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Mutations, Variation and Natural Selection — Answers

Answer sheet with model answers and success criteria for the 10th Grade cells & classification worksheet.

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Success TutoringWorksheetsAnswer sheet10th Grade · Science · Cells & Classification

Mutations, Variation and Natural Selection — Answers

Part A: Mutations

A mutation is a change in the base sequence of DNA. In a substitution, one base is replaced by another. In an insertion, an extra base is added. In a deletion, a base is lost. Because DNA is read in triplets (groups of three bases), insertions and deletions shift every triplet after them.

Mutations happen randomly when DNA is copied, but mutagens such as UV light, X-rays and some chemicals make them more likely. Most mutations are neutral, some are harmful and a few are beneficial.

1.The sequence TAC GGA TTC changes to TAC GGT TTC. What type of mutation is this?
  1. ASubstitution
  2. BInsertion
  3. CDeletion
  4. DNo mutation

Answer: Substitution

One base (A) has been replaced by another (T); the length of the sequence is unchanged.

2.The sequence TAC GGA TTC changes to TAC GAT TC. What type of mutation is this?
  1. ASubstitution
  2. BInsertion
  3. CDeletion
  4. DNo mutation

Answer: Deletion

The fourth base (G) has been lost, so every base after it shifts one place to the left.

3.Explain why a deletion usually has a bigger effect on the protein than a substitution.

Answer: DNA is read in groups of three bases (triplets). A deletion shifts the reading frame, so every triplet after the mutation changes, and many amino acids may be wrong. A substitution changes only one triplet, so at most one amino acid changes (and sometimes none).

4.Some substitution mutations have no effect on the protein. Suggest why.

Answer: More than one triplet can code for the same amino acid, so the new triplet may still code for the same amino acid. The mutation may also be in a part of the DNA that does not code for a protein.

5.Factors that increase the rate of mutation, such as ultraviolet light, X-rays and some chemicals in tobacco smoke, are called (answer space).

Answer: mutagens

6.A mutation in a skin cell can be passed on to a person's children.

Answer: False

Only mutations in the cells that make gametes (in the ovaries or testes) can be inherited. A skin cell mutation affects only that person.

Part B: Natural selection

The theory of evolution by natural selection: individuals in a population show variation. More offspring are produced than can survive, so they compete. Individuals with characteristics best suited to the environment are more likely to survive and reproduce, passing on their alleles. Over many generations, favorable characteristics become more common.

7.Name two sources of genetic variation in sexually reproducing organisms.

Answer: Any two of: mutations (the original source of new alleles), meiosis (shuffling of chromosomes and crossing over), random fertilization (any sperm can fuse with any egg).

8.Write the steps of natural selection in order: (A) individuals best suited survive and reproduce; (B) there is variation within the population; (C) over many generations the favorable trait becomes more common; (D) more offspring are produced than can survive, so there is competition; (E) they pass on their favorable alleles to their offspring.

Answer: B, D, A, E, C

9.Which statement about evolution is correct?
  1. AIndividuals change during their lifetime to suit their environment
  2. BPopulations change over many generations as favorable inherited traits become more common
  3. COrganisms develop new traits because they need them
  4. DAcquired features, such as large muscles from exercise, are passed to offspring

Answer: Populations change over many generations as favorable inherited traits become more common

Natural selection acts on inherited variation that already exists in a population. Individuals do not evolve; populations do.

10.A scar from an injury is an example of variation that can be passed on to offspring.

Answer: False

A scar is caused by the environment, not by a change in DNA in the gametes, so it cannot be inherited.

Part C: Analyzing population data

A species of moth has light and dark forms, controlled by a gene. Birds eat moths that they can see resting on tree trunks. In areas where soot from factories made tree bark dark, dark moths were better camouflaged.

11.In a polluted area, 450 of 500 moths caught were dark. What percentage were dark?

Answer: 90%

  1. 450 ÷ 500 × 100 = 90%
12.Years after the air was cleaned up, only 30 of 600 moths caught in the same area were dark. What percentage is this?

Answer: 5%

  1. 30 ÷ 600 × 100 = 5%
13.Use natural selection to explain the change in the moth population after the air was cleaned up.

Answer: When the tree bark became light again, dark moths were easier for birds to see against it, so more of them were eaten. Light moths were better camouflaged, survived and reproduced more, passing on the allele for light color, so over many generations light moths became more common.

Part D: Antibiotic resistance

14.In a population of 50,000,000 bacteria, about 1 in every 1,000,000 has a mutation that makes it resistant to an antibiotic. How many resistant bacteria are there?

Answer: 50

  1. 50,000,000 ÷ 1,000,000 = 50
15.A patient takes an antibiotic but stops after two days because they feel better. Use natural selection to explain how this can lead to a population of antibiotic-resistant bacteria, and state two ways to slow the spread of resistance.[4 marks]

Model answer: Within the bacterial population there is variation: a few bacteria carry a random mutation that makes them resistant to the antibiotic. The antibiotic kills the non-resistant bacteria first, but if the course is stopped early, the resistant bacteria (and some less-affected ones) survive. With less competition, they reproduce rapidly, passing on the resistance gene, so the new population is mostly resistant and the antibiotic no longer works. To slow resistance: always finish the full course of antibiotics; do not use antibiotics for viral infections such as colds; reduce the use of antibiotics in farming; and use good hygiene so infections spread less.

Success criteria:

  • Variation exists: some bacteria have a resistance mutation
  • The antibiotic kills non-resistant bacteria, so resistant ones survive
  • Survivors reproduce and pass on resistance, so it becomes common
  • Two valid ways to slow resistance
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