Practise explaining accommodation and pupil responses, using muscle changes to link light intensity or viewing distance to vision.
Fur colour in rats is controlled by a gene with two alleles.
One allele codes for black fur colour. The other allele codes for agouti fur colour.
Several female rats with agouti coloured fur are mated with several male rats with black coloured fur.
All of the offspring have agouti coloured fur.
Some rats with white fur also have pink eyes.
These rats have pink eyes because they do not have pigment in their irises.
This means that their irises let light pass through, unlike the coloured irises found in other rats.
Explain how this difference in the iris affects vision in the rats with pink eyes.
An explanation that makes reference to three of the following:
- iris does not block / blocks less light / eq
- so pupil change / dilate / constrict has little / no effect / (iris) cannot regulate / control amount of light entering eye / eq
- too much light / more light enters eye / falls on retina / eq
- poor vision / damages retina / (goes) blind / eq
not just
iris lets
light
through
total 15 marks