6.8 Inherited disorders and embryo screening
5 exam questions worth 13 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Cystic fibrosis is caused by a recessive allele. What does this mean for a person with only one copy of the allele?
Mark scheme
- they do not have the disorder (1)
- they are a carrier / can pass it on (1)
Model answer
They do not have cystic fibrosis, but they are a carrier and can pass the allele on to their children.
Two parents are both carriers of cystic fibrosis (Ff).
Use a Punnett square to find the probability that their child will have cystic fibrosis.
Mark scheme
- gametes F, f and F, f (1)
- offspring FF, Ff, Ff, ff (1)
- 1 in 4 / 25% / 0.25 (1)
Model answer
Gametes F and f from each parent. Offspring: FF, Ff, Ff, ff. The probability of cystic fibrosis (ff) is 1 in 4, or 25%.
Polydactyly is caused by a dominant allele. A man with polydactyly (Dd) has children with a woman without it (dd).
What is the probability that a child has polydactyly?
Mark scheme
- offspring Dd, Dd, dd, dd (1)
- 1 in 2 / 50% (1)
Model answer
Half the children are expected to be Dd, so the probability is 1 in 2 (50%).
Describe what cystic fibrosis does to the body.
Mark scheme
- it is a disorder of cell membranes (1)
- thick / sticky mucus (in lungs / digestive system) (1)
Model answer
It is a disorder of cell membranes that causes thick, sticky mucus to build up in the lungs and digestive system.
Evaluate the use of embryo screening to stop children being born with cystic fibrosis.
Mark scheme
Any four from:
- for: prevents suffering (1)
- for: saves cost of treatment / care (1)
- against: destroys embryos / ethical objection (1)
- against: expensive / may give false results (1)
- against: could lead to designer babies (1)
- a justified conclusion (1)
Model answer
Screening means children are not born with a disorder that causes suffering, and it saves the cost of lifelong treatment. However, embryos are destroyed, which many people think is wrong, and some fear it could lead to choosing embryos for other features. Overall, for a serious disorder like cystic fibrosis, many people think screening is justified.
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