A-LevelBiologyInherited change9700/41 · May/June 2019 · Q215 Marks
The interpupillary distance (IPD) is the distance in millimetres between the centres of the pupils of the eyes. Fig. 2.1 shows how IPD is measured. [Figure 2.1] IPD is one example of a characteristic of human facial structure that shows variation. Fig. 2.2 shows the pattern of variation in IPD in a large sample of adults. [Figure 2.2]
- (a)(i)Name the type of variation shown in Fig. 2.2.[1]
- (a)(ii)Suggest and explain how genes and the environment contribute to variation in IPD in humans.[3]
- (b)(i)Individuals with an IPD of 70mm or more have a mutation in the PAX3 gene that results in less PAX3 protein being made. The normal role of the PAX3 protein is to increase the expression of many other genes involved in embryonic development. These genes affect a range of phenotypic features such as facial structure, hearing and eye colour. State the term that is used to describe a gene, such as PAX3, that controls the expression of other genes and suggest how the PAX3 protein controls the expression of other genes.[3]
- (b)(ii)Describe how microarray analysis could be used to identify the genes switched on by PAX3 in embryonic cells.[5]
- (b)(iii)The chimpanzee, Pan troglodytes, has DNA that is 98.5% similar to humans, including possession of the PAX3 gene. Investigations show that chimpanzees express higher levels of the PAX3 protein during embryonic development than humans. Fig. 2.3 shows a chimpanzee, Pan troglodytes. [Figure 2.3] Suggest how knowledge of the PAX3 gene helps scientists explain how humans and chimpanzees are very different in facial structure, even though they have very similar DNA.[3]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.
- [1] continuous;
- [3] any three from: 1 several / many, genes ; 2 additive effect / gene(s) (products) interact ; 3 environment has, big / significant, effect; 4 (named) environment(al factors) affects gene expression ; 5 e.g. age/diet/ disease / parasites / chemicals / mutagens / alcohol in utero, can affect, development / growth / IPD / phenotype;
- [3] 1 regulator(y) (gene); plus any two from: 2 gene codes for / makes, transcription factor / activator / inducer or protein is, transcription factor / activator / inducer; 3 TF/(PAX3) protein / gene product/inducer / activator, binds to DNA ; 4 (binding site for TF is) promoter / enhancer ; 5 helps / stimulates, RNA polymerase (binding / action);
- [5] any five from: 1 get (total) mRNA from (embryonic) cell (expressing PAX3) ; 2 make, cDNA / ssDNA from mRNA ; 3 add fluorescent, label / tag / dye / chemical, to (this) DNA ; 4 microarray / chip, has DNA, spots / probes / reporters / oligos, from, known / named, genes ; 5 cDNA / ssDNA, hybridises with / binds to / complementary base pairs with, (spots / probes / reporters / DNA, on) chip ; 6 fluorescent spots, indicate / show, (gene / DNA) expression; 7 AVP; e.g. ref. to wash off excess (c)DNA after hybridisation UV/laser (scan), to see fluorescence intensity of fluorescence gives quantitative measure
- [3] any three from: 1 small genetic difference / one gene / PAX3, gives, many / big, phenotypic effect(s) ; 2 because PAX3, regulates / controls, many / other, genes; 3 different, level / magnitude / intensity / pattern, of gene expression (of other genes in chimps) ; 4 higher PAX3 (expression / protein) makes, chimps eyes closer together / chimp IPD smaller / decreases face width ;
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