Complete achromatopsia is a form of complete colour blindness. It is causedby having only rods and no functional cone cells. People with completeachromatopsia have difficulty in seeing detail. Complete achromatopsia iscaused by an autosomal recessive allele and is usually very rare inpopulations with only one in 40 000 being affected. However on the Pacific 5island of Pingelap ten percent of the population are affected.One form of red-green colour blindness is caused by a sex-linked recessiveallele which affects more men than women. People with this red-greencolour blindness are unable to distinguish between red and green, and alsobetween other colours. They have green-sensitive cones but the 10photoreceptive pigment they contain does not function.Scientists investigated the use of gene therapy to correct red-green colourblindness in monkeys. They injected viruses containing the gene for thegreen-sensitive pigment directly into the eyes of the monkeys. Although themonkeys maintained two years of colour vision, there is debate on whether 15this form of gene therapy is worthwhile. No clinical trials of this procedurehave been carried out on humans. Current research into the treatment ofred-green colour blindness involves the use of induced pluripotent stem cells(iPS cells). The use of iPS cells could have advantages over the use of genetherapy. 20Use the information in the passage and your own knowledge to answer the following questions.3(a) People with complete achromatopsia have difficulty in seeing detail (lines 2-3).Explain why.

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