5 Steps to a 5 AP Biology, 2014-2015 Edition

Heredity ❮ 103

If you have ever watched an episode of ERon television, you have heard one of the doctors
frantically scream, “We need to type her and bring some O blood down here stat!” Why is it
important for the physician to determine what type of blood the patient has, and why is it
okay to give the patient O blood in the meantime? People with type A blood produce anti-B
antibodies because the B antigen that is present on type B and type AB blood is a foreign mol-
ecule to someone with type A blood. This is simply the body’s defense mechanism doing its
job. Following the same logic, those with type B blood make anti-A antibodies, and those
with type O blood make anti-A andanti-B antibodies. People who are type AB make none,
and are therefore the universal acceptor of blood. It is important to find out what kind of
blood a person has because if you give type B blood to a person with type A blood, the recip-
ient will have an immune response to the transfused blood. Why is O blood given while they
wait to see what blood type the patient is? This is because type O blood has neither antigen
on the surface of red blood cells. People with type O blood are universal donors because few
people will have an adverse reaction to type O blood. Immune reactions are discussed in fur-
ther detail in Chapter 15, Human Physiology.

Epistasis

Inepistasisthe expression of one gene affects the expression of another gene. A classic
example of epistasis involves the coat color of mice. Black is dominant over brown, and
brown fur has the genotype bb. There is also another gene locus independent of the coat
color gene that controls the deposition of pigment in the fur. If a mouse has a dominant
allele of this pigment gene (Cc or CC), it leads to pigment deposition and the coloring of
the fur according to the coat color gene’s instructions. If a mouse is double recessive for this
trait (cc), it will have white fur no matter what the coat color gene wants because it will not
put any pigment into the fur. It is almost as if the pigment gene were overruling the coat color
gene. If you mate two black mice that are BbCc, the ratio of phenotypes in the offspring
wouldnot be the 9:3:3:1 ratio that Mendel predicts, but rather 9:4:3 black:white:brownbecause
the epistatic gene alters the phenotype.

Pleiotropy

Inpleiotropya single gene has multiple effects on an organism. A good example of
pleiotropy is the mutation that causes sickle cell anemia. This single gene mutation “sickles”
the blood cells, leading to systemic symptoms such as heart, lung, and kidney damage;
muscle pain; weakness; and generalized fatigue. The problems do not stop there; these symp-
tomscan lead to disastrous side effects such as kidney failure. The mutation of a single gene
wreaks havoc on the system as a whole.

Blood Type surface of RBCAntigens on

Antibodies

produced by the

body

Can be

transfused with

which types of

blood?

Can be donated to

individuals of

which type?

A Antigen A Anti B Type A, O Type A, AB

B Antigen B Anti A Type B, O Type B, AB

AB Antigens A & B None All Types Type AB

O No Antigens

Anti A and

Anti B Only O All Types

Figure 10.3 Several human blood type characteristics.

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