AQA GCSE Biology Foundation (8461), Foundation tier · Inheritance, variation and evolution › Reproduction
Practise Genetic inheritance. 4 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.
Key facts
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
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
diagram
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: each box has a probability of 0.25.
Bb × bb gives Bb : bb in the ratio 1 : 1.
Family trees
diagram
Squares usually show males and circles females; shaded symbols show the characteristic. Always read the key.
Reading a family tree: a line between two people joins parents; the line below leads to their children.
How to answer each type of question
Use the genetic terms
1 to 3 marksGrade 4
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 answerHide the model answer
(a) bb (1) (b) Black fur (1) (c) Having two different alleles of a gene (1)
Don’t lose marks
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.
More tips
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.
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.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define dominant and recessive allelesA dominant allele is expressed even with one copy; a recessive allele is expressed only with two copies.
Grade 4
Use genotype, phenotype, homozygous and heterozygous correctlyGenotype is the alleles present, phenotype is the characteristic; homozygous means the same alleles, heterozygous different.
Grade 5
Complete a Punnett squarePut each parent's gametes on the outside and combine one allele from each in every box.
Grade 5
Give cross outcomes as ratios or probabilitiesFor example 3 : 1, or a probability of 0.25, 25%, ¼ or 1 in 4.
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
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 2Medium3 marks
A characteristic is controlled by a single gene with two alleles. In a population, 84% of individuals show the dominant phenotype and 16% show the recessive phenotype.
Explain how you can be certain of the genotype of the 16% showing the recessive phenotype, but cannot be certain of the genotype of the 84% showing the dominant phenotype without more information.[3]
Show the answer and mark scheme
Answer: A recessive phenotype is always caused by two copies of the recessive allele, since one dominant allele would be expressed instead; but the dominant phenotype could be produced by either a homozygous dominant or a heterozygous genotype, which look the same
a recessive phenotype is always caused by two copies of the recessive allele (homozygous recessive)
this is because even one copy of the dominant allele would be expressed instead
individuals with the dominant phenotype could be homozygous dominant or heterozygous, since both genotypes produce the same (dominant) phenotype