ACT science practice test 71

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


Ever since Darwin proposed his theory of natural selection in 1859, biologists have regarded the gene as the sole unit of inheritance, and the discovery of DNA in the twentieth century only served to reinforce this view, known as “hard inheritance.” The idea that environmental factors could produce heritable changes in an organism without altering the organism’s DNA (as occurs in gene mutation due to radiation exposure, for example), known as “soft inheritance,” had long been written off as an impossibility. “Inheritance of acquired traits” became a biological fallacy associated with pre-Darwinians like Jean-Baptiste Lamarck and fanatical Soviet scientists like Trofim Lysenko. But the very recent discovery of what has been termed epigenetics—biological mechanisms that leave DNA unchanged but can alter the ways in which individual genes express themselves—has opened the door on the possibility of “soft inheritance” once again, although many biologists remain skeptical.

Scientist 1

Although study of the so-called “epigenome” is worthwhile and possesses the potential to answer questions about a host of problems from diabetes to cancer, it would be premature to call true “soft inheritance” a reality, at least in animal species. Isolated situations in which environmental factors could affect an organism’s immediate descendants—a mother with a zinc deficiency producing children and grandchildren with weakened immune systems, for example—had already been documented without anyone ever suggesting that there was more to heritability than the gene. Immediate successive generations may exhibit observable effects, but the bloodline always reverts to the true expression of its DNA. A map with dust on it may be hard to read, but the information on the map remains unchanged, and it is simply a matter of how long it takes for a strong wind to blow away the dust.

Scientist 2

Someone who is determined never to see “true soft inheritance” will never see it, but that doesn’t change the fact that it occurs, and indeed has been occurring all along without our realizing it. Epigenetic mechanisms may not alter DNA, but they possess the ability to turn genes “on” or “off” for the duration of an organism’s bloodline, at least until such time as the genes are affected by another epigenetic mechanism. There is already compelling evidence, for example, that tobacco smoking poses risks not only to the individual smoker, but to all of his or her descendants. Imagine a three-way light bulb with three possible brightness settings being put into a three-way lamp on which one of the settings is broken. The fact that the bulb still possesses three settings is immaterial if it is permanently set into a broken lamp. It will continue to “express itself” in a limited way. And the same thing can happen to a gene. If environmental factors can permanently alter the way in which an unaltered gene expresses itself down along a bloodline, resulting in observable effects in the members of that bloodline, then what can we possibly call this other than true soft inheritance?

1. According to the information in the passage, which of the following might constitute an example of true soft inheritance?

A. inherited gene damage due to a parent’s heavy radiation exposure
B. a susceptibility to diabetes as the result of a malnourished grandparent
C. a left-handed parent producing predominantly left-handed offspring
D. a parent who loves books teaching her children to love books

2. Who first proposed the theory of epigenetics?

F. Charles Darwin
G. Trofim Lysenko
H. Scientist 2
J. The information is not included in the passage.

3. In the offspring of which of the following organisms might Scientist 1 currently be willing to concede the documented existence of true soft inheritance?

A. a rhinoceros
B. a radiation-exposed human being
C. an apple tree
D. a zinc-deficient human being

4. Scientist 2 would most likely disagree with the suggestion that genes:

F. are the primary units of inheritance.
G. act in isolation to produce traits.
H. possess the ability to express themselves.
J. are made up of stretches of DNA.

5. Scientist 1 would be most likely to predict that any ill effects on the grandchildren of a tobacco smoker:

A. have nothing to do with the epigenome.
B. are probably psychological rather than biological in origin.
C. must be the result of tobacco use having altered the smoker’s DNA.
D. will cease to be expressed in the bloodline given enough time.

6. Histones are proteins that attach themselves to sequences of DNA and can alter the expression of genes depending on whether they are acetylated or methylated. Given this information, it would appear that histones are analogous to the:

F. sole unit of inheritance.
G. zinc-deficient mother mentioned by Scientist 1.
H. map mentioned by Scientist 1.
J. lamp mentioned by Scientist 2.

7. A study of what would be most likely to resolve the disagreement between Scientist 1 and Scientist 2?

A. the length of time that epigenetic mechanisms tend to remain in a bloodline
B. the unpublished work of Jean-Baptiste Lamarck
C. a comparison of cancer rates among identical twins
D. the inner workings of three-way light bulbs

Figure 1 shows the average sleep pattern of a child, Figure 2 shows the average sleep pattern of a young adult, and Figure 3 shows the average sleep pattern of an elderly person. At Stage 0, the person is awake. As sleep moves from Stage 1 to Stage 4, it grows progressively deeper. REM sleep, commonly associated with dreaming, is predominant in the final third of a sleep cycle.

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Figure 1

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Figure 2

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Figure 3

8. According to Figure 2, a young adult who has been asleep for 5 hours will most likely be in which sleep stage?

F. Stage 0
G. Stage 1
H. Stage 3
J. Stage 5

9. Based on the information in the passage, a child will wake up how many times during a 7-hour stretch of sleep?

A. 7
B. 2
C. 1
D. 0

10. Based on the data presented in Figures 1, 2, and 3, which of the following conclusions can be properly drawn?

F. As people age, they wake up more frequently during the night.
G. As people age, they wake up less frequently during the night.
H. As people age, they spend more time in deep sleep.
J. As people age, they spend more consecutive time in each sleep stage.

11. Based on the information in the passage and in Figure 2, at which of the following hours into a sleep interval will a young adult be most likely to experience REM sleep?

A. 0
B. 1
C. 2
D. 7

12. At which of the following hours of sleep will a child most likely be in the deepest sleep?

F. 2
G. 3
H. 5
J. 7