End-of-topic test: Environmental Effects on Phenotype

Unit 5 · Topic 5.5 end-of-topic test

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 two free-response questions, write your answer in full sentences, then 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

A water buttercup grows with part of its stem under water and part in the air. On the one plant, the leaves under water are cut into fine threads and the leaves in the air are broad and flat.

What do the two kinds of leaf on this one plant show?

A. The plant's underwater cells carry different leaf-shape alleles from its cells in the air B. One genotype gave two leaf shapes because the two parts grew in two conditions C. The water changed the DNA of the leaves it covered D. Leaf shape in this plant is set by genes alone, and the difference is chance

Question 2

Two potato plants grown from pieces of one tuber: the plant at 18 °C with large tubers and the plant at 28 °C with small tubers.

Two potato plants grown from pieces of one tuber, so their DNA is the same, are drawn below. One grew at 18 °C and formed large tubers; the other grew at 28 °C and formed small tubers. Light, water and soil were the same for both.

Which conclusion do the two plants support?

A. The plant at 18 °C carried alleles for large tubers that the other plant lacked B. The warmth changed the DNA of the plant at 28 °C, so its tubers stayed small C. Tuber size is set by genes alone, so the two plants differ by chance D. The temperature changed the tuber size, and the two plants' DNA stayed the same

Question 3

A woman who works outdoors develops thick, hard skin on her palms over many years. She then has a baby.

Predict the skin of the baby's palms at birth.

A. Thick and hard, because a parent's acquired change is passed to the child B. Thick and hard, because the work changed the alleles in the mother's gametes C. Thinner than usual, because the mother's skin used up the alleles for thick skin D. Ordinary, because the change in the mother's skin cells left the DNA in her gametes as it was

Question 4

Cuttings of one grass plant are grown in salty soil and in ordinary soil. The plants in salty soil grow short, with thick leaves. A student writes: the salt changed the plants' leaf alleles.

What is wrong with the student's statement?

A. Nothing; the salt rewrote the leaf alleles, and the cells then used the new alleles B. The leaf cells turned some genes up and others down; the alleles did not change C. Leaf form is set by alleles alone, so the salt had no effect on the leaves D. The cuttings must have carried different leaf alleles from the start

Question 5

Case 1: one clonal line of duckweed grows large fronds in warm water and small fronds in cold water. Case 2: two duckweed strains grown side by side in one warm tank differ in frond size.

Which case shows phenotypic plasticity?

A. Case 2 only, because the difference appeared in one tank B. Both cases, because two frond sizes appear in each C. Neither case, because the DNA did not change in either D. Case 1 only, because one genotype gave two phenotypes

Question 6

A pond holds snails of one species with shells of many colors and patterns. A student writes: this variety shows phenotypic plasticity.

What is wrong with the student's statement?

A. Plasticity is the range one genotype can produce, and the snails' variety may be genetic B. Nothing; variety in a population is what phenotypic plasticity means C. Shell color is never affected by conditions, so it cannot show plasticity D. Plasticity applies only to plants, so snails cannot show it

Question 7

Each option describes one case.

Which case is phenotypic plasticity?

A. A mutation in a pigment gene gives a mouse a white coat B. A red-flowered plant and a white-flowered plant give pink offspring C. One clonal line of a shrub grows tall in a sheltered garden and low in a windy one D. Two breeds of cattle kept on one farm give different amounts of milk

Question 8

On one oak tree, the leaves on a shaded branch grow larger and thinner than the leaves on a branch in full sun.

What differs inside the leaf cells of the two branches?

A. The set of genes in use, and how much each one is used B. The sequence of the DNA, and so the set of genes the cells carry C. Which alleles the cells inherited at fertilization D. The number of chromosomes each cell carries

Question 9

Alligator eggs from one nest are divided between two incubators, one kept at 30 °C and one at 34 °C, with the same moisture and light for every egg. Every hatchling from the 30 °C eggs is female, and every hatchling from the 34 °C eggs is male.

What set the sex of the hatchlings, and how do the results show it?

A. The temperature at which each egg developed, the one condition that differed between the two groups B. The alleles each egg received from its father, because sex is set by the sex chromosomes in every animal C. The moisture around each egg, because eggs in a nest of rotting plants develop in damp conditions D. Chance alone, because about half of the eggs in any clutch develop as males whatever the conditions

Question 10

After two weeks of strong sunshine a person's forearms are noticeably darker than they were in winter.

Which condition changed, and which trait answered?

A. The temperature of the air changed, and the skin's thickness answered B. The person's diet changed, and the skin's melanin answered C. UV light from the sun changed, and the skin's melanin answered D. UV light from the sun changed, and the skin's alleles answered

Question 11

Two children of the same parents grow up with very different amounts of food and reach different adult heights.

Which statement about human height fits the case?

A. Height is set by one gene alone, and the food a child eats has no effect on it B. Nutrition sets height entirely, and a child's genes play no part in it C. Food changes the height alleles a child carries, so the taller child has new alleles D. Many genes set the range a person can reach, and nutrition moves the person within it

Question 12

A cutting of one plant grown in shade has large, thin leaves. The cutting is moved into full sun and grows new leaves.

Predict the new leaves.

A. Large and thin, because the fixed genotype makes the cells use the same genes in any light B. Smaller and thicker, because in sun the leaf cells use some genes more and others less C. Smaller and thicker, because the sun changed the cutting's leaf alleles D. Large and thin, because the leaf form set in shade is kept by every leaf grown afterward

Question 13

The grower's design: two groups of sunflower plantlets from one clonal line, with the temperature and the watering of each group.

A grower wants to know whether temperature changes the height of sunflowers. The design is in the table below. After four weeks the plants in group A are taller.

What is wrong with the design?

A. The two groups came from different plants, so their genes may differ B. Two conditions differ between the groups, so neither can be credited with the difference C. There is no control, because no group grew at the grower's usual conditions D. Nothing; the taller plants in group A show that 20 °C makes sunflowers taller

Question 14

A grower has 24 cuttings of one grapevine and wants to know whether extra water changes the sugar in the grapes. She gives 12 cuttings the vineyard's usual watering and 12 twice as much, with the same soil, light and temperature for every plant, and measures the sugar in each plant's grapes.

Which group is the control, and what does it show?

A. The usual-watering group; it shows the sugar reached under the condition already in use, to compare against B. The double-watering group; it shows the sugar reached under the changed condition, which is the result under test C. Both groups; each shows what one vine's genes can do, because every cutting came from the same plant D. Neither group; a control must receive none of the condition, and both groups were given water

Question 15

Mean leaf length in millimeters of rosemary cuttings from one plant grown at 60 % and at 90 % humidity, 15 cuttings per group; error bars represent ±2SE.

Cuttings of one rosemary plant were grown at two humidities, 15 in each, and the mean leaf length of each group was recorded. The bars below carry ±2SE error bars.

What do the bars show about the two humidities?

A. The two humidities give leaves of exactly the same length B. The higher humidity makes leaves longer, because its mean is higher C. The data do not show a difference between the two humidities, because the bars overlap D. The cuttings must have carried different alleles, because the means differ

Question 16

A gardener sows bean seeds from a mixed packet, 20 in a cold frame and 20 in a warm greenhouse, with the same soil, water and light. The greenhouse beans grow taller.

What can the gardener conclude about the cause of the height difference?

A. The warmth caused it, because every condition except the temperature was held the same for both groups B. The genes caused it, because seeds from a mixed packet always differ from one another C. The cause cannot be separated, because the seeds' genes may differ as well as the temperature D. Nothing, because bean height never answers to temperature

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 40 strawberry plants grown from runners of one plant, so every plant has the same DNA. She wants to know whether the temperature the plants grow at changes the mass of fruit they produce. She grows 20 plants at 18 °C and 20 at 26 °C in one glasshouse divided by a partition, with the same light, water and soil for every plant. At the end of the season she weighs the fruit of each plant and finds the mean fruit mass per plant for each group. The means are drawn below; each error bar represents ±2SE.

Mean fruit mass per plant in grams for strawberry plants of one clonal line grown at 18 °C and at 26 °C, 20 plants per group; error bars represent ±2SE.

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

A full-credit answer: The independent variable is the temperature the plants grow at, 18 °C or 26 °C, the one condition the grower changed. The dependent variable is the mass of fruit per plant, the quantity she measured to see the effect.

Check the box for each point your answer earns

temperature as the independent variable and fruit mass per plant as the dependent variable.

Common slip: Naming the light, water or soil as the independent variable. Those were held the same; the one condition changed was the temperature.

(b) Predict what the two means and their ±2SE error bars would look like if temperature had no effect on fruit mass. (1 point)

A full-credit answer: The two means would be about the same, and the ±2SE error bars of the 18 °C group and the 26 °C group would overlap, so the data would show no difference between the two temperatures.

Check the box for each point your answer earns

means about the same with overlapping ±2SE error bars (the data show no difference).

Common slip: Predicting that the two means would be exactly equal. Two sample means almost always differ a little by chance; the prediction is that the bars overlap.

(c) Justify the claim that the temperature changed the fruit mass, using the error bars drawn above. (1 point)

A full-credit answer: The mean fruit mass was 240 g per plant at 18 °C and 175 g at 26 °C. The error bars represent ±2SE; they run from 222 to 258 g and from 160 to 190 g and do not overlap, so the difference between the means is unlikely to be chance. The plants share one genotype and only the temperature differed, so the temperature 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 the same genes and one condition changed the temperature is credited.

Common slip: Arguing only that 240 is larger than 175. Two means can differ by chance; the non-overlapping error bars are what make chance unlikely.

(d) Explain why the difference in fruit mass is credited to the temperature rather than to a genetic difference between the two groups. (1 point)

A full-credit answer: Every plant grew from runners of one plant, so all 40 carry the same DNA and the two groups cannot differ in their genes. Light, water and soil were the same for every plant, so the temperature is the only condition that differed, and the difference in fruit mass is credited to it.

Check the box for each point your answer earns

one clonal line means one genotype in both groups, so the difference cannot be genetic; with everything else the same, the temperature is the only difference.

Common slip: Pointing to the equal group sizes. Twenty in each group helps the comparison; it is the shared genotype that rules the genes out.

(e) Describe what the result shows about how the plants' genes and the temperature together set the mass of fruit. (1 point)

A full-credit answer: The same genotype gave heavier fruit at 18 °C and lighter fruit at 26 °C, so the temperature changed which genes the plants' cells used and how much; the genes set what the plants can do, and the temperature decided what they did. The DNA is the same in both groups.

Check the box for each point your answer earns

the same genes gave two phenotypes because the condition changed which genes the cells used and how much (gene expression), with the DNA unchanged.

Common slip: Saying the warmth changed the plants' genes. The DNA is the same in both groups; the temperature changed which genes the cells used, not the genes themselves.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

Aphids on a bean plant reproduce without mating, so all the aphids on one plant are a clonal line with the same DNA. When the plant becomes crowded with aphids, the newborn aphids develop wings. On an uncrowded plant, newborn aphids of the same line have no wings. A winged aphid that flies to an empty bean plant produces wingless offspring there.

(a) Identify the condition that changes and the trait that answers to it in the aphids. (1 point)

A full-credit answer: The condition is how crowded the plant is with aphids; the trait that answers is whether a newborn aphid develops wings.

Check the box for each point your answer earns

crowding as the condition and wing development as the trait.

Common slip: Naming the bean plant or the food as the condition. The aphids on the crowded and the uncrowded plants eat the same plant; what differs is how many aphids share it.

(b) Explain how crowding can change whether an aphid develops wings while its DNA stays the same. (1 point)

A full-credit answer: Crowding is a condition outside the aphid that acts as a signal to its cells. A condition outside the organism changes which genes its cells use and how much, so in a crowded plant the developing aphid's cells use the genes that build wings, and in an uncrowded plant they do not; the DNA is the same in both.

Check the box for each point your answer earns

the condition changes which genes the cells use and how much (gene expression), so the phenotype changes while the DNA does not.

Common slip: Saying crowding causes a mutation that gives wings. The wingless offspring of a winged aphid show that its DNA still carries the same genes.

(c) Justify the claim that the winged aphid's wings were a response to conditions rather than a change in its genes, using the winged aphid that flew to the empty plant. (1 point)

A full-credit answer: The winged aphid's offspring on the empty plant have no wings, although they are copies of her DNA. If the wings had come from a change in her genes, her offspring would carry that change and be winged too. They are wingless because the condition, crowding, is gone, so the wings were the same genes used differently under crowding.

Check the box for each point your answer earns

the offspring share the winged aphid's DNA yet are wingless, so the wings were not written in the genes but produced by the condition.

Common slip: Saying the offspring lost the wing allele. Offspring of a clonal line carry the same DNA as their mother; nothing was lost.

(d) Two different clonal lines of aphids feed on identical, uncrowded bean plants in one greenhouse, and the aphids of one line are green while those of the other are red. Classify this color difference as phenotypic plasticity or as a genetic difference, and justify the classification. (1 point)

A full-credit answer: It is a genetic difference. Phenotypic plasticity is one genotype producing different phenotypes under different conditions. Here the conditions are the same, identical uncrowded plants in one greenhouse, and the two lines are two genotypes, so the difference in color comes from their genes.

Check the box for each point your answer earns

a genetic difference, because the conditions are the same and the two lines are two genotypes; plasticity would need one genotype in two conditions.

Common slip: Calling any difference in phenotype plasticity. Plasticity is a property of one genotype; two lines that differ under the same conditions differ in their genes.

Free-response score: 0 of 4

Multiple choice checked: 0 of 16 correct.

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