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Topic 8
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Questions
Q1.
The ability to taste phenylthiocarbamide (PTC) depends on a single gene.
This gene has two alleles. One is a dominant, tasting allele (T), and the other is a recessive, non-tasting
allele (t).
In a survey, it was found that 64% of people could taste PTC.
(a) The Hardy-Weinberg equation is
(p + q)2 = 1
Use this equation to calculate the proportion of the population who would be TT, Tt and tt.
(4)
(b) There is no known selective advantage for being able to taste PTC.
(i) Describe two conditions under which these proportions remain the same over many generations.
(2)
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(ii) In native American populations 98% of individuals can taste PTC.
In native Australian population 58% of individuals can taste PTC.
State why these two populations have different frequencies of the T allele.
(3)
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Q2.
Tomato plants and potato plants are both affected by a fungal disease called blight.
In 2014, a study was carried out to compare the yield of potatoes genetically modified
(GM) for blight resistance with the yield of non-modified (non-GM) potatoes.
The study was carried out using 96 plants of each variety, equally divided between six different plots.
The graph shows the results of this study.
(i) Calculate the percentage change in yield due to blight resistance in plot 2.
(2)
Answer ........................................................... %
(ii) The raw data for potato yield in this study are shown in the table.
A Student's t-test was used to compare the potato yields.
State a suitable null hypothesis for this test.
(1)
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(iii) Calculate the t value for the data to compare GM with non-GM potato yields, using the formula:
(3)
Answer ...........................................................
(iv) The table shows the critical values of t with 10 degrees of freedom.
Use your value of t to test the validity of the null hypothesis.
(3)
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Q3.
A defect in the cone cells of the retina can result in colour blindness.
(a) Which is the visual pigment found in cone cells?
(1)
A
iodopsin
B
opsin
C
retinol
D
rhodopsin
(b) The table below shows the distribution of different cones involved in colour blindness.
(i) The population of London is 12 million, of which 55% are men.
Calculate the number of men in London who are colour blind.
(3)
Answer ...........................................................
(ii) The allele for one defect in cone cells is recessive and carried on the X chromosome.
Explain why more men than women are colour blind.
(2)
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(iii) L-cones respond to long wavelengths of light.
M-cones respond to medium wavelengths of light.
S-cones respond to short wavelengths of light.
The visible light spectrum is shown below.
Analyse the data to explain why red/green colour blindness is the most common throughout the
population.
(4)
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(Total for question = 10 marks)
Q4.
'DELLA' proteins normally prevent transcription factors from binding to important growth-stimulating genes
in the cells of plant stems.
The diagram below shows part of the biochemical pathway leading to the synthesis of an active plant
growth substance called gibberellin (GA1). GA1 reduces the effect of 'DELLA' proteins.
(a) The Le-1 gene codes for the enzyme GA20 oxidase. A mutation of this gene forms a recessive allele
which, when translated, leads to a single amino acid change in this enzyme. This change significantly
reduces the activity of GA20 oxidase.
Explain how a single amino acid change can bring about a change in the structure and activity of GA20
oxidase.
(4)
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(b) (i) Explain why homozygous recessive plants will have short stems.
(4)
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(ii) An ecological survey of a stable population of one species of plant found that 36% had short
stems.
Calculate the percentage of heterozygous individuals in this population.
Assume that there are only two alleles for this gene.
(4)
Answer ...........................................................
(Total for question = 12 marks)
Q5.
Sickle cell anaemia is caused by a mutation in the β-globin gene which codes for part of haemoglobin.
The diagram shows the first 10 amino acids in this part of haemoglobin, in a healthy person and in a
person with sickle cell anaemia.
The allele that leads to sickle cell anaemia is known as HbS.
From 1990 to 1996, all newborn infants in California were screened for the presence of the HbS allele.
The data in the table show the ratio of individuals with and without the HbS allele, in various ethnic
groups.
A chi squared test was used to analyse the raw data.
The null hypothesis was: "There is no difference between the frequency of the HbS allele in African
Americans and in all other ethnic groups combined."
The calculated value of chi squared was 450 615.
The table shows some critical values for the chi squared distribution, with one degree of freedom.
Explain the conclusion that can be drawn from this chi squared test.
(3)
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Mark Scheme
Q1.
Q2.
Q3.
Q4.
Q5.