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.
The language of genetics and how to use Punnett squares and family trees to predict the outcome of a single-gene cross. It comes up on almost every paper: definitions, Punnett squares, and probability or ratio questions.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define dominant and recessive allelesA dominant allele is expressed even with one copy; a recessive allele is expressed only with two copies.
4
Use genotype, phenotype, homozygous and heterozygous correctlyGenotype is the alleles present, phenotype is the characteristic; homozygous means the same alleles, heterozygous different.
5
Complete a Punnett squarePut each parent's gametes on the outside and combine one allele from each in every box.
5
Give cross outcomes as ratios or probabilitiesFor example 3 : 1, or a probability of 0.25, 25%, ¼ or 1 in 4.
6
Extract information from family treesUse the key and the phenotypes of parents and children to work out genotypes.
7
Construct a genetic cross to make predictionsFrom the information given, work out the genotypes and gametes, draw the Punnett square and give probabilities.
8
Deduce genotypes from family tree evidenceFor example, two unaffected parents with an affected child must both be heterozygous.
Notes
Key words
Gamete: a sex cell (e.g. sperm or egg) with a single set of chromosomes.
Chromosome: a structure in the nucleus that contains DNA. Gene: a small section of DNA on a chromosome.
Allele: a different form of the same gene.
Dominant allele: always expressed, even if only one copy is present (written as a capital letter, e.g. B).
Recessive allele: only expressed if two copies are present, so there is no dominant allele (lower case, e.g. b).
Homozygous: the two alleles are the same (BB or bb). Heterozygous: the two alleles are different (Bb).
Genotype: the alleles present (e.g. Bb). Phenotype: the characteristic that is expressed (e.g. black fur).
Single genes and many genes
Some characteristics are controlled by a single gene, e.g. fur colour in mice and red-green colour blindness in humans.
The alleles present (genotype) work at a molecular level to develop the characteristics that are expressed (phenotype).
Most characteristics are the result of multiple genes interacting, not a single gene.
Punnett squares
Split each parent's genotype into its gametes: Bb makes B gametes and b gametes.
Put one parent's gametes across the top and the other's down the side, then fill each box with one allele from each parent.
Each box is an equally likely outcome, so each of the four boxes has a probability of 0.25.
Bb × Bb gives BB : Bb : bb in the ratio 1 : 2 : 1, so the dominant : recessive phenotype ratio is 3 : 1.
Bb × bb gives Bb : bb in the ratio 1 : 1.
The probability is the same for every offspring, because each fertilisation is a separate random event. Real families are small, so actual numbers often differ from the predicted ratio. grade 7+
Family trees
Squares usually show males and circles females; shaded symbols show the characteristic. Always read the key.
If two parents without a characteristic have a child with it, the allele is recessive and both parents must be heterozygous. grade 7+
Cheatsheet
Allele = a different form of a gene
Dominant: expressed with one copy. Recessive: expressed only with two copies
Homozygous = two alleles the same (BB, bb). Heterozygous = two different alleles (Bb)
BB × anything → all offspring show the dominant phenotype
One box out of four = probability 0.25 = 25% = ¼ = 1 in 4
How to answer each type of question
Use the genetic terms
1 to 3 marks4
Learn precise definitions, especially dominant, recessive, homozygous and heterozygous.
If asked for a genotype, give letters; if asked for a phenotype, describe the characteristic in words.
Example. In mice, the allele for black fur (B) is dominant to the allele for brown fur (b). (a) Give the genotype of a mouse that is homozygous recessive. (b) Give the phenotype of a mouse with the genotype Bb. (c) What does heterozygous mean? [3 marks]
Show the model answer
(a) bb (1) (b) Black fur (1) (c) Having two different alleles of a gene (1)
Complete a Punnett square and give a probability
2 to 3 marks6
Write the gametes of each parent on the outside.
Fill in each box with one allele from each parent.
Count the boxes that give the phenotype asked for, out of 4.
Give the answer in the form asked: fraction, decimal, percentage or ratio.
Example. Two heterozygous black-furred mice (Bb) are crossed. Use a Punnett square to find the probability that an offspring will have brown fur. [3 marks]
Show the model answer
B b
B BB Bb
b Bb bb
Gametes B and b from each parent (1). Offspring BB, Bb, Bb, bb (1). Brown fur = bb = 1 out of 4, so the probability is 0.25 (1).
Construct a genetic cross from the information given
3 to 4 marks7
Work out each parent's genotype (the offspring can give clues).
Write the gametes.
Draw the Punnett square.
Link the genotypes to phenotypes and give the ratio or probability.
Example. In pea plants, the allele for tall stems (T) is dominant to the allele for short stems (t). A tall plant is crossed with a short plant. Some of the offspring are short. Construct a Punnett square to show this cross, and give the expected ratio of tall to short offspring. [4 marks]
Show the model answer
The tall parent must be Tt, because some offspring are short (tt) so it must have passed on a t allele (1). The short parent is tt; gametes are T and t, and t and t (1).
T t
t Tt tt
t Tt tt
Offspring: Tt, Tt, tt, tt (1). Ratio tall : short = 1 : 1 (1).
Interpret a family tree
2 to 3 marks8
Read the key.
Look for two parents without the characteristic who have a child with it: this shows the allele is recessive.
Give genotypes: an affected person is homozygous recessive; their unaffected parents are heterozygous.
Example. In a family, neither parent has a particular condition. Their son has the condition. Explain whether the condition is caused by a dominant allele or a recessive allele. [2 marks]
Show the model answer
A recessive allele (1). The parents do not have the condition but passed the allele to their son, so each parent must be heterozygous, with the recessive allele hidden by the dominant allele / if the allele were dominant, at least one parent would have the condition (1).
Shortcuts and memory tricks
Homo = same: homozygous means the same alleles. Hetero = different.
GenoType = the leTTers; PHenotype = the PHysical characteristic.
Learn the two key crosses: Bb × Bb gives 3 : 1, and Bb × bb gives 1 : 1.
Check your Punnett square: every box has exactly two letters, one from each parent.
Where marks are lost
Turning a 3 : 1 ratio into a probability of ⅓. The recessive phenotype is 1 out of 4 (0.25).
Writing a ratio (like 1 : 4 or 1 : 3) when asked for a probability.
Choosing letters that look alike in upper and lower case (such as S and s) and not making them clearly different.
Saying a dominant allele is 'stronger' or 'more common'. It is simply expressed when only one copy is present.
Thinking that if a couple's first child has a recessive condition, the next child cannot. The probability is the same for every child.
Exam technique
Show the gametes and the Punnett square even when the question doesn't insist; they often carry marks.
Give a probability in the form asked. 0.25, 25%, ¼ and '1 in 4' are usually all accepted, but a ratio such as 1 : 3 is not.
Use the letters given in the question. If you choose your own, give a key.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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