Genetics

Question 1

Some cat breeders select for polydactyly. Describe how this would affect the
frequencies of the homozygous genotypes for this gene in their breeding
populations over time.

Answer 1

Homozygous dominant increases and homozygous recessive decreases

Question 2

Explain why the scientists used the same restriction endonuclease enzymes on
each DNA sample.

Answer 2

Cut DNA at same sequence

so you get the required gene.

Question 3

Explain why primer A3 and primer A4 only bind to specific DNA fragments.

Answer 3

They have a specific base sequence;

That is complementary to allele r or R

Question 4

Use all the information given to explain the results in Figure 1. Fragments l n and m

Answer 4

Fragments L from parent rr, because all longer fragments
Fragments N from parent RR, because all shorter fragments
M - offspring heterozygous as they have both 195 and 135 base pair fragments

Question 5

The scientists wanted to know on which chromosome the gene with alleles R and r
was located. From the flies with genotype RR, they obtained cells that were in
mitosis and added a labelled DNA probe specific for allele R. They then looked at
the cells under an optical microscope.
Explain why they used cells that were in mitosis.

Answer 5

Cells in mitosis means that the chromsomes are visible, and so you can see which chromosomes the DNA probe attached to

Question 6

) The scientists concluded that the resistance of the flies to the insecticide is
partly due to increased activity of PM but other factors are also involved.
Explain how these data support this conclusio

Answer 6

1. Few resistant flies die (without inhibitor);
2. More inhibited flies die than resistant flies;
3. (PM) inhibited flies die faster (than resistant flies);
   (Other factors must be involved because)
4. Some resistant flies die;
5. But (with inhibitor) still have greater resistance / die slower than
   non-resistant flies.

Question 7

In fruit flies, the genes for body colour and wing length are linked. Explain what this
means.

Answer 7

gene’s loci on same chromosome.

Question 8

A scientist investigated linkage between the genes for body colour and wing length.
He carried out crosses between fruit flies with grey bodies and long wings and fruit
flies with black bodies and short wings.
Figure 1 shows his crosses and the results.
• G represents the dominant allele for grey body and g represents the recessive
allele for black body.
• N represents the dominant allele for long wings and n represents the
recessive allele for short wings.
Figure 1

Phenotype of parents grey body,
long wings ×
black
body,
short
wings
Genotype of parents GGNN ggnn
Genotype of offspring GgNn
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Page 8
Phenotype of offspring all grey body, long wings
These offspring were crossed with flies homozygous for black body and short wings.
The scientist’s results are shown in Figure 2.
Figure 2
GgNn crossed with ggnn
Grey body,
long wings
Black body,
short wings
Grey body,
short wings
Black body,
long wings
Number of
offspring 975 963 186 194

(b) Use your knowledge of gene linkage to explain these results

Answer 8

GN and gn linked;

2. GgNn individual produces mainly GN and gn gametes;
3. Crossing over produces some / few Gn and gN gametes;
4. So few(er) Ggnn and ggNn individuals.

c=1.1.1.1

Question 9

Which statistical test could the scientist use to determine whether his observed
results were significantly different from the expected results?
Give the reason for your choice of statistical test.

Answer 9

Chi squared test;

2. Categorical data.

Question 10

Use the Hardy–Weinberg equation to calculate the frequency of mosquitoes
heterozygous for the KDR gene in this population in 2003.

Answer 10

0.32

Question 11

Suggest an explanation for the results in the figure above. Mutation w ddt

Answer 11

Mutation produced resistance allele:

2. DDT use provides selection pressure;
3. Mosquitoes with KDR minus allele more likely (to survive) to reproduce;
4. Leading to increase in KDR minus allele in population.

Question 12

Suggest how the KDR minus allele gives resistance to DDT.

Answer 12

(Mutation) changes shape of sodium ion channel

2. DDT no longer complementary / no longer able to bind.

Question 13

Explain the advantage of presenting the results as a ratio.

Answer 13

Allows (valid) comparison;

2. Number / sample size may vary;

Question 14

Explain the high frequency of the HbC

allele in areas where malaria occurs.

Answer 14

Individuals with the HbC
 (allele) reproduce;
2. Pass on HbC
 (allele) which increases in frequency;
3. HbA HbA
 individuals less likely to survive / reproduce / frequency of HbA
(allele) decreases;

Question 15

5.The Hardy-Weinberg equation is
p
2
+ 2pq + q
2 =1
The Hardy-Weinberg equation can be used to estimate the frequency of a recessive allele
in a population. Haemochromatosis is a condition caused by a recessive allele.
In one country, 1 in every 400 people was found to have haemochromatosis.
Describe how you would use the Hardy-Weinberg equation to calculate the frequency of
people who are healthy but carriers (heterozygotes) of the allele for haemochromatosis.

Answer 15

Use 1 in 400 to find frequency of homozygous recessive / q
Note - convention has recessive allele as q and dominant
2. Find square root of q

3. Use of p + q = 1.0 / determine frequency of both alleles
4. Use of 2pq to find carriers / heterozygotes;

Question 16

On islands in the Caribbean, there are almost 150 species of lizards belonging to the
genus Anolis. Scientists believe that these species evolved from two species found
on mainland USA. Explain how the Caribbean species could have evolved

Answer 16

Geographic(al) isolation;
2. Separate gene pools / no interbreeding / gene flow (between
populations);
Accept: reproductive isolation
This mark should only be awarded in context of during the
process of speciation. Do not credit if context is after
speciation has occurred.
3. Variation due to mutation;
4. Different selection pressures / different abiotic / biotic conditions /
environments / habitats;
Neutral: different conditions / climates if not qualified
Accept: named abiotic / biotic conditions
5. Different(ial) reproductive success / selected organisms (survive and)
reproduce;
Accept: pass on alleles / genes to next generation as
equivalent to reproduce
6. Leads to change / increase in allele frequency

Question 17

mark release

Answer 17

Capture / collect sample, mark and release;
2. Method of marking does not harm lizard / make it more visible to
predators;
3. Leave sufficient time for lizards to (randomly) distribute (on island)
before collecting a second sample;
4. (Population =) number in first sample × number in second sample
divided by number of marked lizards in second sample / number
recaptured

Question 18

One farmer stated that the increase in the use of Bt crop plants had caused a
mutation in one of the insect species and that this mutation had spread to other
species of insect. Was he correct?

Answer 18

no

1. Mutations are spontaneous / random;
2. Only the rate of mutation is affected by environment;
3. Different species do not interbreed / do not produce fertile offspring;
4. So mutations cannot be passed on

Question 19

Explain why both types of twin were used in this investigation.

Answer 19

Identical twins show genetic influence / differences between

them show environmental influence;

Question 20

What do these data suggest about the relative effects of genetic and environmental
factors on the development of schizophrenia?

Answer 20

Genes play a greater role / environment plays a lesser role

Question 21

Suggest two factors that the scientists should have taken into account when
selecting the twins to be used in this study.

Answer 21

Age;

2. Sex (non-identical twins);
3. Family / medical history (of mental illness);
4. No use of recreational drugs;
5. Ethnic origins;

Question 22

define genotype

Answer 22

the complete set of genes of an organism

Question 23

define the term sex linked

Answer 23

a gene which is found on the x or y chromosome