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CIE iGCSE Co-Ordinated Science B16.3 Monohybrid inheritance Exam Style Questions Paper 1

CIE iGCSE Co-Ordinated Science Paper 1 - All Chapters

Question

Two parents with genotypes of BB and Bb are crossed.

What is the predicted ratio of genotypes in the offspring?

A. 1 BB : 1 Bb
B. 1 Bb : 1 bb
C. 3 BB : 1 Bb
D. 3 Bb : 1 BB

▶️ Answer/Explanation
The BB parent produces only B gametes, while the Bb parent produces B and b gametes in equal proportions. When crossed, the possible offspring genotypes are BB and Bb in a 1:1 ratio. There are no bb offspring because the BB parent cannot contribute a b allele.
✅ Answer: (A)

Question

What is the correct term for an allele that is always expressed in the phenotype if it is present?

A. dominant
B. heterozygous
C. homozygous
D. recessive

▶️ Answer/Explanation
A dominant allele is always expressed in the phenotype whenever it is present, regardless of whether the organism is homozygous or heterozygous for that allele.
In contrast, a recessive allele is only expressed when two copies are present (homozygous recessive); heterozygous and homozygous describe the genotype, not the dominance of an allele.
✅ Answer: (A)

Question

Albinism is a genetic condition which results in an absence of pigment in the skin, hair and eyes.
If two black mice produce an albino offspring, which Punnett square shows the correct cross?

▶️ Answer/Explanation
Since albinism is recessive (let \(b\) = albino allele, \(B\) = black allele), both black parents must carry one hidden \(b\) allele — making them both \(Bb\); the cross \(Bb \times Bb\) produces offspring in the ratio \(BB : Bb : bb = 1:2:1\), giving a \(\frac{1}{4}\) chance of albino (\(bb\)).
Only a heterozygous cross (\(Bb \times Bb\)) can produce an albino offspring from two visibly black parents, which corresponds to Punnett square B in the diagram.
✅ Answer: (B)

Question

In a plant, the allele for red flowers is dominant to the allele for yellow flowers. A heterozygous, red-flowered plant is crossed with a homozygous, yellow-flowered plant.

Which statement about the offspring is correct?

A. 25% will have red flowers and 75% will have yellow flowers.
B. 50% will have red flowers and 50% will have yellow flowers.
C. 75% will have red flowers and 25% will have yellow flowers.
D. 100% will have red flowers and 0% will have yellow flowers.

▶️ Answer/Explanation
Let \(R\) = red (dominant) and \(r\) = yellow (recessive).
The heterozygous red parent is \(Rr\) and the homozygous yellow parent is \(rr\).
The cross \(Rr \times rr\) gives offspring genotypes \(Rr, Rr, rr, rr\), a 1:1 ratio.
This means 50% of offspring are red (\(Rr\)) and 50% are yellow (\(rr\)).
✅ Answer: (B)

Question

Which statement describes a heterozygous genotype?

A. not pure breeding with two different alleles
B. not pure breeding with two identical alleles
C. pure breeding with two different alleles
D. pure breeding with two identical alleles

▶️ Answer/Explanation
A heterozygous genotype has two different alleles for a gene, for example Aa.
Because the alleles differ, the organism is not pure breeding, as it can pass on either allele.
This is different from a homozygous genotype, which has two identical alleles and is pure breeding.
✅ Answer: (A)
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