14
CASE FILES: BIOCHEMISTRY
COMPREHENSION QUESTIONS
[1.1]
A newly married African-American couple, both from families having
histories of good health, are about to have a child. If the incidence of
the sickle cell trait is approximately 1/12 among persons of African
descent in the United States, what is the chance that they will give birth
to a child that is affected by sickle cell disease?
A. 1/12
B. 1/24
C. 1/96
D. 1/288
E.
1/576
[1.2]
A 25-year-old African-American male with sickle cell anemia, who has
been hospitalized several times for painful sickle cell crises, has suc-
cessfully been free of these crises since he has been on hydroxyurea
therapy. Treatment with hydroxyurea results in which of the following?
A. An increase in the oxygen affinity of HbS
B. An increase in the levels of hemoglobin F (HbF) in red blood cells
C. A decreased cooperativity in oxygen binding by HbS
D. A posttranslational modification of HbS that prevents polymerization
E. A decreased ability of HbS to bind 2,3-bisphosphoglycerate (2,3-
BPG)
[1.3]
A pregnant woman is able to transfer oxygen to her fetus because fetal
hemoglobin has a greater affinity for oxygen than does adult hemoglo-
bin. Why is the affinity of fetal hemoglobin for oxygen higher?
A. The tense form of hemoglobin is more prevalent in the circulation
of the fetus.
B. There is less 2,3-BPG in the fetal circulation as compared to maternal
circulation.
C. Fetal hemoglobin binds 2,3-BPG with fewer ionic bonds than the
adult form.
D. The Bohr effect is enhanced in the fetus.
E. The oxygen-binding curve of fetal hemoglobin is shifted to the right.
Answers
[1.1]
E.
Since sickle cell anemia has an autosomal recessive inheritance
pattern and the incidence in the United States to persons of African
descent is approximately 1/12, then each parent has a 1/12 chance of
being a carrier. Since it is an autosomal recessive gene, then their off-
spring would have a 1/4 chance of being homozygous if both of their
parents were also carriers. Thus, the chances of having a child with
the sickle cell trait is (1/12)(1/12)(1/4) or 1/576.
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