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CIE iGCSE Co-Ordinated Science B16.1 Chromosomes and genes Exam Style Questions Paper 3

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Question

(a) (i) Complete the sentences using words from the list. Each word may be used once, more than once or not at all.
chromosomes      cilia      codes      ribosomes      secretes      targets      walls
A gene is a length of DNA that …………………. for a protein.
Proteins are made in the cell by structures called …………………. .
Genes are found on …………………. in the cell’s nucleus.
(a) (ii) Fig. 3.1 shows a person with dimples in their cheeks. Dimples are inherited.
Dimples are produced when a person has the dominant allele in their genotype.
The dominant allele for dimples is D. The allele for not having dimples is d.
Two people with dimples have a child without dimples. They are expecting a second child.
Predict the probability of the second child having dimples. Complete the genetic diagram to explain your answer.
(b) Fig. 3.2 shows some Columbia sheep.
Columbia sheep are a type of sheep that are selectively bred to have very thick wool.
Describe the process of selective breeding by artificial selection to produce Columbia sheep with very thick wool.

Most-appropriate topic codes:

• Topic B16.1 — Chromosomes and genes (Part (a)(i))
• Topic B16.3 — Monohybrid inheritance (Part (a)(ii))
• Topic B17.2 — Selection (Part (b))

▶️ Answer/Explanation

(a)(i) A gene is a length of DNA that codes for a protein. Proteins are made in the cell by structures called ribosomes. Genes are found on chromosomes in the cell’s nucleus.

A gene is a sequence of DNA that contains the genetic code to build a specific protein. This process occurs at the ribosomes, which are the protein-synthesizing organelles. Chromosomes are thread-like structures made of DNA and protein that house the genes and are located in the nucleus.

(a)(ii) Probability of the second child having dimples = 75% (or 3 in 4).

To determine the genotype of the parents: Both parents have dimples, meaning they must each have at least one dominant D allele. Since they have a child without dimples, who must have the genotype dd (recessive), both parents must be heterozygous (Dd).

The Punnett square is completed as follows:

From this cross, the possible genotypes are: DD, Dd, Dd, and dd. The phenotypes for DD and Dd would be having dimples, while dd would be without dimples. Therefore, 3 out of 4 offspring are predicted to have dimples, giving a probability of 75%.

(b) Select sheep with the desired trait (thick wool) and breed them together. From the offspring, select those with the thickest wool and repeat this process over many generations.

Selective breeding is a process used by humans to develop organisms with desirable characteristics. The steps are:

1. Choose parents that show the desired characteristic (thick wool).
2. Breed these selected individuals together.
3. From the offspring, select the ones that show the desired characteristic to the greatest degree.
4. Repeat this selection and breeding process over several generations to concentrate the desirable genes and produce a breed like the Columbia sheep.

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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