Fig. 1 shows a young patient receiving a leaflet about cystic fibrosis screening. The condition is caused by a recessive allele. The patient's parents are b...
Question 1 Report
Fig. 1 shows a young patient receiving a leaflet about cystic fibrosis screening. The condition is caused by a recessive allele. The patient's parents are both known carriers, but neither has the condition. The leaflet includes a Punnett square showing possible fertilisation events. Cells from the developing fetus could be tested for the allele, but the family is also told that genetic counselling is available before any decision is made. The condition affects mucus in the lungs, so oxygen uptake during exercise can be reduced in some people.
(a) Name the genotype of a carrier parent. Use F and f. [1]
(b) Use the Punnett square to calculate the probability that one child will have cystic fibrosis. [2]
(c) Suggest why a genetic counsellor should not tell the parents what decision to make. [1]
Answer Details
Labelled answer diagram:
(a) A carrier has genotype Ff [1]. The dominant allele means the person does not have cystic fibrosis, but the recessive allele can be passed on.
(b) Two carrier parents produce the possible genotypes FF, Ff, Ff and ff. One of the four possible genotypes is affected, ff [1]. Therefore the probability that one child will have cystic fibrosis is \(\frac14\), or 25% [1]. Each pregnancy is an independent event.
(c) The decision is personal and ethical, so a genetic counsellor should provide unbiased information rather than tell the parents what decision to make [1].
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