Fig. 1 shows a DNA sequence from part of a gene in healthy cells and from cells of a person with an inherited disorder. The sequence is read in groups of th...
Question 1 Report
Fig. 1 shows a DNA sequence from part of a gene in healthy cells and from cells of a person with an inherited disorder. The sequence is read in groups of three bases. The altered protein has one different amino acid, which changes its shape.
(a) Use Fig. 1 to identify the DNA base that has changed. [1]
(b) Name this type of change to genetic material. [1]
(c) Describe how one changed DNA base can lead to altered protein function in human cells. [3]
Table 1 shows results from screening 200 adults for a recessive allele linked to an enzyme disorder. Fig. 1 shows the three possible DNA-test bands used by the laboratory. The adults have no symptoms, but a result may help them plan a family.
| DNA result | Number of adults |
|---|---|
| AA | 128 |
| Aa | 64 |
| aa | 8 |
(a) Use Table 1 to calculate the percentage of adults who are carriers. [2]
(b) Which test-band pattern in Fig. 1 identifies a carrier? [1]
(c) Suggest why an adult with genotype Aa does not show the disorder. [2]
Answer Details
DNA mutation
- (a) In the second triplet, C has changed to A: \(\text{CCT}\) has become \(\text{CAT}\). [1]
- (b) This is a mutation, specifically a base substitution. [1]
- (c) The base sequence determines the amino-acid sequence of a protein. A changed triplet can code for a different amino acid. This can alter protein folding and shape, including the shape of an enzyme's active site, so protein function changes. [3]
Carrier screening
- (a) Carriers have genotype \(Aa\). \[\frac{64}{200}\times100=32\%\] Therefore, 32% of adults are carriers. [2]
- (b) A carrier has the \(Aa\) band pattern with two bands. [1]
- (c) An adult with \(Aa\) does not show the disorder because \(A\) is a dominant functioning allele. Enough enzyme is made, so the recessive \(a\) allele is not expressed in the phenotype. [2]
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