Practise Genetic inheritance. 16 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Some characteristics are controlled by a single gene. What is meant by the term heterozygous?
Having two different alleles of a gene
In mice, the allele for black fur (B) is dominant to the allele for brown fur (b). A heterozygous black mouse was crossed with a brown mouse. Give the genotype of each parent.
Black parent Bb; brown parent bb
What is the probability that an offspring plant will have yellow fruit?
0.25 (1 in 4, 25%)
Genetic terms are used to describe alleles and characteristics. What does the term ‘homozygous’ mean?
Having two identical alleles of a gene (e.g. BB or bb)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Some characteristics are controlled by a single gene.
(a) What is an allele? Tick (✓) one box.[1]
A different form of a gene
A section of a chromosome made of protein
A sex cell
The physical appearance of an organism
(b) What is meant by the term heterozygous?[1]
(c) What is meant by the term phenotype?[1]
(d) Which genotype is homozygous recessive? Tick (✓) one box.[1]
BB
Bb
bb
B
(e) A dominant allele is always expressed, even if only one copy is present. When is a recessive allele expressed?[1]
Show the answer and mark scheme
(a)Answer: A different form of a gene
(b)Answer: Having two different alleles of a gene
two different alleles (of a gene)
(c)Answer: The characteristic that is expressed (the physical appearance)
the characteristic(s) expressed / physical appearance (produced by the alleles)
(d)Answer: bb
(e)Answer: Only when two copies are present (no dominant allele)
only when two copies are present / when there is no dominant allele
Question 2Medium8 marks
In tomato plants, the allele for red fruit (R) is dominant to the allele for yellow fruit (r). Two tomato plants that are both heterozygous for fruit colour were crossed.
(a) Draw a Punnett square to show this cross. Identify the phenotype of each offspring genotype.[3]
(b) What is the probability that an offspring plant will have yellow fruit?[1]
(c) Seeds from this cross grew into 480 plants. Calculate the number of plants expected to have red fruit.[2]
(d) What fraction of the plants with red fruit would be expected to be heterozygous?[2]
Show the answer and mark scheme
(a)Answer: Gametes R and r from each parent; offspring RR, Rr, Rr, rr; RR and Rr red, rr yellow
gametes R and r from each parent
offspring genotypes RR, Rr, Rr, rr
RR and Rr have red fruit; rr has yellow fruit
(b)Answer: 0.25 (1 in 4, 25%)
0.25 / ¼ / 25% / 1 in 4
(c)Answer: 360
480 × ¾ / 480 × 0.75
360
(d)Answer: \(\frac{2}{3}\)
2 of the 3 red genotypes (RR, Rr, Rr) are heterozygous
⅔ / 0.67 / 67%
Question 3Hard8 marks
The kernels (seeds) on a maize cob can be purple or yellow. Kernel colour is controlled by a single gene. The allele for purple (P) is dominant to the allele for yellow (p). A farmer crossed two maize plants and counted the kernels on the cobs produced: there were 1188 purple kernels and 404 yellow kernels.
(a) Calculate the ratio of purple kernels to yellow kernels. Give your answer in the form x : 1, to 2 significant figures.[2]
(b) Deduce the genotypes of the two parent plants. Use a genetic diagram to explain your answer.[3]
(c) Suggest why the ratio is not exactly the same as the expected ratio.[1]
(d) The height of maize plants cannot be predicted using a Punnett square. Suggest two reasons why.[2]
Show the answer and mark scheme
(a)Answer: 2.9 : 1
1188 ÷ 404
2.9 (: 1)
(b)Answer: Both Pp: Pp × Pp gives PP, Pp, Pp, pp, which is 3 purple : 1 yellow, close to the observed 2.9 : 1
both parents Pp
genetic diagram showing gametes P and p from each parent, offspring PP, Pp, Pp, pp
expected ratio 3 purple : 1 yellow, which is close to the ratio observed
(c)Answer: Fertilisation is random, so ratios are only probabilities
fertilisation is random / it is a matter of chance which gametes fuse
(d)Answer: Height is controlled by many genes interacting, not a single gene; height is also affected by the environment, e.g. light, water and mineral ions
height is controlled by multiple genes / not a single gene
height is also affected by the environment, e.g. light / water / mineral ions