Fig. 1 shows a pedigree prepared by a clinic for a family affected by cystic fibrosis. Cystic fibrosis is caused by a recessive allele. Filled symbols show ...
Question 1 Report
Fig. 1 shows a pedigree prepared by a clinic for a family affected by cystic fibrosis. Cystic fibrosis is caused by a recessive allele. Filled symbols show people with the disorder. A square represents a male and a circle represents a female. Individual IV-2 is expecting a baby with a partner who is known not to have cystic fibrosis.
(a) What is the genotype of individual II-2? Use C for the normal allele and c for the cystic fibrosis allele. [1]
(b) Describe the evidence in Fig. 1 that shows cystic fibrosis is recessive. [2]
(c) Complete a genetic diagram to show the possible genotypes of children produced by two carriers of cystic fibrosis. [3]
(d) Give the percentage chance that a child of two carriers will have cystic fibrosis. [1]
(e) When a couple are considering a genetic test, give one reason why a trained counsellor should discuss the result with them. [1]
Answer Details
(a) Individual II-2 is affected and cystic fibrosis is recessive, so their genotype is \(cc\). [1]
(b) The parents in generation I are unaffected but have an affected child. This shows that they can carry the cystic fibrosis allele without having the disorder, so the allele is recessive. [2]
(c) Two carriers are \(Cc\) and \(Cc\). Each produces gametes carrying \(C\) or \(c\).
| \(C\) | \(c\) | |
|---|---|---|
| \(C\) | \(CC\) | \(Cc\) |
| \(c\) | \(Cc\) | \(cc\) |
Parents \(Cc\) and \(Cc\), correct gametes, and offspring \(CC\), \(Cc\), \(Cc\), \(cc\) each gain a marking point. [3]
(d) One of the four equally likely genotypes is \(cc\), so the probability of cystic fibrosis is \(\frac14=25\%\). [1]
(e) Genetic counselling is important because a result may cause anxiety, may affect relatives, and can help the couple understand their reproductive choices. Any one of these is creditworthy. [1]
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