Worksheet

When the Lice Treatment Stopped Working

Grade 8 natural selection practice: a doctor's visit and county lab data on head lice show how an inherited gene version helps some lice survive and reproduce.

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11 questions
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Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Sorting, Short answer

About this worksheet

What students practice

Students use survival data and proportions to explain why lice with one gene version survive treatment more often.
Students explain how inherited variation and repeated treatment change a louse population over many generations.
Students tell apart natural selection from the idea that individuals change their genes when they need to.

How to use

Read the clinic story together and work question 4 as a class with the head of 100 lice drawn as a bar; then let students finish on their own.

Curriculum

NGSS standard covered

MS-LS4-4

Explain how genetic variation in traits makes some individuals more likely to survive and reproduce.

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When the Lice Treatment Stopped Working

Use clinic and lab data to explain why some head lice survive and reproduce.

Part 1: The lab test

Read the passage and Table 1. Then answer questions 1 to 5.

A Second Visit to the Clinic

1Bram Oduya's younger sister came home from summer camp with head lice. Their father bought a lice shampoo at the pharmacy and followed the directions exactly, but a week later the lice were back. At the follow-up visit, the children's doctor, Dr. Esperanza Lindholm, told the family this was common. "Lice have nothing to do with being dirty," she said. "They spread when heads touch. And in our county, this shampoo often does not work anymore." Bram, who was taking eighth-grade science, asked why.

2Head lice are tiny wingless insects that live only on the human scalp and feed on small amounts of blood. A female glues several eggs a day to hairs near the scalp. The eggs hatch in about a week, and the young lice can lay eggs of their own about two weeks after that, so a new generation appears roughly every three weeks. Young lice inherit their gene versions from their parents.

3The shampoo contains an insecticide that acts on a protein in the nerves of lice and kills them. In many lice, though, the gene for that nerve protein is a changed version. Lice with the changed version build a slightly different protein, and the insecticide affects it much less.

4Dr. Lindholm showed Bram data from the county hospital lab, which has tested lice collected at the clinic for years, with permission from each family. In one test, lab workers place live lice on filter paper soaked with the insecticide, then count how many are still alive one hour later. Table 1 shows the results by gene version. Each test began with live, active lice, and every louse was handled by trained lab staff, not by families. The lab also tested lice from a family that lives on a remote farm and had never used any lice treatment. Some of those lice already carried the changed version.

5Every few years the lab also checks what share of the lice brought to the clinic carry the changed version, and the clinic records how often the shampoo cleared a child's lice in one treatment (Table 2). The same shampoo was the most common treatment in the county during all of those years.

Table 1: Lice on Insecticide Paper for 1 Hour
Nerve protein gene	Lice tested	Alive after 1 hour	Survival (%)
Typical version	200	10	5
Changed version	200	180	90

Use Table 1. Compare the survival of the two kinds of lice.

Lice with the changed version were times as likely to survive the hour as lice with the typical version.

Use paragraph 3. Why did most lice with the changed version survive the insecticide paper?

A. Their bodies broke down the insecticide faster each time they touched it during the test.
B. They hid under the other lice, so less of the insecticide touched their bodies.
C. Their nerve protein is built a little differently, so the insecticide affects it less.
D. They were older and larger than the other lice, so a dose of that size was too small.

Use paragraph 4. What do the lice from the remote farm suggest about where the changed version came from?

A. Some lice carried the changed version even where no treatment had been used.
B. The changed version appears mainly in lice that live far from towns.
C. The shampoo caused the gene change in lice that were exposed to it.
D. Lice on the farm built the changed version because they expected to be treated.

A child has 100 lice. 20 of them carry the changed version and 80 carry the typical version. Suppose the shampoo has the same effect on each kind of louse as the insecticide paper in Table 1.

About lice with the changed version and lice with the typical version would survive.

Use your answers to question 4. Before the treatment, 20% of the child's lice carried the changed version.

To the nearest whole percent, % of the surviving lice carry the changed version.

Part 2: Year after year at the clinic

Use paragraphs 2 and 5, Table 2 and what you found in Part 1 to answer questions 6 to 11.

Table 2: County Lice Records
Year of records	Lice samples tested	Changed version (%)	Shampoo cleared lice (%)
Year 1	120	35	70
Year 4	140	58	50
Year 7	110	74	35
Year 10	150	86	20
Year 13	130	95	10

Which statement best describes the pattern in Table 2?

A. The share with the changed version rose from 35% to 95% as the shampoo worked less often.
B. The share of lice with the changed version fell as the shampoo cleared more children's lice.
C. The share with the changed version rose quickly at first, then fell back after Year 7.
D. The number of lice samples tested rose each time, which made the share rise too.

Sort each statement by whether it describes what happened to the county's lice.

Some lice carried the changed version before the shampoo was used on them.Treatments killed a much larger share of typical lice than of changed-version lice.Surviving lice laid eggs, and their young inherited their gene versions.Each louse that touched the shampoo changed its own gene to survive.Lice with the changed version became rarer as the years went by.

Fits the evidence	Does not fit the evidence

Use paragraph 2 and Table 2. Why could the share of lice with the changed version rise so much in about 12 years?

A. Lice lay eggs only in years when the shampoo is used, so treated years add more lice.
B. The lab tested more lice in later years, so it found more of the changed version.
C. Each louse lives for about 12 years, so it has a long time to change its genes.
D. A new generation appears about every three weeks, so selection repeats many times.

Use Table 2.

From Year 1 to Year 13, the share of children whose lice were cleared by the shampoo fell by percentage points.

10.

Suppose the clinic switches to a treatment that kills lice by blocking their breathing holes, so it works just as well on both gene versions. Over the next few years, what would most likely happen to the share of lice with the changed version?

A. It would rise even faster, since the new treatment causes more gene changes in the lice.
B. It would likely stop rising, since the changed version no longer helps lice survive.
C. It would drop to 0% in the first year, since the lice no longer need the changed version.
D. It would keep rising quickly, since the new treatment still kills mostly typical lice.

11.

A parent in the waiting room says, "The lice just got used to the shampoo. Each louse built up a tolerance after being treated." Write a claim that answers the parent. Give evidence from both tables and the passage, and explain your reasoning about variation, survival and reproduction.