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Read the following passage - AQA - A-Level Biology - Question 10 - 2018 - Paper 2
Question 10
Read the following passage. Complete achromatopsia is a form of complete colour blindness. It is caused by having only rods and no functional cone cells. People wit... show full transcript
Worked Solution & Example Answer:Read the following passage - AQA - A-Level Biology - Question 10 - 2018 - Paper 2
Step 1
People with complete achromatopsia have difficulty in seeing detail (lines 2–3). Explain why.
Answer
Individuals with complete achromatopsia possess only rod cells and lack functional cone cells. Rod cells are responsible for vision in low-light conditions and detecting motion, but they do not enable color discrimination or detail perception. As a result, people with this condition cannot perceive details since they rely solely on the limited capabilities of rod cells.
Step 2
Ten percent of the population on the Pacific island of Pingelap are affected by complete achromatopsia (lines 3–8). Use the Hardy-Weinberg equation to calculate the percentage of this population who are heterozygous for this disorder. Show your working.
Answer
To apply the Hardy-Weinberg principle, we assume that the population is in equilibrium. Let 'q' represent the frequency of the recessive allele (a), which corresponds to achromatopsia. Given that 10% of the population are affected (aa), we can write:
Thus, taking the square root of both sides:
Now, using the formula for heterozygotes (2pq):
2pq = 2(0.684)(0.316) \approx 0.432 $$ Multiplying by 100 to get the percentage gives: $$ 0.432 imes 100 \approx 43.2 \% $$Step 3
Red-green colour blindness affects more men than women (lines 7–8). Explain why.
Answer
Red-green colour blindness is linked to the X chromosome. Males have one X and one Y chromosome (XY), meaning that a single recessive allele on the X chromosome will express the phenotype for the blindness. In contrast, females have two X chromosomes (XX), so a woman would need two copies of the recessive allele to express the trait. This genetic configuration results in a higher prevalence of red-green colour blindness in men.
Step 4
People with red-green colour blindness are unable to distinguish between red and green, and also between other colours (lines 8–10). Explain why.
Answer
This inability arises from the malfunctioning of cone cells that are responsible for detecting light wavelengths corresponding to red and green. Those with red-green colour blindness lack the necessary pigments in their cone cells that differentiate between these colors, leading to a potential overlap in the wavelengths they perceive, ultimately resulting in an inaccurate or absent color perception.
Step 5
Current research into the treatment of red-green colour blindness involves the use of induced pluripotent stem cells (iPS cells) (lines 17–19). Suggest how iPS cells could correct red-green colour blindness.
Answer
Induced pluripotent stem cells (iPS cells) can potentially be differentiated into functional photoreceptor cells that contain the necessary pigments for color perception. By replacing non-functional cells in the retina with healthy, differentiated iPS cells, normal function could be restored, allowing proper detection of red and green wavelengths.
Step 6
The use of iPS cells could have advantages over the use of gene therapy to correct red-green colour blindness (lines 19–20). Using the information from the passage, suggest and explain reasons why.
Answer
iPS cells offer several advantages over traditional gene therapy. Firstly, they can be cultivated in large quantities and potentially provide a continuous supply of cells for treatment. Secondly, iPS cells can differentiate into various cell types, including fully functional cone cells, enhancing the chance for the successful integration into the retina. Lastly, unlike gene therapy, which may require repeated treatments or face issues of immune rejection, iPS cells can be tailored from a patient's own tissues, minimizing complications related to immune responses.