IB Biology SL D1.3 Mutation and gene editing questions

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Question 1

Multiple choiceMedium1 mark

Two versions of a gene occur in a population. In version X, one nucleotide is replaced by another; in version Y, one nucleotide is inserted within the coding sequence, before its final codon. No other sequence changes are present. Which statements are correct?

I. Version X is a single-nucleotide polymorphism (SNP) resulting from a base substitution.
II. The replacement in X must change the amino acid encoded at that position.
III. The insertion in Y shifts the reading frame of subsequent codons, making it likely that the polypeptide ceases to function.

Show answer and explanation

Answer: B. A base substitution can give rise to an SNP, but degeneracy of the genetic code means it may or may not change the amino acid. A one-nucleotide insertion within a coding sequence shifts the reading frame of subsequent codons, making loss of polypeptide function likely. Thus I and III are correct; II is not.

A: accepts the misconception in II that every substitution changes an amino acid, and omits the consequence of the insertion.
C: accepts II and fails to identify the base substitution as an SNP.
D: includes II; degeneracy means the replacement need not change the amino acid.

Remember: substitutions may be silent, while a one-nucleotide insertion can shift the reading frame.

Question 2

Paper 2Medium16 marks

Phenylketonuria (PKU) is caused by the recessive mutated allele p. The allele P is dominant, and only individuals with genotype pp have PKU. Two unaffected parents both have genotype Pp.

Their daughter also has genotype Pp. As an adult, she develops a skin cancer. A mutation, T, in a gene involved in regulating cell division is detected in her tumour cells, but not in her non-tumour skin cells, blood cells or gametes.

Non-tumour skin cells and tumour cells from the daughter were cultured under the same conditions. The table shows the mean percentage of cells that underwent division in 48 hours, from three replicate cultures.

Cell sampleMean percentage of cells undergoing division in 48 hours
Non-tumour skin cells8%
Tumour cells35%
(a)

Determine the expected genotype ratio among the parents’ offspring and the probability that an offspring will have PKU.

[4]
(b)

Explain why the daughter with genotype Pp does not have PKU, and how she could pass allele p to a child who has PKU.

[3]
(c)

Describe the difference in cell division between the two cell samples in the table.

[2]
(d)

Using the information about mutation T, explain how it could contribute to cancer and whether it could be inherited by the daughter’s offspring.

[5]
(e)

Give one limitation of the cell-culture results in deciding whether mutation T caused the higher rate of cell division.

[2]

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