During a genetic counselling appointment, a couple brought records from several years of their family. The counsellor is investigating an inherited hearing ...
Question 1 Report
During a genetic counselling appointment, a couple brought records from several years of their family. The counsellor is investigating an inherited hearing condition caused by recessive allele d. The dominant allele D gives normal hearing. Fig. 1 is a pedigree prepared from the records. A square represents a human male and a circle represents a human female. A filled symbol shows a person with the condition. The couple labelled A and B are planning another child. Neither A nor B has the condition, but their first child is affected.
The counsellor explains that the allele is carried in the DNA of body cells and can be passed to eggs or sperm. The information helps the family decide whether to have further genetic tests. The pedigree does not show the mass, blood group or temperature of any person because these are not needed for this investigation.
(a) Describe evidence from Fig. 1 that the hearing condition is recessive. [2]
(b) Use D and d to give the genotypes of parents A and B. [2]
(c) Use a genetic diagram to calculate the probability that a further child of A and B will be unaffected but carry allele d. [4]
(d) Suggest two reasons why genetic counselling may be useful to this couple. [2]
Answer Details
(a) A and B are both unaffected, but they have an affected child. For a recessive condition, this means each unaffected parent must have passed on a recessive allele. [2]
(b) Parent A is \(Dd\), and parent B is \(Dd\). They are unaffected because each has dominant allele \(D\), but each carries \(d\). [2]
(c) Genetic cross:
Parents: Dd × Dd
Gametes: D, d × D, d
D d
D DD Dd
d Dd ddThe unaffected carriers are the two \(Dd\) outcomes. Therefore:
\[\frac{2}{4}=\frac{1}{2}=50\%\]
The probability of a further child being unaffected but carrying \(d\) is \(\frac{1}{2}\), or 50%. [4]
(d) Genetic counselling can explain the chance of an affected child and help the couple understand carrier status and testing options. It can also support informed decisions about future pregnancies or provide information about available support. Any two valid points gain credit. [2]
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