This diagram is used in a clinic to show inheritance of cystic fibrosis. The condition is caused by recessive allele f. Both prospective parents have had a ...
Question 1 Report
This diagram is used in a clinic to show inheritance of cystic fibrosis. The condition is caused by recessive allele f. Both prospective parents have had a DNA test and are known to be carriers. Fig. 1 shows the genetic cross prepared by the nurse.
(a) Use Fig. 1 to state the probability that one child will have cystic fibrosis. [1]
(b) Which genotype gives a child who is a carrier but does not have cystic fibrosis? [1]
(c) Describe why the result for one pregnancy does not change the probability for the next pregnancy. [3]
A sports medicine team investigates an inherited condition that affects collagen in tendons. Fig. 1 shows normal collagen fibres and fibres from a person with a mutation. The person had repeated tendon injuries during exercise, despite following the same training plan as teammates.
(a) Describe one visible difference between the two sets of fibres in Fig. 1. [1]
(b) Suggest how a mutation in a collagen gene can increase the chance of tendon injury. [2]
(c) Use the term protein to describe why the gene change affects the tendon. [2]
Answer Details
Cystic fibrosis inheritance
- (a) One of the four outcomes is \(ff\), so the probability of cystic fibrosis is \(\frac{1}{4}\), or 25%. [1]
- (b) \(Ff\) gives a carrier who does not have cystic fibrosis. [1]
- (c) Each pregnancy is independent because gametes are made independently each time. Each parent can again pass on either \(F\) or \(f\), and fertilisation is random. Previous offspring do not change the parents' genotypes or the probability for a new pregnancy. [3]
Collagen mutation
- (a) The mutated fibres are less regularly aligned and include an altered pattern compared with the regular normal fibres. [1]
- (b) Altered collagen fibres have reduced strength or elasticity. The tendon is therefore more likely to tear when a force acts on it. [2]
- (c) A gene contains instructions for making a protein. A change in the DNA can produce collagen protein with a changed structure, affecting the tendon. [2]
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