iG Biology - Question paper 1BR ( Jun 2025)

Question 1

The photograph shows a fungus called Mucor growing on an agar plate. Cells within Mucor contain nuclei and mitochondria.

(a) (i) Which of these is a correct statement about the fungus, Mucor?

A) Mucor is a eukaryotic organism with cell walls made of chitin
B) Mucor is a eukaryotic organism with cell walls made of cellulose
C) Mucor is a prokaryotic organism with cell walls made of chitin
D) Mucor is a prokaryotic organism with cell walls made of cellulose

(ii) Plant cells contain chloroplasts, cytoplasm and starch. How many of these are also present in a fungus such as Mucor?

A) 0
B) 1
C) 2
D) 3

(iii) The fungus in the diagram is circular with a radius of 45 mm. Calculate the area, in mm², of this circle.
Give your answer to 2 significant figures. [area = πr² π = 3.14]

(b) Mucor can reproduce using asexual or sexual reproduction.

(i) State two differences between asexual and sexual reproduction.
(ii) Explain why it is an advantage for Mucor to be able to reproduce using sexual reproduction.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — parts (a)(i), (a)(ii)
• 2(b): Cell structure — part (a)(ii)
• Appendix 3: Mathematical skills — part (a)(iii)
• 3(a): Reproduction — parts (b)(i), (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) A (Mucor is a eukaryotic organism with cell walls made of chitin)
B is incorrect as Mucor does not have cellulose
C is incorrect as Mucor is not a prokaryote
D is incorrect as Mucor is not a prokaryote

(a)(ii) B (1)
A is incorrect as Mucor has cytoplasm
C is incorrect as Mucor does not have chloroplasts or starch
D is incorrect as Mucor does not have chloroplasts or starch

(a)(iii)
• \(3.14 \times 45^2 = 6358.5\)
• to two sig figs = 6400

(b)(i) An answer that makes reference to two of the following:
• asexual produces genetically identical offspring / clones (1)
• asexual has no fertilisation / no gametes (1)
• asexual only has one parent cell (1)
• asexual uses (only) mitosis / asexual does not use meiosis (1)

(b)(ii) An explanation that makes reference to the following:
• sexual reproduction produces (genetic) variation (1)
• some may survive environmental changes / not all killed by disease / are more adaptable / natural selection can occur (1)

Question 2

The diagram shows parts of the human respiratory system with some structures labelled.

(a) (i) Which of these is the structure labelled X?

A) alveolus
B) bronchiole
C) bronchus
D) trachea

(ii) State why a lung is described as an organ.
(iii) Explain how contraction of the diaphragm causes air to move into the lungs.

(b) A scientist compared the rate at which oxygen is used during exercise by a person who is a non-smoker with a person who is a smoker. The rate is measured in cm³ per minute for each kilogram of body mass. The table shows the scientist’s results.

Human	Rate of oxygen use during exercise in cm³ per minute for each kilogram of body mass
non-smoker	35
smoker	27

(i) Calculate the volume of oxygen used in 30 minutes by a non-smoker with a mass of 70 kg.
(ii) Explain the effect of cigarette smoking on the rate at which oxygen is used.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(g): Gas exchange — parts (a)(i), (a)(iii), (b)(ii)
• 2(a): Level of organisation — part (a)(ii)
• 2(f): Respiration — part (b)(i), (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) B (bronchiole)
A is incorrect as it is not an alveolus
C is incorrect as it is not a bronchus
D is incorrect as it is not a trachea

(a)(ii) An answer that makes reference to the following:
• it has more than one tissue / is a group of tissues / is a collection of tissues / made of multiple tissues (1)

(a)(iii) An explanation that makes reference to the following:
• diaphragm moves down / flattens (1)
• (chest / thorax) volume increases (1)
• (chest / thorax) pressure decreases / air flows from high pressure to low pressure / down a pressure gradient (1)

(b)(i) • \(35 \times 70 \times 30 = 73,500\) cm³ (2)
One mark for ×35 or ×70 or ×30 or 2450 or 1050 or 2100
Accept 74,000 or 73,000 for two marks
Correct answer gains both marks

(b)(ii) An explanation that makes reference to three of the following:
1. oxygen use lower in smokers / lower rate of oxygen use (1)
2. tar (in cigarettes) (1)
3. causes emphysema / mucus builds up / mucus not removed / cilia damaged (1)
4. less surface area (of alveoli) (1)
5. less diffusion of oxygen (into blood) (1)
6. carbon monoxide binds to haemoglobin (reducing oxygen) (1)
7. causes narrowing of arteries / blocks blood vessels (reducing flow of oxygenated blood) (1)

Question 3

The diagram shows a human skin cell.

(a) (i) In which of the labelled structures does protein synthesis occur?

A) cell membrane
B) mitochondrion
C) nucleus
D) ribosome

(ii) All cells have cell membranes. Which of the labelled structures are also found in plant cells?

A) mitochondrion and nucleus only
B) mitochondrion and ribosome only
C) mitochondrion, nucleus and ribosome
D) nucleus and ribosome only

(b) The table shows information about the number of mitochondria in different types of human cell.

Type of human cell	Number of mitochondria in one cell	Number of mitochondria in 10 μm³ of cell	Volume of cell in μm³
skin	510	3	1700
villus	2000	8	2500
sperm	75	12

(b) (i) The volume of a villus cell is 2500 μm³. Calculate the ratio of the volume of a villus cell to the volume of a sperm cell. Give your answer in the form n:1
(ii) Using the information in the table, comment on the importance of mitochondria to the activities of different cell types.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(b): Cell structure — parts (a)(i), (a)(ii)
• 2(f): Respiration — part (b)(ii)
• 2(e): Nutrition — part (b)(ii) villus cells
• 3(a): Reproduction — part (b)(ii) sperm cells

▶️ Answer/Explanation

Solution

(a)(i) D (ribosome)
A is incorrect as cell membrane does not synthesise protein
B is incorrect as mitochondria does not synthesise protein
C is incorrect as the nucleus does not synthesise protein

(a)(ii) C (mitochondrion, nucleus and ribosome)
A is incorrect as ribosomes are also present in both
B is incorrect as nuclei are also present in both
D is incorrect as mitochondrion is also present in both

(b)(i) 40:1
Example calculation:
• Number of mitochondria per 10 μm³ for sperm = 12
• Volume of sperm cell = (75 ÷ 12) × 10 = 62.5 μm³
• Ratio = 2500:62.5 = 40:1

(b)(ii) An explanation that makes reference to four of the following:
• villus has highest number of mitochondria per cell / sperm has lowest number of mitochondria per cell
• sperm has highest number of mitochondria per 10 μm³ / skin has lowest number per 10 μm³
• mitochondria perform aerobic respiration / make ATP / release energy
• villus cells do active transport (so need lot of energy)
• sperm cells move / swim (so need lot of energy)
• skin cells have few active processes / use little energy

Question 4

Fruit juice drinks contain a range of nutrients.

The table shows some of the nutrients in a fruit juice drink and the percentage of the recommended daily allowance (RDA) of these nutrients for a 16 year old.

Nutrient	Mass of nutrient in 400 cm³ of drink in g	Percentage of RDA in 400 cm³ of drink
starch	2.8	2
sugar	6.4	4
fat	7.0	10
fibre	9.2	35

(a) (i) Vitamins and minerals are not listed in the table. Name one other component of a balanced diet that is not listed in the table.
(ii) Calculate the RDA of fibre for a 16 year old. Give your answer in grams and to the nearest whole number.
(iii) State the function of fibre in the human diet.

(b) Describe how starch is digested into glucose in the human alimentary canal.

(c) DCPIP is a blue solution that turns colourless when enough vitamin C is added.

A student uses this apparatus to compare the concentrations of vitamin C in different fruit juices.

This is the student’s method.

place 5 cm³ of blue DCPIP solution in a test tube
fill a dropping pipette with fruit juice
add drops of fruit juice to the DCPIP solution
record the number of drops of fruit juice added until the DCPIP solution turns colourless

The table shows the student’s results.

Fruit juice	Number of drops of fruit juice needed to turn DCPIP solution to colourless
apple	18
grape	22
lemon	4
lime	7
orange	3

(c)(i) The graph shows the effect of concentration of vitamin C on the number of drops needed to change DCPIP solution to colourless.

Determine the concentration of vitamin C in lime juice.

(ii) Give the fruit juices in order of increasing concentration of vitamin C.

One has been done for you.

(iii) Explain how the student could modify the experiment to give a more accurate measure of the concentrations of vitamin C in the fruit juices.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(e): Nutrition — parts (a)(i), (a)(ii), (a)(iii), (b)
• 2(e): Digestive enzymes — part (b)
• 5(a): Food production — part (c)
• Practical Investigation: Experimental skills — part (c)(iii)

▶️ Answer/Explanation

Solution

(a)(i) • water / protein (1)
Accept amino acids

(a)(ii) 26 (2)
Example calculation: \( 9.2 \div 35 \times 100 = 26.28571 \)
One mark for correct answer to any number of decimal places e.g. 26.28571…
OR
One mark for \( 9.2 \div 35 \)
OR
One mark for \( 0.26(28571) \)
Correct answer gains both marks

(a)(iii) An answer that makes reference to one of the following: (1)
• helps peristalsis / eq (1)
• prevents constipation / eq (1)
• helps intestines push food / eq (1)
• helps release faeces / eq (1)
• helps egestion / eq (1)
• helps in movement of food / eq (1)
Reject excrete faeces

(b) An answer that makes reference to three of the following: (3)
• amylase (1)
• from salivary glands / from pancreas / eq (1)
Accept (released into / digested in) mouth
Accept (released into / digested in) small intestine / duodenum / ileum
• starch / it, is digested into maltose (1)
Accept broken down / hydrolysed / turned into for digested
• maltase digests maltose (into glucose) (1)

(c)(i) An answer that makes reference to the following: (1)
• 38 (mg per cm³) (1)

(c)(ii) An answer that makes reference to the following order: (1)
• orange lemon (lime) apple grape (1)
Accept numbers from table i.e. 3 4 (lime = 7) 18 22

(c)(iii) An answer that makes reference to two of the following: (2)
• use a syringe / graduated pipette / measuring cylinder / burette / eq (1)
• to measure volume / ml / cm³ / dm³ / mm³ / eq (1)
• as drop sizes vary / masses of drops vary / volumes of drops vary / eq (1)
• mix / swirl / stir (solution after adding drops) / eq (1)
Ignore pipette alone
Accept Automatic pipette
Accept volumetric pipette
Accept balance / scale
Accept use titration
Accept mass / weight (of juice)
Accept idea that method does not control drop size
Ignore references to repeats / replicates

(Total for Question 4 = 11 marks)

Question 5

The diagram shows the human female reproductive system with some structures labelled.

(a) (i) In which structure does ovulation occur?

A) W
B) X
C) Y
D) Z

(ii) Explain the importance of progesterone in the menstrual cycle.
(iii) Explain how the placenta enables a developing embryo to obtain nutrients.

(b) For fertilisation to occur, a sperm needs to reach an ovum.

(i) Which of these describes the number of chromosomes in the nucleus of a human sperm?

A) diploid number of 23 chromosomes
B) diploid number of 46 chromosomes
C) haploid number of 23 chromosomes
D) haploid number of 46 chromosomes

(ii) Scientists are developing a contraceptive drug to reduce fertility. The drug is tested on 25 male mice.

Graph 1 shows the effect that different concentrations of the drug have on the mean concentration of sperm in semen.

Graph 2 shows the effect that different concentrations of the drug have on the percentage of sperm that can move.

The scientists conclude that the drug could be used as a contraceptive to reduce fertility in humans.

Discuss the scientists’ conclusion. Use information in the graphs and your own knowledge to support your answer.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 3(a): Reproduction — parts (a)(i), (a)(ii), (a)(iii), (b)(i)
• 2(j): Co-ordination and response — part (a)(ii) – hormonal control
• 3(b): Inheritance — part (b)(i) – chromosome numbers
• 5(a): Use of biological resources — part (b)(ii) – drug development and testing
• Assessment Objective A02: Application of knowledge and understanding — part (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) B (X)
A is incorrect as ovulation occurs in the ovary
C is incorrect as ovulation occurs in the ovary
D is incorrect as ovulation occurs in the ovary

(a)(ii) An answer that makes reference to two of the following:
• (progesterone is released) after ovulation / in second half of cycle / eq (1)
• maintains uterus lining / maintain endometrium / prevents menstruation / eq (1)
• so embryo can implant in the lining / to support an embryo / to support fetus / eq (1)

(a)(iii) An answer that makes reference to three of following:
• villi / villus (1)
• large surface (area) (1)
• (blood / capillaries) maintains gradient / makes steep gradient / eq (1)
• thin / short distance / maternal and fetal blood are close / eq (1)
• diffusion / active transport (1)

(b)(i) C (haploid number of 23 chromosomes)
A is incorrect as sperm are not diploid
B is incorrect as sperm are not diploid
D is incorrect as sperm have 23 chromosomes

(b)(ii) An answer that makes reference to five of the following:
• sperm movement decreases with concentration / eq (1)
• no further effect between 0.4 and 0.8 / levels off after 0.4 / eq (1)
• no effect on sperm concentration / small reduction / slight effect / unclear effect / fluctuates / eq (1)
• fertilisation would not occur (if sperm cannot swim) / eq (1)
• (sperm) needs to swim to oviducts / Fallopian tubes / fertilisation occurs in oviducts / Fallopian tubes / eq (1)
• (may not work as) some sperm can (always) move / reach eggs / reach oviducts (with any concentration) / eq (1)
• sample size is small / no repeats (so not reliable) / eq (1)
• may not have same effect in humans / may have side effects / health impacts / humans would needs bigger doses / eq (1)
• no mention of age of mice / eq (1)

Question 6

A student uses this apparatus to investigate osmosis in a non-living system.

This is the student’s method:

tie partially permeable tubing at one end with string to make a bag
fill the bag with sugar solution
tie the bag at the other end with string
measure the mass of the bag, string, and sugar solution
place the bag into a test tube filled with pure water
after 5 minutes remove the bag and measure the mass
return the bag to the test tube
repeat the measurement of mass every 5 minutes for 30 minutes in total

The table shows the student’s results.

Time in minutes	Mass of bag in g
0	15
5	17
10	20
15	23
20	26
25	27
30	27

(a) (i) Draw a line graph to show the student’s results. Join your points with straight lines.
(ii) Describe the change in mass of the bag during the 30 minutes.
(iii) Explain the change in mass of the bag during the 30 minutes.

(b) The student decides to investigate how increasing the temperature would affect the results.
(i) Describe how the student could repeat the experiment safely at a higher temperature.
(ii) Explain how repeating the experiment at a higher temperature would affect the results.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(d): Movement of substances into and out of cells — parts (a)(iii), (b)(ii)
• 2(d): Practical: investigate diffusion and osmosis using living and non-living systems — parts (a)(i), (a)(ii), (b)(i)
• 2(d): Understand how factors affect the rate of movement of substances into and out of cells — part (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) Graph requirements:

Linear scale for both axes with at least two large squares on y-axis and at least half of x-axis
Straight lines between points
Axes with time on horizontal and mass on vertical
Plots correct (± half square)
Axes labelled and with units (minimum: mass/g and time/min)

(a)(ii) A description that makes reference to the following:

Mass increases (1)
Levels off after 25 minutes / at 27 g / eq (1)

(a)(iii) An explanation that makes reference to two of the following:

Water moved into bag (by osmosis/diffusion) / eq (1)
(Water moves) from a high water potential to a low water potential / from high concentration (of water) to low concentration (of water) / from dilute to concentrated / eq (1)
Until pressure prevented more water entering / bag was full / bag was turgid / eq (1)

(b)(i) A description that makes reference to two of the following:

Use a water bath (1)
With a thermostat / use a (Bunsen) burner and thermometer / eq (1)
Eye protection / eq / use tongs / gloves / eq (1)

(b)(ii) An explanation that makes reference to three of the following:

Osmosis is faster / eq (1)
Tubing fills faster / mass increases faster / so increase in mass will be steeper / will level off earlier / eq (1)
More (kinetic) energy (1)
So particles / water, will move faster / eq (1)

Question 7

Limpets are marine animals that attach to rocks on coastal shores.
The diagram shows a food web that contains limpets.

(a) (i) Which are the trophic levels of the starfish in this food web?

A) primary consumer and secondary consumer
B) primary consumer and tertiary consumer
C) secondary consumer and tertiary consumer
D) secondary consumer only

(ii) State what is meant by the term population.
(iii) Explain why only 1% of the energy in the algae is transferred to the starfish.

(b) Some scientists think that food webs on rocky shores are at risk from an increase in greenhouse gases.

The scientists record these two factors during August every year for 17 years:

the number of limpets per m² on a rocky shore
the mean temperature at the same rocky shore

The graph shows the scientists’ results.

(i) Describe how the scientists could determine the number of limpets on the rocky shore.
(ii) Give one factor the scientists should control when comparing the number of limpets per m² each year.
(iii) The scientists conclude that the combustion of fossil fuels is a risk to the food web. Discuss the scientists’ conclusion.
Use the information in the graph and your own knowledge to support your answer.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(b): Feeding relationships — parts (a)(i), (a)(iii)
• 4(a): The organism in the environment — parts (a)(ii), (b)(i), (b)(ii)
• 4(d): Human influences on the environment — part (b)(iii)
• 2(f): Respiration — part (a)(iii)
• 4(c): Cycles within ecosystems — part (b)(iii)

▶️ Answer/Explanation

Solution

(a)(i) C (secondary consumer and tertiary consumer)
A is incorrect as it is not a primary consumer
B is incorrect as it is not a primary consumer
D is incorrect as it is also a tertiary consumer

(a)(ii) • all of (the organisms of) one / a species in an area (at one time)

(a)(iii) An explanation that makes reference to three of the following:
• (energy lost) from movement
• (energy lost) from respiration / heat loss
• (energy lost) as not some not eaten / are inedible
• (energy lost) as some not digested / indigestible / faeces / egestion
• (energy loss) due to death and decay / (energy lost) to other food chains

(b)(i) A description that makes reference to three of the following:
• quadrat / square in gridded area
• place randomly / choose random coordinates
• repeats
• count limpets (in quadrat / square)
• scale up for whole area multiply by area

(b)(ii) • time of day / location / area / position on shore / species of limpet / same day in August

(b)(iii) An answer that makes reference to five of the following:
1. fossil fuel use releases carbon dioxide
2. carbon dioxide is a greenhouse gas / may cause global warming
3. increased temperature reduces limpet number
4. less food / energy for starfish / dogwhelks / gulls
5. dogwhelks / gulls / starfish / secondary consumers would decrease
6. algae would increase / less algae eaten / more food for periwinkles
7. there are anomalies / outliers / not all close to line of best fit
8. other factors may affect limpet population / other greenhouse gases may have effect
9. not done for long enough time so less reliable OR done for long time / 17 years so reliable
10. data only for one month / only August / only one location / only one shore / only looks at one species

Question 8

Golden rice is an example of a genetically modified plant. Golden rice has been given a gene to make a substance called carotene.
This gene was taken from daffodil plants.

(a)(i) State the term for a genetically modified organism that has been given a gene from a different species.
(ii) Describe how named enzymes are used to transfer a gene from one species to another species.

(b) Carotene is converted into vitamin A after being consumed by humans. Non-genetically modified rice does not usually contain carotene.

(i) Vitamin A deficiency is common in some countries. Explain why growing golden rice may benefit people in these countries.
(ii) Suggest why some people may not agree with growing genetically modified crops such as golden rice.

(c) Chemical pesticides can be used in rice fields to kill insect pests. Scientists investigate the effect that spraying pesticides in a rice field has on three types of organism.

The three types of organism investigated are:

insect pests
carnivorous beetles (predators of insect pests)
lizards (higher trophic level predators)

The scientists count the number of each type of organism in the rice field.
The scientists then spray the field with pesticide.
The number of each type of organism in the field are then counted every three months for a year.
The table shows the scientists’ results:

(i) Comment on the effects on the numbers of different organisms of spraying pesticides in a rice field.
(ii) State two reasons why using biological control is better for the environment than using chemical pesticide.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 5(c): Genetic modification (genetic engineering) — parts (a)(i), (a)(ii), (b)(i), (b)(ii)
• 5(a): Food production — parts (b)(i), (c)(ii)
• 4(b): Feeding relationships — part (c)(i)
• 4(d): Human influences on the environment — part (c)(i)

▶️ Answer/Explanation

Solution

(a)(i) • transgenic (1)

(a)(ii) A description that makes reference to three of the following:

plasmid (is used) (1)
restriction enzyme cuts gene / DNA / plasmid / eq (1)
ligase joins plasmid and gene / joins DNA / eq (1)
forming a recombinant plasmid / forming recombinant DNA / eq (1)

(b)(i) An answer that makes reference to two of the following:

less night blindness / fewer eye problems / better vision / eq (1)
rice is a common food / part of many diets / common crop / cheap (for consumers) to buy / eq (1)
cheap to grow / easy to grow / easy to get large yields / eq (1)
less health care needed / less damage to economy / eq (1)

(b)(ii) An answer that makes reference to two of the following:

(may grow in wild and) affect food chains / food webs / ecosystems / eq (1)
may compete with other species (in wild) / eq (1)
may interbreed with wild plants / genes may transfer into wild plants / eq (1)
people may think they are a risk to health / eq (1)
may be better to give people a more balanced diet / should use natural sources of carotene / eq (1)

(c)(i) An explanation that makes reference to four of the following:

all species decrease / eq (1)
insects increase after 3 months / from 6 months / eq (1)
beetles increase after 9 months / from 12 months / eq (1)
lizard numbers do not recover / stay low / do not increase / level off after 9 months / eq (1)
pesticide washes away / breaks down / eq (1)
pesticide passes along food chain to beetles / to lizards / eq (1)
insects become resistant / eq (1)
insects increase as there are fewer beetles eating them / beetles decrease as few insects to eat / lizards decrease as few beetles to eat / eq (1)
insects have short life cycles / lizards have long life cycle / eq (1)

(c)(ii) An answer that makes reference to two of the following:

(biological is) more specific / does not kill other species / eq (1)
(biological is) not toxic to humans / no residue left on plants / crops / eq (1)
(biological is) no development of resistance / eq (1)
(biological) lasts longer / no need to keep reapplying / eq (1)
no bioaccumulation / no biomagnification / does not pass along food chains / eq (1)

Question 9

The ability of humans to taste a chemical called PTC is controlled by a single gene.

The ability to taste PTC is coded for by a dominant allele, T. Not being able to taste PTC is coded for by a recessive allele, t.

(a) State what is meant by the term recessive allele.

(b) The pedigree diagram shows the individuals in a family who can taste PTC and those who cannot taste PTC.

(i) What are the genotypes of individuals 1 and 2?

A) TT and TT
B) Tt and TT
C) tt and Tt
D) tt and tt

(ii) Determine the probability of individuals 3 and 4 having another child that can taste PTC and is male. Use a genetic diagram to show your working.

(c) People who can taste PTC say that it tastes bitter. Many poisonous plants also taste bitter.

(i) Explain why there are many people who can taste PTC in areas where there are many poisonous plants. Refer to natural selection in your answer.
(ii) People who can taste PTC often find the taste of vegetables very bitter. Suggest why the recessive allele for not tasting PTC still exists in human populations.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 3(b): Inheritance — parts (a), (b)(i), (b)(ii)
• 3(b): Natural selection and evolution — part (c)(i)
• 3(b): Variation and mutation — part (c)(ii)

▶️ Answer/Explanation

Solution

(a) An answer that makes reference to one of the following:

only has an effect if two alleles are present (1)
no effect if dominant allele is present (1)
only has effect if dominant allele not present (1)
no effect in heterozygous (1)
only has an effect when homozygous / eq (1)

(b)(i) C (tt and Tt)

A is incorrect as both would be tasters
B is incorrect as both would be tasters
D is incorrect as neither would be tasters

(b)(ii) An explanation that makes reference to the following:

correct parental genotypes (Tt and Tt) (1)
correct gametes (T or t and T or t) (1)
offspring: TT, Tt, Tt, tt (1)
0.375 / 3/8 / 37.5 (%) (1)

Genetic diagram:

Parents: Tt × Tt

Gametes: T, t and T, t

Offspring:

	T	t
T	TT	Tt
t	Tt	tt

Probability of heterozygous (Tt) = 2/4 = 0.5 or 50%

(c)(i) An explanation that makes reference to four of the following:

mutation occurs (1)
variation (in population) (1)
people do not eat poisonous plants / can detect poisonous plants / eq (1)
survive / eq (1)
reproduce / produced offspring / breed / eq (1)
pass on allele (to offspring) / eq (1)

(c)(ii) An answer that makes reference to two of the following:

people who taste PTC eat fewer vegetables / do not like taste of vegetables / eq (1)
(PTC tasters) get fewer vitamins / fewer minerals / less fibre / eq (1)
(PTC tasters) may develop scurvy / night blindness / constipation / eq (1)
heterozygotes carry a recessive allele / tasters may carry a non-tasting allele / heterozygotes can pass on a recessive allele / eq (1)
some humans lived in areas with few poisonous plants / had diets with fewer plants / had mainly meat-based diets / eq (1)

Question 10

Fly larvae will continuously move around the edge of a Petri dish when placed under a lamp.

The diagram shows a fly larva in a Petri dish.

Design an investigation to find out if changing the colour of light from the lamp affects the speed that the larvae move.

Include experimental details in your answer and write in full sentences.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(j): Co-ordination and response — investigating response to light stimulus
• 3: Assessment information: Experimental skills — designing investigations and practical skills
• Appendix 5: Suggested practical investigations — investigating responses to external stimuli

▶️ Answer/Explanation

Solution

An answer that makes reference to the following:

• C – change colour of light / wavelength of light / use different filters (1)

• O – same species / type / age / size / food (1) Ignore same larva unqualified

• R – use several larvae with each light colour / repeat (1) Ignore repeat with different colours

• M1 – measure distance larvae move / measure arc of dish / number of circuits (1) Accept measure how long it takes to move set distance for M1 and M2

• M2 – time / stated time (1) Accept time taken to do a number of laps for two marks (M1 and M2)

• S1 & S2 (two from) – same humidity / food / light intensity / temperature / of dish / surface of Petri / same dish (2) Accept clean dish between tests

Full sentence example: “I would change the colour of light using different filters while keeping the same species, age, and size of larvae, using several larvae for each colour and repeating the experiment. I would measure the distance moved along the Petri dish edge and time this movement, while controlling variables like temperature, humidity, and light intensity.”

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