Genetic Variants and Phenotype (Higher tier)

1. 1 State [1 mark]

   State what is meant by gene expression.

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   Model answer

   The way that a gene is used to make its protein.

   Mark scheme

   • Gene used to make a protein — 1 mark
2. 2 State [2 marks]

   State two ways in which a genetic variant can influence the phenotype of an organism.

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   Model answer

   A variant in coding DNA can alter the activity of a protein. A variant in non-coding DNA can alter how genes are expressed.

   Mark scheme

   • Coding DNA, altering a protein — 1 mark
   • Non-coding DNA, altering gene expression — 1 mark
3. 3 Explain [3 marks]

   A variant changes one base in a gene that codes for an enzyme. Explain how this could stop the enzyme from working.

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   Model answer

   The change in the base can change an amino acid in the enzyme. This changes the shape of the protein, including the active site. The substrate no longer fits in the active site, so the enzyme cannot catalyse the reaction.

   Mark scheme

   • Amino acid changed — 1 mark
   • Shape of the active site changes — 1 mark
   • Substrate does not fit, so no reaction — 1 mark
4. 4 Explain [3 marks]

   Explain how a variant in non-coding DNA, next to a gene, could stop a protein from being made, when the gene itself is normal.

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   Model answer

   The variant could stop the transcription of the gene, so that no mRNA is made. Without mRNA, the protein cannot be made, although the coding DNA is normal.

   Mark scheme

   • Variant in non-coding DNA affects expression — 1 mark
   • Transcription is stopped, so no mRNA — 1 mark
   • So the protein is not made — 1 mark
5. 5 Describe [2 marks]

   Describe why a variant in the DNA does not always change the phenotype.

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   Model answer

   Some variants are in non-coding DNA that does not control any gene. Others are in coding DNA, but do not change the amino acid, because more than one triplet codes for the same amino acid.

   Mark scheme

   • Variant in DNA with no effect — 1 mark
   • Same amino acid is coded — 1 mark
6. 6 Evaluate [6 marks]

   Compare the effects on an organism of a variant in the coding DNA of a gene and a variant in non-coding DNA next to the gene.

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   Model answer

   A variant in the coding DNA of a gene can change one or more amino acids in the protein. This can change the shape of the protein and its activity, and if it is an enzyme, the shape of the active site, so the substrate no longer fits. The protein that is made is different. A variant in non-coding DNA next to the gene does not change the amino acids, so the protein itself is normal. Instead it can change how the gene is expressed, for example by stopping transcription of the mRNA, so that less of the protein is made. In both cases the phenotype may change, but the cause is different, either a different protein or a different amount of it.

   Mark scheme

   • Coding variant changes the amino acid — 1 mark
   • Shape or activity of the protein changes — 1 mark
   • Enzyme active site example — 1 mark
   • Non-coding variant leaves the protein normal — 1 mark
   • Gene expression changes, such as stopped transcription — 1 mark
   • Different cause, same type of result — 1 mark

Quick check

1. 1

   What does coding DNA contain?

   1. AOnly mutations
   2. BGenes that code for proteins
   3. COnly RNA
   4. DOnly chromosomes
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   B: Genes that code for proteins

   Genes that code for proteins.

2. 2

   How can a variant in coding DNA affect an enzyme?

   1. AIt has no effect
   2. BIt makes the cell bigger
   3. CIt can change the shape of the active site
   4. DIt makes the enzyme live longer
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   C: It can change the shape of the active site

   It can change the shape of the active site.

3. 3

   What can a variant in non-coding DNA affect?

   1. AHow genes are expressed
   2. BThe number of chromosomes
   3. CThe shape of the cell
   4. DThe type of cell division
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   A: How genes are expressed

   How genes are expressed.

4. 4

   Which stage can be stopped by a variant in non-coding DNA?

   1. ADigestion
   2. BRespiration
   3. CMeiosis
   4. DTranscription of mRNA
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   D: Transcription of mRNA

   Transcription of mRNA.

5. 5

   What is gene expression?

   1. AThe mutation of a gene
   2. BThe use of a gene to make its protein
   3. CThe copying of DNA before mitosis
   4. DThe death of a cell
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   B: The use of a gene to make its protein

   The use of a gene to make its protein.

6. 6

   Why might a change in a base have no effect on the protein?

   1. ABases are not important
   2. BProteins are not made from amino acids
   3. CMore than one triplet codes for the same amino acid
   4. DThe cell repairs all mutations
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   C: More than one triplet codes for the same amino acid

   More than one triplet codes for the same amino acid.

7. 7

   A variant changes the active site of an enzyme. What happens?

   1. AThe substrate no longer fits
   2. BThe substrate fits better
   3. CThe enzyme becomes a substrate
   4. DThe enzyme becomes DNA
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   A: The substrate no longer fits

   The substrate no longer fits, so the reaction is not catalysed.

8. 8

   Sickle cell disease is caused by a variant in which type of DNA?

   1. ANon-coding DNA
   2. BmRNA
   3. CThe cell membrane
   4. DCoding DNA
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   D: Coding DNA

   It is a variant in the coding DNA for haemoglobin.

9. 9

   A variant stops the transcription of a gene. What is the most likely result?

   1. AA different protein is made
   2. BNo protein is made from the gene
   3. CMore protein is made
   4. DThe gene is deleted
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   B: No protein is made from the gene

   No mRNA is made, so the protein is not made, though the gene itself is normal.