During a newborn screening programme, a hospital laboratory investigated an inherited condition that damages light-sensitive cells in the eye. The condition...
Question 1 Report
During a newborn screening programme, a hospital laboratory investigated an inherited condition that damages light-sensitive cells in the eye. The condition is caused by allele c. A normal allele is written as C. Fig. 1 gives information about one family. Filled symbols show people with the condition. Squares represent males and circles represent females. Blood tests show that both unaffected parents, P and Q, carry allele c. Their affected daughter is R. The laboratory uses this family information before offering a genetic test to relatives. The condition is not caused by a lack of oxygen in the blood, although affected cells cannot make the protein needed for normal vision.
(a) Use Fig. 1 and the blood-test information to state which allele is recessive and give evidence for your answer. [2]
(b) Use the parents' genotypes to calculate the probability that their next child will have the condition. [3]
(c) Describe how a change in a gene can result in eye cells making a non-functioning protein. [3]
(d) Suggest two reasons why the laboratory asks relatives for a blood sample before giving genetic advice. [2]
Answer Details
(a) Allele c is recessive [1]. Both P and Q are unaffected carriers, yet their daughter R is affected [1]. An affected child from unaffected carrier parents shows that the condition needs two copies of c.
(b) The parents are \(Cc\) and \(Cc\) [1]. Their possible offspring genotypes are \(CC\), \(Cc\), \(Cc\), and \(cc\) [1]. Only \(cc\) is affected, so the probability is \(\frac14=25\%\) [1].
(c) A mutation changes the base sequence of DNA in a gene [1]. This may change codons and therefore the amino-acid sequence [1]. The protein may fold into a different shape and not function in eye cells [1].
(d) A blood sample can identify whether a relative carries allele c or determine genotype accurately [1]. This allows the chance of an affected child to be estimated and appropriate genetic counselling or further testing to be offered [1].
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