This free quiz has 12 real AQA A-level Biology exam questions on inheritance and genetic crosses (21 marks, about 25 minutes). It marks itself: tick-box and table questions are marked for you, and for written answers you check your work against the real AQA mark scheme.
What the quiz covers (AQA 3.7.1)
where a sex-linked mutation is found, and why male fruit flies are more likely to have white eyes
predicting the offspring of a sex-linked cross
naming codominance, and a genetic diagram for a dihybrid cross with codominant alleles
using an epistasis diagram to work out fruit colour from genotypes
ABO blood group genotypes from a family tree, and the chi-squared test with its degrees of freedom
the probability of parents having a son with an autosomal recessive condition
a test cross to find out whether a fly is homozygous or heterozygous
a sex-linked cross with epistasis in cats, and explaining autosomal linkage results (3 marks)
How to use it
Write each answer in full before you open the mark scheme. For genetic diagrams, set out parental genotypes, gametes, offspring genotypes, phenotypes and ratio in that order, and work the ratio out from your own diagram rather than from memory. Always use the word allele, and state the sex of each offspring in sex-linked crosses.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
What phenotype means. Students who scored nothing gave a GCSE-style answer that the phenotype is just an observable feature or appearance. Many mentioned the genotype but left out the environment.
✔ Gains credit: the phenotype is the expression of the genotype (genetic constitution) and its interaction with the environment.
Epistasis or codominance. Common wrong names for an interaction between two genes were codominance, dihybrid and epigenetics, and some called the relationship between two alleles of one gene epistasis.
✔ Gains credit: an interaction where one gene affects the expression of another gene is epistasis; two alleles that are both expressed in a heterozygote, such as red and white giving pink, are codominant.
One allele in males. Students who gained no credit often wrote only about sex chromosomes and never mentioned alleles. Others explained why males are affected but said nothing about females.
✔ Gains credit: males need only one recessive allele, whereas females need two recessive alleles (homozygous recessive) and can be heterozygous carriers.
The probability of a son. The most frequent wrong answer was 25% because students worked out the chance of the genotype but did not include the chance of the child being a boy. A ratio such as 1 : 8 was rejected.
✔ Gains credit: multiply the probability of the genotype by the probability of a son (0.5), and give the answer as a decimal, fraction or percentage.
State the sex. In sex-linked crosses, many lost a mark by leaving the sex out of the offspring phenotypes, or by combining white males and white females into one group, which gave 2:1:1 instead of 1:1:1:1.
✔ Gains credit: each phenotype names the sex as well as the characteristic, e.g. tortoiseshell female, with males and females counted separately in the ratio.
Ratios from your diagram. A significant minority wrote 9:3:3:1 even when their genotypes and phenotypes were correct, and others wrote 1:1:1:1. In a cross with a lethal genotype, many counted flies that die as embryos, giving 3:1 or 1:2:1.
✔ Gains credit: a ratio worked out from the offspring genotypes in the diagram, leaving out lethal genotypes that do not survive, e.g. 2:1.
Genes are linked, not alleles. Most students could not explain autosomal linkage clearly, referring to alleles (unqualified) or traits rather than genes. Very few gave the gametes, describing offspring proportions instead, and some said crossing over happens in mitosis.
✔ Gains credit: the genes are on the same chromosome (autosome), so most gametes keep the parental combinations and crossing over produces only a few recombinant gametes.
Keep practising
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Questions © AQA, from past A-level and AS Biology papers, with their mark schemes and examiners’ reports.