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Practise 33 exam-style questions on Genetic inheritance for AQA GCSE Biology. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Explain what is meant by a dominant allele.
an allele that is expressed in the phenotype even if only one copy is present
A dominant allele shows its effect in the organism's appearance (phenotype) even if the other allele is different (heterozygous).
an allele expressed when only one copy is present (masks recessive allele)
Accept 'masks recessive allele'.
A single-gene trait in pea plants has alleles: tall (T) dominant, short (t) recessive. Two heterozygous tall plants are crossed. (a) Give the genotypic ratio. (b) Give the phenotypic ratio.
Genotypic ratio 1 TT : 2 Tt : 1 tt. Phenotypic ratio 3 tall : 1 short.
Crossing Tt × Tt gives offspring genotypes TT, Tt, Tt, tt so 1:2:1 genetically and 3 tall : 1 short by appearance.
Genotypic ratio 1:2:1 (TT:Tt:tt)
Phenotypic ratio 3 tall : 1 short
In mice, black fur (B) is dominant to white fur (b). A black mouse heterozygous for fur colour is crossed with a white mouse. (a) Complete the Punnett square for this cross and (b) give the expected phenotype ratio of offspring.
Punnett square: top row B and b (gametes from Bb); side column b and b (gametes from bb). Offspring genotypes: Bb, bb, Bb, bb. Phenotype ratio: 1 black : 1 white or 50% black, 50% white.
A heterozygous black (Bb) crossed with white (bb) produces two genotypes: Bb (black) and bb (white) in equal numbers so the expected phenotypic ratio is 1:1.
Correct Punnett square showing Bb / bb / Bb / bb (or equivalent placement)
Allow equivalent arrangement; award if all four genotypes correct.
Phenotype ratio 1 black : 1 white (or 50% black, 50% white)
Must follow correct square.
A known heterozygote (Aa) is crossed with a homozygous dominant (AA). State the probability that an offspring will be homozygous dominant.
Cross Aa × AA gives offspring genotypes AA and Aa in equal proportions ? probability offspring is AA = 50% or 1/2.
A cross of Aa with AA produces half the offspring AA and half Aa, so the chance of an AA child is 50%.
Correct Punnett logic: gametes A,a × A,A giving AA, AA, Aa, Aa (or equivalent)
Award for correct set-up/derivation.
Probability = 1/2 or 50%
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