Download Lab 5 Prelab Reading

The Genetics of Codominance
Prelab Reading
During next week’s lab, we will explore one of the oldest kinds of genetic tests—blood typing. In lab, you and your
lab group will visit different stations set up around the room and solve short “case studies” by performing blood
typing tests with artificial blood.
Table 1. Frequency of ABO Blood
The biological basis for blood typing is carbohydrates that “decorate” the surface
Types in the U.S. Population
of our red blood cells. In humans, there are two carbohydrates in four
combinations that determine our blood type. Individuals that have the “A”
carbohydrate are blood type A. Individuals that have the “B” carbohydrate are
blood type B. Certain individuals have both “A” and “B” and fall into the category
of blood type AB. Lastly, people with neither “A” nor “B” are blood type O.
These four phenotypes are controlled by three alleles (Table 2). The “A” allele
produces the A carbohydrate. The “B” allele produces the B carbohydrate. Both of
these alleles are codominant (“dominant together”) over the recessive “O” allele
that does not make any carbohydrate. These alleles are found on the autosomal
chromosomes and therefore everyone has two alleles (for example: AA, OO, AB,
etc.). Because both A and B are codominant, when an individual has both A and
B, they exhibit the combined phenotype for each (type AB blood). Neither the A
nor the B dominate over each other, so both phenotypes are visible.
Table 2. ABO Blood Type Alleles
Alleles
Phenotype
Dominant/Recessive
A
“A” carbohydrate on red blood cells
dominant*
B
“B” carbohydrate on red blood cells
dominant*
O
neither “A” nor “B”
recessive
*Both A and B are codominant, which means that neither dominates over the other. O is recessive; both A and B
dominate over O.
Some Frequently Asked Questions about blood typing:
Q. Why is blood typing important?
A. Although blood typing is a nice genetic phenomenon, the inheritance of blood
type isn’t really what makes blood types important. Imagine you are blood type B.
Your red blood cells are covered with the “B” carbohydrate. From the moment your
cardiovascular system developed, your body has become “used” to the fact that your
“B” carbohydrate belongs on your cells, and the cells of your immune system
recognize the “B” carbohydrate as a “self ” (as opposed to an invading “nonself ”)
molecule. Throughout your life, however, you have been exposed to the “A”
carbohydrate (primarily through your diet), and your immune system has built up
antibodies (protective proteins) against the “A” carbohydrate, because they’re
recognized as “nonself.”
This is a big deal if you need to get a blood transfusion. Before someone gives you
Fig. 1. On left: a drop of type B blood
on a microscope slide, treated with
anti-A antibodies. On right: type B
blood cells agglutinate when treated
with anti-B antibodies.
blood, they have to check your blood type. If you have blood type B, it’s important that you NOT receive blood with
any “A” carbohydrate on it. Otherwise, the antibodies in your blood will attack these new blood cells and cause them
to agglutinate (clump; see Fig. 1). Clumped blood cells can become trapped in your capillaries and cause serious
medical problems.
If you are blood type B, you can only receive two kinds of blood: blood type B or blood type O. Blood type O has
neither “A” nor “B” and therefore will not produce an immune response.
Q. What does it mean if I’m B– or AB+? What do the positive and negative mean?
A. The positive and negative that is often reported on blood typing tests is actually a test for another molecule on the
surface of red blood cells not related to the “A” and “B” carbohydrates. These symbols stand for the presence (+) or
absence (–) of a marker called Rh factor. Because the test for Rh factor is almost identical for the test for the ABO
blood types, both tests are generally performed simultaneously and their results are reported together. They are,
however, independent issues.
While ABO blood types display the unique phenomenon of codominance, Rh factor is inherited in a normal
dominant/recessive manner. Most of the population (~85%) is Rh+, which is the dominant condition.
Q. Why should someone care if they are Rh+ or Rh–?
A. In most cases, it’s not important. It only becomes important if an Rh– mother is pregnant with an Rh+ baby.
During pregnancy, the placenta ensures that the mother’s blood (which is not accustomed to Rh factor and would
view it as foreign) does not mix with the baby’s blood. During birth, however, it is possible for some of the baby’s
blood to get into the mother’s bloodstream. Her immune system will begin to make small amounts of antibodies
against the Rh factor, but generally won’t cause her any trouble. But if the woman becomes pregnant a second time
with an Rh+ baby, those antibodies can cross the placenta and cause problems for the baby. This sounds serious but
can be easily dealt with by prescribing certain medications to the woman while pregnant with the second child.