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This passage is adapted from A.F. Young, "Pathogenesis of Hemoglobinopathies."
There are few biochemical compoundsas familiar to us as hemoglobin, and as theprimary transporter of oxygen in our blood,the celebrity of this curious little compound05is not without just cause. Vital to almostevery known vertebrate, hemoglobin appearswithin the very first week of embryogenesis,and while its role may not change throughoutdevelopment, its molecular structure undergoes10a series of significant transformations.Within the red blood cell, hemoglobinexists as a four-subunit complex, or "tetramer,"each subunit of which is made up ofone "heme" metalloprotein, and one of several15varieties of "globin." Comprised of ironand a carbon-nitrogen ring, heme is responsiblefor both the oxygen-binding capacityof hemoglobin, and for the red coloration ofblood. Globin, meanwhile, refers to a folded20chain of polypeptides, and it is the combinationof these chains that imparts each type ofhemoglobin with its unique characteristics.In humans, six globin chains are expressedsequentially throughout development.25Embryonic hemoglobin, or HbE, is composedof two ζ chains and two ε chains, bothof which are expressed exclusively duringthe embryonic period. In the fetal period,another tetramer of two α chains and two30γ chains emerges, and persists for the firstsix months of postnatal life. Due to its highaffinity for binding gases, this fetal hemoglobin,or HbF, is able to extract oxygen fromlow-affinity maternal hemoglobin, and thus35plays a crucial role in the oxygenation of fetaltissues. Like HbF, the final two physiologichemoglobins, HbA and HbA2, also requirea pair of a chains, and differ only in beingcoupled to two β chains, and two δ chains,40respectively. Typically, both HbA and HbA2are synthesized at fairly stable concentrations,though HbA is produced in far greaterabundance.Given the tremendous import of these45complexes, it should hardly be surprisingthat errors in their production can yielddevastating results. What may be surprising,however, is that these errors—includingsickle-cell disease and thalassemia—are50among the most common of all inheritedgenetic disorders, with an estimated 7% ofthe world's population as carriers, two-thirdsof whom reside in Africa.Thalassemia describes a group of disorders55in which either the α or β chain isquantitatively reduced. Depending on themutation, these defects can present with awide range of anemia-related symptoms, andare particularly prevalent throughout Africa,60Southeast Asia, and the Mediterranean. Thisgeographical distribution is anything butrandom. Many studies have demonstratedthat the production of suboptimal hemoglobinconfers a degree of protection against65malaria, a potentially deadly infectious diseasecaused by members of the?Plasmodiumgenus, which parasitize red blood cells. Itfollows, then, that whereas in many regionsthroughout the world thalassemia may70merely constitute disease, in those wheremalaria is endemic, it represents a favorableevolutionary advantage.Owing to a redundancy in the humangenome, there are four copies of the α globin75gene, with two α-coding regions on eachcopy of chromosome 16. For this reason,the spectrum of severity in α thalassemia isparticularly broad. For instance, deletion ofa single gene will result in a carrier state, and80is unlikely to cause clinically acute symptoms.Deletion of all four, meanwhile, leadsto a precipitation during the fetal periodof nonfunctional γ tetramers, also calledHb Barts, and is universally lethal in utero.85Similarly, a deletion of three copies typicallyresults in a serious but survivable anemia,and is characterized by the formation ofHb Barts in the fetal period, and nonfunctionalβ tetramers, termed HbH, throughout90adulthood. Predictably, a deletion oftwo copies produces a still milder anemia,but can be subclassified based on whetherthe deletions occur on the same chromosome,termed "cis" deletion, or on opposite95chromosomes, termed "trans" deletion. Thetrans subtype appears more commonly inthe Mediterranean, while cis is more oftenfound in Asia. Notably, it has been suggestedthat the cis deletion may contribute to the100relatively higher rates of failed pregnanciesobserved in this part of the world.Like its α counterpart, β thalassemia alsoimpairs the production of HbA. However, thesymptoms of β thalassemia will not become105evident until after the first six months oflife, when the concentration of HbF wanesto a critical threshold. Often, a compensatoryupregulation in the expression of HbA2occurs in affected individuals, the effects of110which can be pharmaceutically augmentedby a drug called hydroxyurea, which inducesthe 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
2. As used in line 4, the word "celebrity" most closely means
3. Based on lines 11-22 and the information in the graph, what makes the hemoglobin varieties distinct?
4. Which option gives the best evidence for the answer to the previous question?
5. The paragraph in lines 44-53 most strongly suggests that it is surprising that
6. As used in line 50, the word "common" most closely means
7. The purpose of lines 60-72 is to connect
8. Lines 73-78 ("Owing . . . broad") most directly imply that the intensity of thalassemia would be more uniform if there were
9. The paragraph in lines 73-101 suggests that the relationship between the number of globin genes deleted and the severity of disease is
10. What evidence from the passage gives the best justification as to why the graph begins along the x-axis where it does?
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?
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