A-LevelBiologyInherited change9700/42 · May/June 2017 · Q512 Marks
Cancer is a disease in which normal controls over cell division are lost and malignant tumours form. An early diagnosis of many types of cancer can result in successful treatment. The BRCA2 protein is involved in suppressing the development of tumours. The gene that codes for this protein is on chromosome 13. Several different dominant alleles of this gene, BRCA2, code for faulty versions of the protein. The presence of any one of these faulty alleles leads to an increased chance of developing several types of cancer, including breast cancer. Not everyone with one of these alleles develops cancer. This is because environmental factors, including lifestyle, are also involved. Fig. 5.1 is a pedigree (family tree) showing the occurrence of cancers in four generations of a family. The presence of a faulty BRCA2 allele was confirmed in person 15. The other individuals with cancer were not tested for the presence of the allele. Individuals 24–30 are all under twelve years old. [Figure 5.1]
- (a)Discuss the extent to which Fig. 5.1 provides evidence that a faulty BRCA2 allele increases the risk of a person developing cancer.[4]
- (b)(i)People whose families are suspected of having a faulty BRCA2 allele may choose to be tested for its presence in their own genome. A company based in the USA sells a microarray containing DNA probes for 20 different alleles that are associated with an increased risk of cancer, including the faulty BRCA2 alleles. This microarray can be used in a medical facility or research laboratory to test blood samples for the presence of these alleles. Explain the meaning of the term genome.[1]
- (b)(ii)Suggest a type of cell from a blood sample that is suitable for testing for the presence of this allele and explain your choice.[1]
- (b)(iii)Outline how a microarray enables the detection of particular alleles.[4]
- (b)(iv)Suggest one advantage and one disadvantage of screening for faulty alleles of BRCA2 before any symptoms occur. advantage disadvantage[2]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.
- [max 4] 1. individual 8 or 11 has, BRCA2 / allele, but does not have cancer ; 2. no evidence / unknown, that individuals (apart from 15) with cancer have, BRCA2 / allele or individuals with cancer (apart from 15) may have a different mutation; 3. no children of individual 15, (known to) have the allele / have cancer ; 4. individuals in fourth generation / children of individual 15, may develop cancer later in life ; 5. individual 15 has cancer and, BRCA2/allele; 6. (some) individuals with cancer in third generation had a parent with cancer or (some) individuals with cancer in third generation had a parent with, BRCA2/allele; ora 7. individual 3 or 4 may have had the, BRCA2 / allele or any individual from 8 to 11 may have inherited, BRCA2 / allele, from 3 or 4 ; 8. idea that overall data inconclusive;
- [1] all the, DNA/genetic material (in a person's cell);
- [1] (named) white cell, because it contains a nucleus ;
- [max 4] 1. ref. to probes are (short) lengths of ssDNA ; 2. complementary to the, alleles / DNA, being tested for ; 3. many copies of one type of probe placed in each cell (of the microarray); 4. (target), alleles / DNA, made single-stranded or single-stranded DNA made from mRNA ; 5. (target), alleles / DNA, labelled, (with fluorescent 'tags') ; 6. (target), alleles / DNA, hybridises / binds, with, probes / ssDNA; 7. unbound (target), alleles / DNA, washed off or bound (target), alleles / DNA, will not be washed off; 8. laser/UV light, used to detect presence of, fluorescence / hybridised probes / alleles / DNA ;
- [max 2] advantage max 1 1. if present, enables lifestyle change / early treatment / regular check-ups; 2. if not present removes worry ; 3. preventative treatment may be cheaper than treating disease itself ; disadvantage max 1 4. if present may cause worry ; 5. if present person may not develop cancer ; 6. test is expensive; 7. may have implications for life insurance / AW; 8. may decide to not have children / may be tested after they have children ;
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