AS & A Level Biology - 9700 Paper 4 2020 Summer Zone 3

(a) (i) The varieties Golden Dawn and Tondo di Piacenza were grown in the same garden and cross-pollination occurred between them. The gardener grew these cross-pollinated F1 seeds into plants that formed fruits.
The gardener did not know the genotypes of the parent plants and did not know that cross-pollination had occurred.
State the phenotype of the fruits of the F1 plants and explain why it was unexpected for the gardener.
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(ii) The gardener crossed two of these F1 plants.
Complete Fig. 1.1 with the F1 gametes, F2 genotypes and F2 phenotypes.
State the ratio of fruit phenotypes in the F2 offspring.
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ratio of fruit phenotypes ......................................................................................... [4]
(b) Watermelons, Citrullus lanatus, are plants in the same family as courgettes. They produce large round edible fruits that usually contain many hard seeds. Seeds are the structures formed when the male and female gametes fuse at fertilisation.
In the 1990s a triploid (3n) watermelon plant was developed. To produce the triploid watermelon plant, a normal diploid parent plant (2n = 22) was crossed with an artificially created tetraploid plant (4n = 44).
Triploid watermelon plants develop edible fruits but these are sterile and do not contain seeds, making them more enjoyable to eat.
Explain why the fruits of the triploid plants are sterile and do not contain seeds.
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(c) (i) Watermelons are attacked by watermelon mosaic virus (WMV).
In 1965, a WMV-resistant plant in the same family, Cucurbita ecuadorensis (2n = 40), was found growing wild in South America.
State why a WMV-resistant variety of watermelon cannot be obtained by breeding Cucurbita ecuadorensis with a normal diploid watermelon.
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(ii) Outline how the WMV-resistant trait of Cucurbita ecuadorensis could be transferred to watermelon plants.
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(a) Diabetes insipidus (DI) is a condition in humans that causes a person to have an excessive thirst, which leads to increased drinking. One form of DI is caused by a tumour in the region of the hypothalamus concerned with osmoregulation.

(i) Explain how a tumour develops in the hypothalamus.
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(ii) Suggest how a tumour in the hypothalamus can lead to a person producing a large volume of dilute urine.
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(b) The nervous and endocrine systems of a mammal co-ordinate responses to changes in the internal and external environment. Table 2.1 compares some features of the nervous and endocrine systems.

Complete Table 2.1.

[Table_1: Table 2.1]

feature | nervous system | endocrine system
signal | impulse | .........................................
method of transmission of signal | ............................ | in blood
type of communication | ............................ | chemical
duration of effect | ............................ | .........................................

[4]

(a) The collared flycatcher, ̱Ficedula albicollis̱, and the pied flycatcher, ̱F. hypoleuca̱ are two closely related species of bird. DNA analysis has shown that speciation from a common ancestor occurred approximately 1 million years ago.

A study was carried out on the island of Öland, Sweden. In Öland, the breeding areas of the two bird species overlap and small numbers of hybrid flycatchers are produced.

• Birds were captured and their DNA was analysed to identify whether each bird was ̱F. albicollis̱, ̱F. hypoleuca̱ or a hybrid.
• Sperm samples were taken from the male birds.

Table 3.1 shows the percentage of males of each bird type with normal sperm.

Table 3.1

bird type	percentage of males with normal sperm
̱F. albicollis̱	68
̱F. hypoleuca̱	78
male hybrid	0

• The researchers observed that female birds mostly choose mates of their own species based on plumage (feathers) and song.
• Hybrid flycatchers are produced when female ̱F. albicollis̱ mate with male ̱F. hypoleuca̱ that have a song that is similar to ̱F. albicollis̱.
• Analysis showed that all female hybrids were sterile.

The group of eggs a female bird lays at a single time in its nest is called a clutch. The offspring in the nest are looked after by a male-female pair. Sometimes the male in the male-female pair does not provide the sperm that fertilise the eggs of the female.

Table 3.2 shows:
• the percentage of clutches with eggs that hatched
• the percentage of extra-pair nestlings (offspring in the nest fathered by a male that was different from the male of the male-female pair).

Table 3.2

parents of nest	percentage of clutches with eggs that hatched		percentage of extra-pair nestlings
male	female
̱F. albicollis̱	̱F. albicollis̱	94.5	17.2
̱F. hypoleuca̱	̱F. hypoleuca̱	89.3	22.4
hybrid	̱F. albicollis̱ or ̱F. hypoleuca̱	38.0	100.0

(i) Discuss the pre-zygotic and post-zygotic isolating mechanisms that maintain ̱F. albicollis̱ and ̱F. hypoleuca̱ as separate species.
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(ii) Explain how the two species ̱F. albicollis̱ and ̱F. hypoleuca̱ could have evolved from one original ancestral population.
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(b) A single-nucleotide polymorphism (SNP) is caused by a base pair substitution mutation in a specific region of DNA.

One method of identifying whether two individuals have the same SNP is to:
• use a specific primer and polymerase chain reaction (PCR)
• add a restriction enzyme
• carry out gel electrophoresis
• stain with a dye to compare banding patterns.

(i) Explain why:
a specific primer is used
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a restriction enzyme is added
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gel electrophoresis is carried out.
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(ii) The method of identifying whether two individuals have the same SNP method was carried out to compare species A and species B.

Fig. 3.1 shows the banding patterns that were observed.

[Image_1: Diagram of banding patterns]

Describe ̱anḏ suggest an explanation for the results obtained in Fig. 3.1.
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Lung epithelial cells have a thin layer of watery mucus on their surface.
The normal allele of the CFTR gene codes for a transport protein that transports chloride ions out of epithelial cells.
Fig. 4.1 is a diagram of part of the cell surface membrane and the mucus layer of an epithelial cell with normal CFTR proteins.

Cystic fibrosis (CF) is a genetic disorder caused by having two recessive alleles of CFTR. In severe cases of CF, the transport proteins are not added to the cell surface membrane. This causes the mucus layer to be thick and sticky.
(a) Explain why the absence of CFTR proteins will cause the mucus layer to be thick and sticky.
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[2]
(b) The probability of a baby having CF when both parents are heterozygous carriers for CF is 25%.
It is possible to carry out prenatal screening to check for CF by using one of these tests:
• amniocentesis, using cells from the amniotic fluid
• chorionic villus sampling, using cells from the placenta.
Both tests slightly increase the probability of the pregnancy failing (miscarriage).
Outline the advantages of carrying out prenatal screening for CF.
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(c) Embryos produced by IVF may be screened for genetic abnormalities:
• to test for a specific genetic disease, such as cystic fibrosis
• to check whether there is an abnormal number of chromosomes present.
To improve the success of implantation and pregnancy, only embryos without any form of genetic abnormality are transferred to the woman’s uterus.
A new double screening method was trialled where a single embryo biopsy was taken and used to test for a specific genetic disease and to check the number of chromosomes. In the trial, 1122 embryos were tested using this double screening method.
In the trial, of the 1122 embryos tested:
• 50.6% did not have a genetic disease
• 27.5% did not have a genetic disease and did not have an abnormal number of chromosomes (normal embryos).
Only normal embryos were transferred into the women. The percentage of embryo transfers that resulted in pregnancy was calculated.
The results of the trial using double screening of a single biopsy were compared to the results of IVF procedures that used standard screening methods, as shown in Table 4.1.
[Table_1]
Using the data in Table 4.1, discuss the social and ethical considerations of double screening for cystic fibrosis and chromosomal abnormalities in a single biopsy.
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(a) (i) With reference to Fig. 5.1, identify one letter corresponding to:
a structural gene .............
a control (regulatory) sequence .............
a repressor molecule ............. [3]

(a) (ii) Explain why enzyme D is described as inducible.
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(b) (i) Calculate the percentage of Mig1 molecules in the nucleus when glucose is present.
Show your working and write your answer to two significant figures.
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(b) (ii) When glucose is absent, 13\% of the available Mig1 molecules are present inside the nucleus.
Explain why this figure is different from your answer to (i).
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(c) Yeast cells are unable to take in and metabolise the disaccharide sugar lactose.
Some strains of yeast have been genetically engineered to overcome this, by inserting two genes from the bacterium \textit{Escherichia coli} into yeast cells.
Name the two bacterial genes that have been inserted into the yeast cells.
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(a) Structures and compounds involved in the light dependent stage of photosynthesis are listed, A to J.
A - thylakoid membrane   F - NADP
B - starch grain   G - electron
C - chlorophyll a   H - proton
D - chlorophyll b   I - ATP
E - water   J - chloroplast envelope
Complete Table 6.1 by matching each description with one letter chosen from A to J to show the correct structure or compound.
You may use each letter once, more than once or not at all.

Table 6.1
| description | letter |
| ----------- | ------ |
| accessory pigment |
| location of ATP synthase |
| acts as reaction centre |
| transports hydrogen atoms |
| diffuses through ATP synthase |
| broken down in photolysis |

(b) GP (PGA) and TP (triose phosphate) are intermediates of the Calvin cycle of the light independent stage. Some GP and TP are used for the synthesis of organic compounds.
(i) Name a polysaccharide synthesised as a result of TP production.
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(ii) Name the additional element that is required for the production of amino acids from Calvin cycle intermediates.
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(a) Fig. 7.1 shows part of the process of respiration in a mitochondrion.
[Image_1: Fig. 7.1 diagram]
(i) With reference to Fig. 7.1, name:
area A
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process B
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substance C
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(ii) State the type of chemical compound that is represented by D.
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(iii) State the process by which ATP can be synthesised directly during glycolysis or the Krebs cycle.
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(b) Elephant seals can stay underwater for up to two hours. During this time, respiration continues. Fig. 7.3 shows the mass of blood in the body, as a percentage of total body mass, for the elephant seal and for three other mammals.
[Image_2: Fig. 7.3 graph]
(i) Fig. 7.3 shows that elephant seals have a higher mass of blood as a percentage of total body mass than humans. Calculate how many times greater this figure is for elephant seals compared to humans. Show your working and write your answer to two decimal places.
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(ii) Suggest why an elephant seal needs such a large mass of blood as a percentage of total body mass.
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(c) Elephant seals have a very thick layer of adipose tissue under their skin. Adipose cells are rich in fat molecules. Suggest why 1 g of fat will produce more ATP than 1 g of carbohydrate as a respiratory substrate in aerobic respiration.
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(d) (i) Adipose tissue plays an important role in thermoregulation in elephant seals. State the role of adipose tissue in thermoregulation.
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(ii) The hypothalamus in the brain is the control centre for thermoregulation. Outline how a change in temperature of the external environment results in an impulse arriving at the hypothalamus.
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(iii) When the blood temperature of a mammal decreases, one response is that its body secretes more adrenaline. Suggest how an increase in adrenaline results in an increase in blood temperature.
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(a) Sampling can be used to find the distribution and abundance of species in an area.

Students sampled a rocky shore from a high tide area to a low tide area. They decided to use a belt transect.

Describe how you would carry out a belt transect to assess the distribution and abundance of organisms in an area.

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(b) The results of an ecological investigation may suggest that there is a relationship between two variables, for example the abundance of a plant species and light intensity.

Scattergraphs can be drawn between the two variables.

(i) Fig. 8.1 shows three scattergraphs, A, B and C.



State which scattergraphs show a relationship between the two variables, x and y.
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(ii) Name a statistical test you would carry out to assess the strength of any relationship between two variables.
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(a) Using named examples, describe and explain the differences between continuous variation and discontinuous variation. [8]
(b) Outline how selective breeding (artificial selection) has improved the yield of crops, such as wheat and maize. [7]

(a) Describe and explain how the stimulation of sensory hair cells of a Venus fly trap plant leads to an insect being trapped. [8]
(b) Explain what is meant by the term homeostasis and describe the principles of homeostasis in mammals. [7]