Free Monohybrid inheritance questions for IGCSE Biology (0610), part of Inheritance & variation. Each question has the answer and the marking points an examiner would look for.
Generate a full Monohybrid Inheritance worksheet →All Inheritance & variation questionsIn seed shape in pea plants, round (R) is dominant to wrinkled (r). Two plants with genotypes Rr and rr are crossed. What percentage of the offspring are expected to be wrinkled?
Answer: C
Haemophilia is caused by a recessive allele on the X chromosome.
Explain why haemophilia is more common in males than in females.
| Part | Answer and marking points | Marks |
|---|---|---|
| allele on X; males have one X the allele is on the X chromosome B1 males have only one X so a single recessive allele causes the condition B1 | 2 |
In mice, the allele for brown fur (B) is dominant to the allele for white fur (b).
A brown individual of unknown genotype is crossed with a white individual. Half of the offspring have white fur.
State the genotypes of the two parents.
Draw a genetic diagram to explain the results of this cross.
Two of the brown offspring are crossed with each other. Calculate the probability that their first offspring is white. Show your working.
Explain why the observed ratio in a real cross may differ from the expected ratio.
In humans, the ABO blood groups are controlled by three alleles, IA, IB and Io. IA and IB are codominant and Io is recessive to both. A man of blood group A and a woman of blood group B have a child of blood group O. State the genotypes of all three people.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | Bb and bb Bb B1 bb B1 | 2 |
| (b) | gametes B, b and b; offspring Bb : bb = 1 : 1 parental genotypes B1 gametes correct and circled B1 offspring genotypes B1 phenotypes with 1 : 1 ratio B1 | 4 |
| (c) | 0.25 (1 in 4) Bb × Bb shown M1 0.25 / ¼ / 25% A1 | 2 |
| (d) | Fertilisation is random; small sample size random fusion of gametes / chance B1 small number of offspring B1 | 2 |
| (e) | man IAIo; woman IBIo; child IoIo IAIo B1 IBIo B1 IoIo B1 | 3 |
This question is about the inheritance of fur colour in mice. The allele for black, B, is dominant to the allele for grey, b.
State what is meant by the term allele.
State the genotype of a heterozygous organism and give its phenotype.
Two heterozygous organisms are crossed. Use a genetic diagram to show the possible genotypes of the offspring, and state the expected ratio of phenotypes.
Describe how a test cross can be used to find out whether a black organism is homozygous or heterozygous. Explain how the results are interpreted.
Explain why the observed ratio of offspring in a cross is often different from the expected ratio.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | alternative form of a gene an alternative form of a gene B1 | 1 |
| (b) | Bb; black Bb B1 black B1 | 2 |
| (c) | Bb × Bb; gametes B, b; BB, Bb, Bb, bb; 3 black : 1 grey parental genotypes Bb × Bb B1 gametes B and b from each B1 offspring BB, Bb, Bb, bb B1 3 black : 1 grey B1 | 4 |
| (d) | cross with bb; all black → homozygous; some grey → heterozygous cross the organism with a grey (homozygous recessive, bb) organism B1 if all offspring are black, the organism is homozygous BB B1 if any (about half) are grey, it is heterozygous Bb B1 because the grey offspring must have received b from both parents B1 | 4 |
| (e) | fertilisation is random; small sample size fertilisation is random B1 the number of offspring is small B1 | 2 |
This question is about inheritance of purple and white flowers. The allele for purple (P) is dominant to the allele for white (p).
Define the terms allele and homozygous.
Two heterozygous purple parents are crossed. Draw a genetic diagram to show the genotypes and phenotypes of the offspring, and state the expected ratio.
The cross produced 400 offspring. Calculate the expected number of white offspring.
Describe how a test cross could be used to find out whether a purple individual is homozygous or heterozygous.
Explain why the observed ratio of offspring is often different from the expected ratio.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | allele: alternative form of a gene; homozygous: two identical alleles allele: an alternative form of a gene B1 homozygous: having two identical alleles of a gene B1 | 2 |
| (b) | Pp × Pp; PP, 2 Pp, pp; 3 purple : 1 white parental genotypes Pp × Pp B1 gametes P and p from each B1 offspring genotypes PP, Pp, Pp, pp B1 phenotypic ratio 3 purple : 1 white B1 | 4 |
| (c) | 100 400 × ¼ = 100 B1 | 1 |
| (d) | cross with homozygous recessive white; all purple → homozygous; some white → heterozygous cross with a homozygous recessive (pp, white) individual B1 if all offspring are purple the parent is homozygous B1 if about half are white it is heterozygous B1 | 3 |
| (e) | fertilisation is random; small sample fertilisation is random / probabilities are only close with large numbers of offspring B1 | 1 |
Some genetic conditions are caused by recessive alleles.
Two parents without the condition have a child with it. Explain how this is possible.
State the probability that their next child has the condition.
Define homozygous.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | both carriers Aa both parents are heterozygous (carriers, Aa) B1 each passes on a recessive allele B1 the child is homozygous recessive (aa) B1 | 3 |
| (b) | 25 % 0.25 (1 in 4) B1 | 1 |
| (c) | two identical alleles having two identical alleles of a gene B1 | 1 |
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