designation Rh) is injected
into rabbits, the rabbits produce so-called Rh antibodies that
will agglutinate not only the red blood cells of the monkey but
the cells of a large proportion of human beings as well. Some
people (Rh-negative individuals), however, lack the Rh antigen;
the proportion of such persons varies from one human population
to another. Akin to data concerning the ABO system, the evidence
for Rh genes indicates that only a single chromosome locus (called
r) is involved and is located on chromosome 1. At least 35 Rh
alleles are known for the r location; basically the Rh-negative
condition is recessive.
Rick
Weiss
|