Fig. 1 shows part of a family tree used by a cancer genetics team. Some people have developed breast or ovarian cancer linked to a changed BRCA1 allele. The...
Question 1 Report
Fig. 1 shows part of a family tree used by a cancer genetics team. Some people have developed breast or ovarian cancer linked to a changed BRCA1 allele. The changed allele increases risk but does not guarantee that cancer will develop, because cancer also involves further mutations in body cells and environmental factors. Individual III-2 is an adult who has not had cancer and is considering a test for the family allele.
(a) What type of allele is most consistent with the pattern in Fig. 1? [1]
(b) Describe two features of Fig. 1 that support your answer. [2]
(c) Give the probability that III-2 inherited the changed BRCA1 allele if their affected parent is heterozygous. [1]
(d) Explain why a positive BRCA1 result cannot predict with certainty that III-2 will develop cancer. [3]
(e) Describe two ways in which regular screening could help a person with a changed BRCA1 allele. [2]
(f) When a result is shared, give one reason why it may also be important to inform close blood relatives. [1]
(g) Give one ethical issue that a genetics team should consider before testing an adult for this allele. [2]
Answer Details
(a) The pattern is most consistent with a dominant allele. [1]
(b) Affected people occur in successive generations, and an affected parent has an affected child. The pedigree also shows that both males and females can inherit the allele. Any two relevant observations gain credit. [2]
(c) If the affected parent is heterozygous, there is a \(50\%\) probability that III-2 inherited the changed allele. [1]
(d) A changed BRCA1 allele increases susceptibility to cancer; it does not guarantee cancer. Further mutations in body cells may be needed before cancer develops, and environmental or lifestyle factors also affect risk. [3]
(e) Regular screening can detect a tumour at an early stage. Earlier treatment may improve the outcome or allow treatment before the tumour spreads. [2]
(f) Close blood relatives may also have inherited the changed allele, so informing them allows them to seek testing or screening. [1]
(g) The adult must give informed consent. A genetics team should also consider that a result may cause anxiety or affect insurance and family relationships. [2]
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