Download An example of Codominance: Human Blood

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Textbook: pages 85-86
There are more than 20 genetically determined blood group
systems known today, but the ABO and Rh systems are the
most important ones used for blood transfusions.
The ABO blood group was discovered in 1900-01 at the
University of Vienna by an Austrian named Karl
Landsteiner. He wanted to know why some people
benefited greatly from receiving a blood transfusion from
someone else, while others seemed to die from it…
How the human body’s immune system works:
Antigens: a molecule on a cell surface that elicits an
immune response (white blood cells attacking the
perceived foreign cell containing the antigen). The
antigen of interest found on blood cells is two
carbohydrates (A and B) that may be found on
blood cell surfaces.
Antibody: a protein your white blood cells produce
to attach to antigens and attempt to counter their
effect. You may have antibodies in you that are
seeking out antigen A or B.
If successful, the antibodies can cause blood
clumping (agglutination). The agglutinated red cells
can clog blood vessels and stop the circulation of the
blood to various parts of the body. They also crack
and its contents leak out in the body. The red blood
cells contain hemoglobin, which becomes toxic
when outside the cell. This can have fatal
consequences for the patient.
Rh blood types were discovered in 1940 by Karl Landsteiner and
Alexander Wiener. The Rh system was named after rhesus
monkeys, since they were initially used in the research to make the
antiserum for typing blood samples. If the antiserum agglutinates
your red cells, you are Rh+. If it doesn'
t, you are Rh-. People are
either Rh- or Rh+. Those who are Rh+ have an Rh antigen present
on the red blood cell surface. Rh- do not automatically have an
Rh+ antibody, but they will develop if any Rh+ blood is
introduced. Rh+ blood patients will not have any antibodies (can’t
have an antibody against something that’s not there: Rh- only
means no Rh+ antigen)
Clinically, the Rh factor, like ABO factors, can lead to serious
medical complications. The greatest problem with the Rh group is
not so much incompatibilities following transfusions (though they
can occur) as those between a mother and her developing fetus.
Mother-fetus incompatibility occurs when the mother is Rh- (dd)
and the father is Rh+ (DD or Dd). Maternal antibodies can cross
the placenta and destroy fetal red blood cells (Hemolitic anemia,
or Rh Disease). The risk increases with each pregnancy.
Europeans are the most likely to have this problem--13% of their
newborn babies are at risk. Actually only about ½ of these babies
(6% of all European births) have complications. With preventive
treatment, this number can be cut down even further. Less than 1%
of those treated have trouble. However, Rh blood type
incompatibility is still the leading cause of potentially fatal blood
related problems of the newborn. In the United States, 1 out of
1000 babies are born with this condition.