Table 1 shows results from a metabolic clinic that tests the DNA of adults before they plan a family. The enzyme is needed to break down a substance found i...
Question 1 Report
Table 1 shows results from a metabolic clinic that tests the DNA of adults before they plan a family. The enzyme is needed to break down a substance found in some food. A person with no working copies of the allele produces almost no enzyme and develops the condition. Nia and Omar are both healthy, but their cell tests and DNA results are shown in the table.
| Genotype | Enzyme made by cells | Health status |
|---|---|---|
| AA | high amount | unaffected |
| Aa | medium amount | unaffected carrier |
| aa | very low amount | affected |
| Nia: Aa | medium amount | unaffected carrier |
| Omar: Aa | medium amount | unaffected carrier |
(a) State the genotype of a child who develops the enzyme condition. [1]
(b) Explain why Nia does not show symptoms of the condition. [2]
(c) Complete a genetic cross for Nia and Omar. Include the gametes, all four possible genotypes of their children and the phenotype ratio. [4]
(d) Calculate the probability that one child of Nia and Omar will be an unaffected carrier. [2]
(e) Suggest why the clinic explains that an affected child is possible even though both parents are healthy. [1]
Answer Details
(a) A child who develops the condition has genotype \(aa\). [1]
(b) \(A\) is the dominant, working allele. Nia has one \(A\) allele, which allows sufficient enzyme to be made, so she does not show symptoms. [2]
(c) Both Nia and Omar are \(Aa\), so each produces \(A\) and \(a\) gametes.
| Omar \(A\) | Omar \(a\) | |
|---|---|---|
| Nia \(A\) | \(AA\), unaffected | \(Aa\), unaffected carrier |
| Nia \(a\) | \(Aa\), unaffected carrier | \(aa\), affected |
The possible genotypes are \(AA\), \(Aa\), \(Aa\) and \(aa\). The phenotype ratio is \(3\) unaffected : \(1\) affected. [4]
(d) Two of the four possible offspring genotypes are \(Aa\):
\[\frac{2}{4}=\frac{1}{2}=50\%\]
The probability of an unaffected carrier is \(\frac{1}{2}\), or \(50\%\). [2]
(e) Both healthy parents can pass their recessive \(a\) allele to the same child. That child could inherit \(aa\) and be affected. [1]
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