Practice questions · Topic 5.5

Unit 5 · Practice for the Topic 5.5 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: Environmental effects on phenotype, summed up

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The same genes, different looks; conditions change which genes the cells use, not the genes; phenotypic plasticity; the six named cases; how clones in two conditions separate genes from environment.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback tells you what a wrong choice assumed. For the free-response questions, write your answer in full sentences. The first free-response question walks you through one investigation one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Where a question shows means with error bars, read the legend first: every error bar here represents ±2SE.

Question 1

Poplar trees can be grown from cuttings of one tree, so a plantation of them shares one set of DNA. Cuttings of one poplar planted in a wet field grow large leaves; cuttings of the same tree planted in a dry field grow small leaves.

What do the two fields of trees show?

A. The trees in the wet field carried alleles for large leaves B. The dry field changed the DNA of the trees planted in it C. One genotype gave two leaf sizes under two conditions D. Leaf size in poplars is set by genes alone

Question 2

A sea snail grows a thicker shell in water where crabs that eat snails live than in water without crabs. Snails of one clonal line do this.

What does the presence of the crabs change inside the snail's cells?

A. The sequence of the DNA in the snail's shell-building genes B. Which alleles for shell thickness the snail's cells carry C. The number of chromosomes in each cell of the snail D. How strongly the cells turn on their shell-building genes

Question 3

A man who plays the guitar for many years has thick, hard skin on his fingertips. He has a daughter.

Predict the skin of the daughter's fingertips at birth.

A. Thick and hard, because a parent's acquired change is passed to the child B. Ordinary, because the change in his fingertip cells did not change the DNA in his gametes C. Thick and hard, because the playing changed the alleles in his gametes D. Thinner than usual, because his fingertips used up the alleles for thick skin

Question 4

Case 1: one clonal line of a fern grows broad, thin fronds in shade and narrow, thick fronds in sun. Case 2: two varieties of the same fern grown side by side in one shaded greenhouse differ in frond shape.

Which case shows phenotypic plasticity?

A. Case 1 only, because one genotype gave two frond shapes B. Case 2 only, because the two varieties differ C. Both cases, because two frond shapes appear in each D. Neither case, because frond shape is set by genes alone

Question 5

Cuttings of one hydrangea were planted in two beds. The soil in the first bed is acid, and its plants flower blue; the soil in the second bed is alkaline, and its plants flower pink. A gardener takes cuttings from the pink-flowered plants in the alkaline bed and plants them in her own garden, where the soil is acid.

Predict the flower color of the gardener's plants.

A. Pink, because a cutting keeps the alleles of the plant it was cut from B. Blue, because the soil decides the color and the gardener's soil is acid C. Blue, because the acid soil changes the plants' color alleles D. Pink, because a plant keeps the color of the bed it came from, whatever soil it is moved to

Question 6

The grower's design: two groups of twelve spider-plant plantlets from one plant, with the hours of light each group gets; water, soil and temperature are the same for every plant.

A grower tests whether the hours of light a day change the leaf length of spider plants. Her design is in the table below.

Which group is the control, and why?

A. Group A, because it grows under the parent plant's usual light B. Group B, because it gets the most light C. Both groups, because both are plantlets of one plant D. Neither group, because a control must get no light at all

Question 7

Mean shoot length in centimeters of willow cuttings from one tree grown in plain water and in water with fertilizer, 15 cuttings per group; error bars represent ±2SE.

Cuttings of one willow were grown in plain water and in water with fertilizer, 15 in each, and the mean shoot length of each group was recorded. The bars below carry ±2SE error bars.

What do the bars show?

A. The fertilizer lengthened the shoots, because the mean of its group is higher than the other's B. The two groups' shoots are exactly the same length, because the two means are so close C. The cuttings in fertilizer carried different alleles for growth, because their mean differs D. The data do not show a difference between the two waters, because the bars overlap

Question 8

Cuttings of one plant are grown in two conditions with everything else the same, and a trait is measured in each group. The two means carry ±2SE error bars, and the bars stay clear of each other.

What can be concluded?

A. The two groups must have carried different alleles, because their means differ B. The difference between the two means is exactly its true size, because the bars are clear C. The difference is unlikely to be chance, so the condition is credited D. Nothing about the condition, because error bars only describe the spread of one group

Question 9

A student says that because the environment can change a phenotype, an organism's genotype must change over its lifetime too.

What is wrong with the student's statement?

A. The set of genes in use changes, not the genes themselves; the genotype stays fixed B. Nothing; when the cells change which genes they use, the genotype must change with them C. The environment never changes a phenotype, only a genotype, over a lifetime D. A genotype changes over a lifetime only in plants, never in animals

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Scientific Investigation · 5 points

A grower has 24 plantlets taken from one spider plant, so every plantlet has the same DNA. She wants to know whether watering with slightly salty water changes leaf length. She grows 12 plantlets with tap water and 12 with slightly salty water, with the same light, soil and temperature for every plant, and after eight weeks she measures the length of the longest leaf on each plant and finds each group's mean. The means are drawn below; each error bar represents ±2SE.

Mean leaf length in centimeters of spider-plant plantlets from one plant grown with tap water and with slightly salty water, 12 plantlets per group; error bars represent ±2SE.

(a) Identify the independent variable in the grower's test. (1 point)

Hint: Which one condition did the grower deliberately change between the two groups?

A full-credit answer: The independent variable is the water the plants are given, tap water or slightly salty water, because it is the one condition the grower deliberately changed.

Check the box for each point your answer earns

the water (tap or slightly salty) as the independent variable.

Common slip: Naming the leaf length. That is what was measured, the dependent variable.

(b) Identify the dependent variable and one controlled variable. (1 point)

Hint: What did the grower measure, and which conditions did she hold the same in both groups?

A full-credit answer: The dependent variable is the leaf length, the quantity measured to see the effect. Light, soil and temperature are controlled variables, kept the same for every plant; any one of them counts.

Check the box for each point your answer earns

leaf length as the dependent variable and light, soil or temperature as a controlled variable.

Common slip: Giving the salty water as the dependent variable. The water is what the grower changed; the leaf length is what she measured.

(c) Identify the control group and describe what it shows. (1 point)

Hint: Which group lacks the factor under test, and what does its result let the grower compare against?

A full-credit answer: The control group is the 12 plantlets watered with tap water. It shows the leaf length the plantlets reach without the salt, so the salty group's result can be compared against it and any difference credited to the salt.

Check the box for each point your answer earns

the tap-water group as the control, giving the leaf length without salt to compare against.

Common slip: Calling the salty-water group the control. The control lacks the factor under test.

(d) Justify the grower's use of plantlets from one spider plant rather than seedlings from a packet of seeds. (1 point)

Hint: What would seedlings from many parents differ in that plantlets of one plant do not?

A full-credit answer: Plantlets from one plant all have the same genotype, so a difference in leaf length between the two groups cannot come from their genes. Seedlings from a packet would carry different genotypes, and a difference could be genes or water.

Check the box for each point your answer earns

one genotype throughout, so a difference between the groups cannot be genetic and is credited to the water.

Common slip: Saying plantlets are used because they grow faster. The point is that their genes are the same.

(e) Justify the claim that the salty water changed the leaf length, using the error bars drawn above. (1 point)

Hint: Read where each error bar starts and ends, and ask whether the two bars overlap.

A full-credit answer: The mean leaf length was 8.2 cm with tap water and 6.1 cm with salty water. The ±2SE bars run from 7.6 to 8.8 cm and from 5.5 to 6.7 cm and do not overlap, so the difference between the means is unlikely to be chance. The plants share one genotype and only the water differed, so the salty water is the cause left.

Check the box for each point your answer earns

the ±2SE bars do not overlap, so the difference is unlikely to be chance, and with one genotype and one condition changed the salty water is credited.

Common slip: Arguing only that 8.2 is larger than 6.1. Two means can differ by chance; the non-overlapping bars are what rule chance out.

Free-response score: 0 of 5

Free response 2 · Scientific Investigation · 4 points

A grower has 30 rose plants grown from cuttings of one rose, so every plant has the same DNA. He wants to know whether the brightness of the light changes the size of the flowers. He grows 15 plants in bright light and 15 in dim light, with the same water, soil and temperature for every plant, and at flowering he measures the diameter of the first flower on each plant and finds each group's mean. The means are drawn below; each error bar represents ±2SE.

Mean flower diameter in centimeters of rose plants from one clonal line grown in bright light and in dim light, 15 plants per group; error bars represent ±2SE.

(a) Describe the feature of the grower's design that lets a difference between the groups be credited to the light rather than to the plants' genes. (1 point)

A full-credit answer: Every plant is a cutting of one rose, so all 30 plants have the same genotype. A difference between the bright-light group and the dim-light group therefore cannot come from their genes, and with water, soil and temperature the same the light is the only condition that differs.

Check the box for each point your answer earns

cuttings of one plant give one genotype in both groups, so a difference cannot be genetic; with everything else the same the light is the only difference.

Common slip: Naming the equal group sizes as the feature. Fifteen in each group helps the comparison, but it is the shared genotype that rules out the genes.

(b) Identify the independent variable and the dependent variable. (1 point)

A full-credit answer: The independent variable is the brightness of the light, bright or dim, the one condition changed. The dependent variable is the flower diameter, the quantity measured.

Check the box for each point your answer earns

light brightness as the independent variable and flower diameter as the dependent variable.

Common slip: Swapping the two. The grower changed the light and measured the flowers.

(c) Justify the claim that these data leave open whether flower size differs between bright and dim light, using the error bars drawn above. (1 point)

A full-credit answer: The mean flower diameter was 7.2 cm in bright light and 7.6 cm in dim light. The error bars represent ±2SE; they run from 6.6 to 7.8 cm and from 7.0 to 8.2 cm and overlap, so the difference between the means could be chance, and the data do not show a difference between the two light levels.

Check the box for each point your answer earns

the ±2SE bars overlap, so the difference between the means could be chance and the data do not show a difference.

Common slip: Arguing that 7.6 is larger than 7.2, so dim light gives bigger flowers. Two means can differ by chance; the overlapping bars mean chance has not been ruled out.

(d) Predict what the two means and their ±2SE error bars would look like if dim light did give larger flowers. (1 point)

A full-credit answer: The dim-light mean would sit above the bright-light mean, and the two ±2SE error bars would stay clear of each other: the bottom of the dim-light bar above the top of the bright-light bar. Then the difference would be unlikely to be chance, and with one genotype and one condition changed the light would be credited.

Check the box for each point your answer earns

the dim-light mean higher and the two ±2SE bars not overlapping.

Common slip: Drawing the means farther apart with the bars still overlapping. Overlapping bars leave the difference open however far apart the means look; the bars must clear each other.

Free-response score: 0 of 4

Multiple choice checked: 0 of 9 correct.

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