A mobile genetics laboratory tested DNA from three people after a newborn screening result suggested cystic fibrosis. The DNA test separates fragments linke...
Question 1 Report
A mobile genetics laboratory tested DNA from three people after a newborn screening result suggested cystic fibrosis. The DNA test separates fragments linked to two alleles of the CFTR gene. Allele C is the usual allele and allele c causes cystic fibrosis. Fig. 1 shows the bands obtained from cells in saliva samples. Each parent is healthy, but person C has cystic fibrosis.
Fig. 1
(a) State the genotype of person C. [1]
(b) Use the information in Fig. 1 to complete a genetic diagram for a child of persons A and B. Show the gametes and all possible offspring genotypes. [3]
(c) Calculate the probability, as a percentage, that the next child of persons A and B will have cystic fibrosis. [2]
(d) Explain why a person with genotype Cc does not usually have this condition. [2]
(e) Suggest two reasons why the clinic may offer a DNA test to healthy relatives of persons A and B. [2]
Answer Details
(a) Person C has only the allele c band, so the genotype is cc. [1]
(b) Persons A and B each have both bands, so both are Cc. They each produce gametes C and c. [1]
| Gamete C | Gamete c | |
|---|---|---|
| Gamete C | CC | Cc |
| Gamete c | Cc | cc |
The possible offspring genotypes are CC, Cc, Cc and cc. [2]
(c) One of four possible genotypes is \(cc\): \(\frac14\). [1] \[\frac14\times100=25\%\] The probability is 25%. [1]
(d) C is a dominant functional allele and produces sufficient functional CFTR protein. [1] The c allele is recessive, so cystic fibrosis usually occurs only when two copies are inherited. [1]
(e) Two reasons are to identify healthy relatives who are carriers [1] and to support informed decisions about having children. [1] Testing partners and providing counselling or early testing in future pregnancies are also valid reasons.
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