New SAT Reading Practice Test 86: Hemoglobin

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Hemoglobin

This passage is adapted from A.F. Young, "Pathogenesis of Hemoglobinopathies."

There are few biochemical compounds
as familiar to us as hemoglobin, and as the
primary transporter of oxygen in our blood,
the celebrity of this curious little compound
05is not without just cause. Vital to almost
every known vertebrate, hemoglobin appears
within the very first week of embryogenesis,
and while its role may not change throughout
development, its molecular structure undergoes
10a series of significant transformations.
Within the red blood cell, hemoglobin
exists as a four-subunit complex, or "tetramer,"
each subunit of which is made up of
one "heme" metalloprotein, and one of several
15varieties of "globin." Comprised of iron
and a carbon-nitrogen ring, heme is responsible
for both the oxygen-binding capacity
of hemoglobin, and for the red coloration of
blood. Globin, meanwhile, refers to a folded
20chain of polypeptides, and it is the combination
of these chains that imparts each type of
hemoglobin with its unique characteristics.
In humans, six globin chains are expressed
sequentially throughout development.
25Embryonic hemoglobin, or HbE, is composed
of two ζ chains and two ε chains, both
of which are expressed exclusively during
the embryonic period. In the fetal period,
another tetramer of two α chains and two
30γ chains emerges, and persists for the first
six months of postnatal life. Due to its high
affinity for binding gases, this fetal hemoglobin,
or HbF, is able to extract oxygen from
low-affinity maternal hemoglobin, and thus
35plays a crucial role in the oxygenation of fetal
tissues. Like HbF, the final two physiologic
hemoglobins, HbA and HbA2, also require
a pair of a chains, and differ only in being
coupled to two β chains, and two δ chains,
40respectively. Typically, both HbA and HbA2
are synthesized at fairly stable concentrations,
though HbA is produced in far greater
abundance.
Given the tremendous import of these
45complexes, it should hardly be surprising
that errors in their production can yield
devastating results. What may be surprising,
however, is that these errors—including
sickle-cell disease and thalassemia—are
50among the most common of all inherited
genetic disorders, with an estimated 7% of
the world's population as carriers, two-thirds
of whom reside in Africa.
Thalassemia describes a group of disorders
55in which either the α or β chain is
quantitatively reduced. Depending on the
mutation, these defects can present with a
wide range of anemia-related symptoms, and
are particularly prevalent throughout Africa,
60Southeast Asia, and the Mediterranean. This
geographical distribution is anything but
random. Many studies have demonstrated
that the production of suboptimal hemoglobin
confers a degree of protection against
65malaria, a potentially deadly infectious disease
caused by members of the?Plasmodium
genus, which parasitize red blood cells. It
follows, then, that whereas in many regions
throughout the world thalassemia may
70merely constitute disease, in those where
malaria is endemic, it represents a favorable
evolutionary advantage.
Owing to a redundancy in the human
genome, there are four copies of the α globin
75gene, with two α-coding regions on each
copy of chromosome 16. For this reason,
the spectrum of severity in α thalassemia is
particularly broad. For instance, deletion of
a single gene will result in a carrier state, and
80is unlikely to cause clinically acute symptoms.
Deletion of all four, meanwhile, leads
to a precipitation during the fetal period
of nonfunctional γ tetramers, also called
Hb Barts, and is universally lethal in utero.
85Similarly, a deletion of three copies typically
results in a serious but survivable anemia,
and is characterized by the formation of
Hb Barts in the fetal period, and nonfunctional
β tetramers, termed HbH, throughout
90adulthood. Predictably, a deletion of
two copies produces a still milder anemia,
but can be subclassified based on whether
the deletions occur on the same chromosome,
termed "cis" deletion, or on opposite
95chromosomes, termed "trans" deletion. The
trans subtype appears more commonly in
the Mediterranean, while cis is more often
found in Asia. Notably, it has been suggested
that the cis deletion may contribute to the
100relatively higher rates of failed pregnancies
observed in this part of the world.
Like its α counterpart, β thalassemia also
impairs the production of HbA. However, the
symptoms of β thalassemia will not become
105evident until after the first six months of
life, when the concentration of HbF wanes
to a critical threshold. Often, a compensatory
upregulation in the expression of HbA2
occurs in affected individuals, the effects of
110which can be pharmaceutically augmented
by a drug called hydroxyurea, which induces
the expression of HbF in children and adults.

The graph shows varying concentrations of globin chains during human development.

1. The general purpose of this passage is to

  • A. make an argument.
  • B. raise vital questions.
  • C. introduce a concept.
  • D. call for a course of action.

2. As used in line 4, the word "celebrity" most closely means

  • A. notoriety.
  • B. infamy.
  • C. personage.
  • D. festivity.

3. Based on lines 11-22 and the information in the graph, what makes the hemoglobin varieties distinct?

  • A. Whether there is a carbon-nitrogen ring
  • B. Whether there is a red coloration of the blood
  • C. Variation in the arrangement of metalloprotein chains
  • D. Variation in the arrangement of polypeptide chains

4. Which option gives the best evidence for the answer to the previous question?

  • A. Lines 11-15 ("Within . . . globin")
  • B. Lines 15-16 ("Comprised . . . ring")
  • C. Lines 16-19 ("heme . . . blood")
  • D. Lines 19-22 ("Globin . . . characteristics")

5. The paragraph in lines 44-53 most strongly suggests that it is surprising that

  • A. the vast majority of inherited genetic disorders occur due to malfunctions in hemoglobin production.
  • B. biochemical compounds so important to human development can so frequently have errors in their production.
  • C. such a critical part of human health is susceptible to widespread pandemic contagion.
  • D. sickle-cell disease and thalassemia, despite being genetically inherited diseases, can be responsible for the majority of early deaths in Africa.

6. As used in line 50, the word "common" most closely means

  • A. communal.
  • B. lowly.
  • C. widespread.
  • D. famous.

7. The purpose of lines 60-72 is to connect

  • A. hemoglobin development to migration patterns.
  • B. cultural characteristics to evolutionary traits.
  • C. geographic particularities to evolutionary adaptation.
  • D. environmental forces to medical innovations.

8. Lines 73-78 ("Owing . . . broad") most directly imply that the intensity of thalassemia would be more uniform if there were

  • A. fewer copies of the α globin gene in humans.
  • B. the same number of copies of the α globin gene in humans.
  • C. more copies of the α globin gene in humans.
  • D. more information is needed than is given in the selected sentences.

9. The paragraph in lines 73-101 suggests that the relationship between the number of globin genes deleted and the severity of disease is

  • A. inverse.
  • B. proportional.
  • C. equivalent.
  • D. random.

10. What evidence from the passage gives the best justification as to why the graph begins along the x-axis where it does?

  • A. Lines 5-7 ("Vital . . . embryogenesis")
  • B. Lines 19-22 ("Globin . . . characteristics")
  • C. Lines 47-53 ("What . . . Africa")
  • D. Lines 81-84 ("Deletion . . . utero")

11. Based on the graph, a rough measurement of which of the following globin chains would give the clearest indication that a child was born two months premature?

  • A. Alpha
  • B. Beta
  • C. Gamma
  • D. Zeta