Because natural selection acts against genes that cause inherited disorders, lethal genetic diseases should be, and generally are, very rare. Thus it seems surprising that certain inherited disorders of red blood cells, notably sickle cell anemia and thalassemia, occur in some populations at unusually high frequencies. We have been able to explain this phenomenon through natural selection by assuming that the same variant gene that causes the lethal diseases in homozygous individuals (who inherit two abnormal genes, one from each parent) protects heterozygous individuals (who inherit one abnormal and one normal gene) against another potentially lethal, and more prevalent, disease—in this case-malaria, which is produced by a parasite that infects red blood cells. That protection maintains the high frequencies of these otherwise deleterious genes. The strength of malaria as a selective force derives from its powerful effects on the health and reproductive capacity of human populations. Malaria has been a major cause of death throughout history, contributing in Africa today to early-childhood mortality rates that are as high as 50 percent. It kills about 10 percent of its victims directly and contributes to the death of others by decreasing the ability of their immune systems to fight off other infections. Its high mortality rate ensures that a significant number of individuals will not live to reproduce; thus, any genetic mutation that provides resistance to malaria must have a high selective advantage. That the sickle cell gene might confer such resistance was first indicated by the coincidence of the geographic ranges of sickle cell disease and malaria. Clinical evidence was harder to come by, but in 1954 Allison showed that children with the sickle cell gene had much milder cases of malaria than did children without it. Because the biochemical mechanism of this resistance to Malaria could not, however, be established, the role of the sickle-cell gene could not be unequivocally demonstrated. This state of affairs persisted until 1977, when, after fifty years of attempts, a procedure was devised that allowed researchers to maintain malaria parasites in a laboratory culture. Using this technique, scientists could finally investigate how sickle cell blood cells protect a heterozygous carrier against malaria. They soon discovered that the parasite in an infected sickle cell develops normally up to the time that the cell is sequestered in tissue. There the low-oxygen environment and the low intracellular pH induce the characteristic sickling in the host cell, causing its potassium level to 'drop; this in turn causes the parasite to die. Such a process can protect against malaria even if not all of the parasites are affected because any meaningful reduction in the rate of multiplication of the parasite gives the immune system the time necessary to mount a protective response of its own. 1 1. The primary purpose of the passage is to A. announce that the mechanism of sickle cell anemia has been discovered and to explain why the discovery was so difficult B. explore the limits of natural selection and the way in which certain diseases can transcend them C. describe how sickle cell anemia is produced and where it is most prevalent D. explain why the gene for sickle cell anemia has persisted and how this explanation was confirmed E. analyze the effects of malaria on human populations and to suggest a way in which these effects can be minimized 2 2. According to the passage, which of the following is typically true of lethal genetic disorders? A. They protect against some other lethal disease. B. They are diseases of the red blood cells. C. They have extremely low frequencies of occurrence. D. They kill both homozygous and heterozygous individuals. E. They occur only in certain geographical areas. 3 3. According to the passage, most malaria-related deaths occur A. in areas only recently exposed to the disease B. among young children C. because of a mutation in the victim's genes D. in individuals heterozygous for the sickle cell gene E. because victims become more susceptible to other infections 4 4. It can be inferred from the passage that the number of individuals homozygous for the sickle cell gene is A. approximately equal to the number who contract malaria B. slightly smaller than the number who are heterozygous for the sickle cell gene C. significantly smaller than the number who contract malaria D. significantly larger than the number who are heterozygous for the sickle cell gene E. Approximately equal to the number who die of malaria 5 5. According to the passage, the immediate cause of death of a malaria parasite in a red blood cell undergoing sickling is A. a drop in the pH level of the cell B. a drop in the potassium level of the cell C. a drop in the oxygen content of the environment surrounding the cell D. the sequestering of the cell in tissue E. the protective response of the organism's immune system 6 6. The author suggests that in a lethal attack of malaria, which of the following occurs? A. The intracellular pH drops much too slowly to induce sufficient sickling of red blood cells. B. The malaria parasites spread much more rapidly than the host's immune system can respond. C. Additional homozygous red blood cells are produced far more slowly than the malaria parasites spread. D. The introduction of the malaria parasite prevents the host's immune system from responding. E. The host's red blood cells art not sequestered in tissue in quantities sufficient to produce adequate supplies of sickled cells. 7 7. Which of the following best describes the organization of the first paragraph? A. A generalization is presented and a seeming anomaly is reconciled with it. B. A thesis is advanced and a counterexample is introduced to refute. C. An argument is advanced and a general conclusion is derived from IL D. An explanation is proposed and examined in E. Opposing views are presented and their differences clarified. 8 8. Which of the following is most dearly an example of the way in which the majority of fatalities from malaria occur, as the process is described in the passage? A. After an attack of malaria, the victim dies from a case of pneumonia. B. Weakened from a case of the flu, the victim dies after contracting malaria. C. The victim contracts malaria and dies shortly after exhibiting the first symptoms. D. The victim inherits a blood disorder and quickly dies when infected with malaria. E. The victim contracts malaria and, weakened by repeated bouts with it, eventually dies.

Last visit was: 19 Nov 2025, 13:10

It is currently 19 Nov 2025, 13:10

Toolkit

Reading Comprehension (RC)

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Go to My Error Log Learn more

Request Expert Reply

Confirm Cancel

Nov 19
GMAT 705 in 30 days: Best Study Strategy for Working Professionals
12:30 PM EST
-
01:30 PM EST
Learn how Keshav, a Chartered Accountant, scored an impressive 705 on GMAT in just 30 days with GMATWhiz's expert guidance. In this video, he shares preparation tips and strategies that worked for him, including the mock, time management, and more
Nov 20
UCR MBA Impact - Live Q&A with UCR AdCom & Student
07:30 AM PST
-
08:30 AM PST
Learn what truly sets the UC Riverside MBA apart and how it helps in your professional growth
Nov 20
GMAT 675 with 96th percentile in DI- Kamakshi’s Winning Strategy
01:30 PM EST
-
02:30 PM IST
Learn how Kamakshi achieved a GMAT 675 with an impressive 96th %ile in Data Insights. Discover the unique methods and exam strategies that helped her excel in DI along with other sections for a balanced and high score.
Nov 22
GMAT RC Masterclass
11:00 AM IST
-
01:00 PM IST
Do RC/MSR passages scare you? e-GMAT is conducting a masterclass to help you learn – Learn effective reading strategies Tackle difficult RC & MSR with confidence Excel in timed test environment
Nov 23
Conquer Algebra on the GMAT Focus Edition
11:00 AM IST
-
01:00 PM IST
Attend this free GMAT Algebra Webinar and learn how to master the most challenging Inequalities and Absolute Value problems with ease.
Nov 24
FREE Focus Edition Mock!
07:00 PM PST
-
08:00 PM PST
Full-length FE mock with insightful analytics, weakness diagnosis, and video explanations!
Nov 25
Try OnDemand GMAT Masterclass
10:00 AM EST
-
11:00 AM EST
Prefer video-based learning? The Target Test Prep OnDemand course is a one-of-a-kind video masterclass featuring 400 hours of lecture-style teaching by Scott Woodbury-Stewart, founder of Target Test Prep and one of the most accomplished GMAT instructors.

Back to Forum

Create Topic

Because natural selection acts against genes that cause inherited diso

1 2

Sajjad1994

GRE Forum Moderator

Joined: 02 Nov 2016

Last visit: 19 Nov 2025

Posts: 17,304

Own Kudos:

49,310

Given Kudos: 6,180

GPA: 3.62

Products:

Posts: 17,304

Kudos: 49,310

16 Sep 2023, 12:36

Kudos

Bookmarks

kittle

Sajjad1994 - there are so many typos in the passage. This leads to massive confusion for a reader, eventually leading to improper understanding of the passage. Request you to please vet every passage before posting here. I have noticed this multiple times. Writing it out now as this is GMAT OG passage where we can not have such mistakes

My personal confidence got impacted because I was not able to understand it. What is 'sickle all anemia' - it is Sickle cell anaemia

Can you please point out more typos except the one you mentioned, I think this was the only typo which is fixed now.

Thank you.

kittle

Joined: 11 May 2021

Last visit: 19 Nov 2025

Posts: 318

Own Kudos:

161

[1]

Given Kudos: 618

Products:

Posts: 318

Kudos: 161

[1]

17 Sep 2023, 00:58

Kudos

Bookmarks

Sajjad1994

kittle

Can you please point out more typos except the one you mentioned, I think this was the only typo which is fixed now.

Thank you.

Sajjad1994 - see below for instance "art" ? Should be are - during passage reading this makes it a bit confusing
D. significantly larger than the number who art heterozygous for the sickle cell gene

Moreover, look at this sentence - looks like a fragment in the passage. Definitely something is missing:
"Thus it seems surprising that certain inherited disorders of red blood cells." in the passage itself

nikkimah

Joined: 20 Jul 2023

Last visit: 08 Jan 2025

Posts: 22

Own Kudos:

Given Kudos: 16

Posts: 22

Kudos: 1

15 Oct 2024, 03:08

Kudos

Bookmarks

Can someone explain Q4 in detail? As the passage states that sickle cell gene in homozygous individuals causes the lethal diseases while the same gene protects in heterozygous individuals, can we infer that the population for homozygous individuals will be much smaller who have sickle cell gene because it will cause lethal disease?

karishma GMATNinja marty.

Thanks

Nsp10

Joined: 22 May 2023

Last visit: 19 Nov 2025

Posts: 122

Own Kudos:

[1]

Given Kudos: 112

Location: India

Schools: IE Schulich

GPA: 3.0

Products:

Schools: IE Schulich

Posts: 122

Kudos: 88

[1]

Updated on: 03 Mar 2025, 07:35

Originally posted by Nsp10 on 03 Mar 2025, 07:14.
Last edited by Nsp10 on 03 Mar 2025, 07:35, edited 1 time in total.

Kudos

Bookmarks

bb
@[color=#ff0000]Sajjad1994[/color]
Bunuel
"I think there might be a spelling mistake here.
Could you please check if 'milder' was intended instead of 'raider'?
Para 3 line 6.

Bunuel

Math Expert

Joined: 02 Sep 2009

Last visit: 19 Nov 2025

Posts: 105,390

Own Kudos:

778,346

[1]

Given Kudos: 99,977

Products:

Expert

Expert reply

Posts: 105,390

Kudos: 778,346

[1]

03 Mar 2025, 07:24

Kudos

Bookmarks

Nsp10

bb
@[color=#ff0000]Sajjad1994[/color]
@Bunnel
"I think there might be a spelling mistake here.
Could you please check if 'milder' was intended instead of 'raider'?
Para 3 line 6.

Fixed the typo. Thank you!

1 2

Moderators:

GMATNinja

GMAT Club Verbal Expert

7443 posts

GMATNinjaTwo

GMAT Club Verbal Expert

231 posts

Sajjad1994

GRE Forum Moderator

17304 posts