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CIE iGCSE Co-Ordinated Science B17.1 Variation Exam Style Questions Paper 3

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Question

Pea plants produce seeds. The seeds are either green or yellow.
(a) Complete the sentences about pea plants. Use words from the list:
asexual        continuous        discontinuous        genotypes        mutations        phenotypes
The colour of pea seeds is an example of              variation. This is because there are only two              with no intermediates.
(b) Fig. 2.1 is a diagram of a flower from a pea plant.
(i) Using letters A–F in Fig. 2.1, identify the:
    ovary 
    sepal
(ii) Using letters A–F in Fig. 2.1, identify the parts of the flower where pollen grains are transferred during pollination.
(c) A scientist investigates the inheritance of colour in pea seeds. The inheritance of colour is controlled by a single gene.
  • The allele for yellow pea seeds is Y.
  • The allele for green pea seeds is y.
All of the scientist’s peas are yellow.
(i) Table 2.1 shows the different possible combinations of alleles, a description of the combination and the colour of the pea seeds. Complete Table 2.1.
(ii) The scientist uses plants with yellow pea seeds to produce plants with green pea seeds.
Complete the Punnett square in Fig. 2.2 to show how plants with yellow pea seeds produce plants with green pea seeds.
(iii) State the expected ratio of seed colours from this cross.
yellow pea seeds        : green pea seeds       

Topic codes:

• Topic B16.3 — Monohybrid inheritance (All parts)
• Topic B17.1 — Variation (Part (a))

▶️ Answer/Explanation

(a) discontinuous ; phenotypes
Seed colour in pea plants shows discontinuous variation because there are only two distinct categories (green or yellow) with no intermediate forms. Discontinuous variation is controlled by genes and results in a limited number of phenotypes.

(b)(i) ovary = E ; sepal = F
The ovary is the swollen base of the carpel containing ovules. The sepal is the small green leaf-like structure at the base of the flower that protects the flower bud.

(b)(ii) from C (anther) to A (stigma)
Pollen grains are transferred from the anther (part C) to the stigma (part A) during pollination. The anther produces pollen, while the stigma is the receptive surface where pollen grains land and germinate.

(c)(i) Table completed:

YY is homozygous dominant because both alleles are dominant (Y). Yy is heterozygous because the alleles are different (one dominant Y and one recessive y).

(c)(ii) Punnett square completed:

Both parent plants are heterozygous (Yy). Their gametes each contain either Y or y. The offspring genotypes are YY (yellow), Yy (yellow), Yy (yellow), and yy (green).

(c)(iii) yellow : green = 3 : 1
From the Punnett square, 3 out of 4 offspring have the dominant allele Y (YY or Yy) and are yellow. 1 out of 4 offspring is homozygous recessive (yy) and is green. The expected phenotypic ratio is therefore 3 yellow : 1 green.

Question

(a) Scientists record the mass of each baby at birth in one country.
The results are shown in Fig. 10.1
(i) Complete the sentences about the mass of babies at birth as shown in Fig. 10.1.
The lowest mass at birth was …………………. g.
The most frequent mass at birth was ………………. g.
Fig. 10.1 shows that mass of babies at birth is an example of continuous variation. This is because there is a …………………. of phenotypes between two extremes.
A phenotype is defined as the …………………. features of an organism.
(ii) State one other example of continuous variation in humans.
(b) The statements A–E describe stages of natural selection. They are not in the correct order.
A There is competition for resources.
B Those with adaptations best suited to their environment survive and reproduce.
C This process is repeated over several generations.
D There is variation in a population.
E Alleles are passed on to the next generation.
Complete the answer spaces to show the correct order. One has been done for you.
……. → A → ……. → ……. → …….
(c) Alleles are made of DNA.
(i) Define the term allele.
(ii) State where alleles are found in a cell.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B17.1 — Variation (Part (a))
• Topic B17 — Variation and selection / natural selection (Part (b))
• Topic B16.1/B16.3 — Chromosomes, genes and alleles (Part (c))

▶️ Answer/Explanation

(a)(i) lowest mass = 800 g; most frequent mass = 2400 g; range; observable

Reading the graph, the curve starts at approximately 800 g (lowest value plotted).
The peak of the curve (highest frequency) occurs at approximately 2400 g.
Continuous variation shows a full range of values between two extremes, with no distinct categories.
A phenotype refers to the observable features of an organism.

(a)(ii) Height (or another example such as foot size, hand span)

Height in humans varies continuously across a population, with no fixed discrete categories — making it a valid example.

(b) D → A → B → C → E

There must first be variation (D) in a population for competition (A) for resources to matter.
Those best adapted survive and reproduce (B), and this is repeated over generations (C), with alleles passed on (E).

(c)(i) An allele is an alternative form of a gene

Different alleles of the same gene can produce different versions of a characteristic (e.g., different alleles for eye colour).

(c)(ii) In the nucleus

Alleles are sections of DNA found on chromosomes, which are located in the nucleus of the cell.

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