Monohybrid, multiple allele and sex-linked crosses - AQA A-Level Biology (Paper 2) Exam Questions

Practise 20 exam-style questions on Monohybrid, multiple allele and sex-linked crosses for AQA A-Level Biology (Paper 2). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

20 questions
1–8 marks each
Instant marking
No sign-up needed

20 questions, marked as you go

Sample questions with model answers

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.

Question 1

2 marks

Tay-Sachs disease is caused by a recessive allele of a gene on an autosome. A couple who are both heterozygous for this gene plan to have three children.

Calculate the probability that at least one of their three children will have Tay-Sachs disease. Give your answer as a decimal to 2 significant figures.

Model answer

Probability that one child is unaffected = 3/4
Probability that all three are unaffected = (3/4)³ = 27/64 = 0.4219
Probability that at least one is affected = 1 - 27/64 = 37/64 = 0.578 = 0.58 (2 s.f.)

Explanation

Each child has a probability of 1/4 of being homozygous recessive, whatever happens to the others, because fertilisation is random. 'At least one' is easiest to work out as 1 minus the probability that none of the children is affected. Adding 1/4 three times is wrong: it counts some outcomes more than once and would exceed 1 for five children.

Mark scheme

1 mark
Correct method, e.g. 1 - (3/4)³ or 1 - 27/64; or a correct final answer of 0.58 with no working
A correct answer of 0.58 with no working scores 2 marks. Accept for 1 mark: 0.42 or 27/64 (the probability that no child is affected) OR 37/64 or 0.578 (correct but not given as a decimal to 2 significant figures). Do not award this mark for 0.75 (1/4 added three times) or 0.016 (1/4 cubed: all three affected).
1 mark
0.58
Do not accept 0.578, 0.6 or 58% for this mark.

Question 2

3 marks

Red-green colour blindness is caused by a recessive allele of a gene on the X chromosome. The diagram shows the inheritance of red-green colour blindness in one family.

Give the genotype of person 4. Use XB for an X chromosome with the dominant allele and Xb for an X chromosome with the recessive allele. Explain how the family tree shows that this is her genotype.

Calculate the probability that person 7 is a carrier.

Family tree with three generations, labelled I, II and III from top to bottom. Squares are males and circles are females; horizontal lines join couples, and lines down from a couple lead to their children. The key shows: an unshaded square = Unaffected male; an unshaded circle = Unaffected female; a shaded square = Male with red-green colour blindness; a shaded circle = Female with red-green colour blindness. In generation I, person 1 (male, shaded: has red-green colour blindness) and person 2 (female, unshaded: unaffected) are a couple; their two children in generation II, from left to right, are person 3 (male, unshaded: unaffected) and person 4 (female, unshaded: unaffected). Person 4 and person 5 (male, unshaded: unaffected), whose parents are not shown, are a couple; their three children in generation III, from left to right, are person 6 (male, shaded: has red-green colour blindness), person 7 (female, unshaded: unaffected) and person 8 (male, unshaded: unaffected).

Model answer

Person 4 is XBXb.
Her father, person 1, is colour blind (XbY) and every daughter receives her father's X chromosome, so she must have Xb. Her son, person 6, is also colour blind and received his X chromosome from her. She is not colour blind herself, so her other X chromosome carries XB.

Person 7 receives XB from her father (5) and either XB or Xb from her mother (4), so the probability that she is a carrier = 1/2.

Explanation

For X-linked recessive conditions, a woman whose father or son is affected must carry the recessive allele, because a father gives his X chromosome to every daughter and a son gets his only X chromosome from his mother. If she is unaffected she must also have a dominant allele, so she is a heterozygous carrier. A daughter of a carrier mother and an unaffected father has a probability of 1/2 of being a carrier. A common error is to give a ratio instead of a probability.

Mark scheme

1 mark
Person 4 is XBXb
Point 3 is the calculation: a correct answer of 1/2 (0.5 or 50%) with no working scores that mark. Genotypes for the marker (the figure does not give them): 1 XbY, 2 XBXB or XBXb, 3 XBY, 4 XBXb, 5 XBY, 6 XbY, 7 XBXB or XBXb, 8 XBY. Accept the alleles in either order. Reject XbXb or any genotype with a Y chromosome.
1 mark
Her father (person 1) is colour blind and passed on his (only) X chromosome, which carries Xb OR her son (person 6) is colour blind and received his X chromosome (with Xb) from her; AND she is not colour blind so she also has XB
Either piece of evidence, with the idea that she is not colour blind, gains the mark. Award only if point 1 is awarded.
1 mark
1/2 / 0.5 / 50% (a correct answer with no working scores this mark)
A correct answer of 1/2 with no working scores 1 mark. Accept 1 in 2. Reject 1 : 1 or 1 : 2. Person 7's mother (4) is a carrier and her father (5) is not colour blind, so person 7 receives XB from her father and XB or Xb (equally likely) from her mother.

Question 3

5 marks

In chickens, a gene for feather colour has two codominant alleles, FB and FW. Chickens with the genotype FBFB have black feathers and chickens with the genotype FWFW have white feathers. Heterozygous chickens are called erminette: they have mostly white feathers with some black feathers scattered among them.

A farmer crossed erminette chickens with each other.

Give the genotypes of the parents, the gametes, and the genotypes, phenotypes and ratio of the offspring expected from this cross.

Model answer

Parents: FBFW × FBFW (erminette × erminette)
Gametes: FB, FW and FB, FW
Offspring genotypes: FBFB, FBFW, FBFW, FWFW
Phenotypes: black, erminette, erminette, white
Ratio: 1 black : 2 erminette : 1 white

Explanation

Codominant alleles are both expressed in the phenotype of a heterozygote, so the heterozygote has a third phenotype (erminette) and the three genotypes give three phenotypes. Each heterozygous parent produces FB and FW gametes in equal numbers, and random fertilisation gives FBFB, FBFW and FWFW in a 1 : 2 : 1 ratio. A common error is to treat one allele as dominant and give a 3 : 1 ratio.

Mark scheme

1 mark
Parents: FBFW × FBFW
Max 2 if the genotypes given for the parents are wrong. In that case award only point 2 (gametes that follow correctly from the student's parental genotypes) and point 3 (offspring genotypes that follow correctly from the student's gametes). If the parents' genotypes are correct but the gametes are wrong, point 3 may still be awarded for offspring genotypes that follow correctly from the student's gametes. Points 4 and 5 are not consequential: they need the correct phenotypes and ratio for this cross. Accept the alleles within a genotype in either order, and a Punnett square written out in words or rows. Accept B and W, or other clearly defined symbols, for FB and FW.
1 mark
Gametes: FB and FW (from each parent)
Ignore circles or the absence of circles around gametes.
1 mark
Offspring genotypes: FBFB, FBFW and FWFW
Accept FBFW shown twice, or FWFB.
1 mark
Phenotypes: black (FBFB), erminette (FBFW) and white (FWFW)
The phenotypes must be matched to the correct genotypes, or listed in the same order as the genotypes. Accept 'black and white feathers' or 'speckled' for erminette. Reject grey.
1 mark
Ratio: 1 black : 2 erminette : 1 white
Accept 1 : 2 : 1 only if the order of the phenotypes is clear. Accept 25% : 50% : 25% or 1/4 : 1/2 : 1/4. Reject 3 : 1 or 1 : 1 : 1.

Question 4

5 marks

A gene that controls coat colour in a species of mammal has three alleles. Two of the alleles are codominant.

Explain what is meant by each of the following terms:
- genotype
- phenotype
- codominant
- multiple alleles.

Model answer

Genotype: the genetic constitution of an organism - the alleles it has.
Phenotype: the expression of the genotype together with its interaction with the environment.
Codominant: both alleles are expressed in the phenotype of a heterozygous organism.
Multiple alleles: a gene that has more than two alleles in the population, although each diploid organism carries only two of them.

Explanation

The genotype is the genetic constitution (the alleles present); the phenotype is how it is expressed, which also depends on the environment, so a definition of phenotype needs both ideas. Codominant alleles are both expressed in a heterozygote, so the heterozygote has its own phenotype, unlike dominant and recessive alleles where only the dominant allele affects the phenotype. 'Both alleles are dominant' is not enough, and codominance is not blending. Multiple alleles exist in a population, but a diploid organism carries at most two of them; do not confuse multiple alleles with characteristics controlled by many genes.

Mark scheme

1 mark
Genotype: the genetic constitution of an organism / the alleles an organism has
Accept 'the combination of alleles of a gene in an organism'. Insufficient: 'the genes' or 'the DNA' alone. Reject 'the characteristics of an organism'.
1 mark
Phenotype: the expression of the genotype / the characteristics produced by the genotype (alleles)
Points 2 and 3 are the phenotype: 1 mark for the genetic idea and 1 mark for the environment. Insufficient: 'what an organism looks like' with no reference to the genotype or the environment (this scores neither point).
1 mark
(Phenotype also results from) interaction (of the genotype) with the environment
Accept 'the phenotype is also affected by the environment'.
1 mark
Codominant: both alleles are expressed in the phenotype of a heterozygote / neither allele is dominant or recessive to the other
Accept 'the heterozygote shows the effect of both alleles'. Insufficient: 'both alleles are dominant' alone. Reject 'the alleles blend'.
1 mark
Multiple alleles: a gene that has more than two alleles (in a population)
Accept 'three or more alleles of one gene'. Reject 'many genes control the characteristic' (that is polygenic inheritance) or 'an organism has more than two alleles'.

Ready to try the rest? Every answer you write is marked against the mark scheme, so you can see exactly where the marks are.

Study Helper Hints first, answers after you try

Stuck on this one? Ask for a hint and I’ll point you in the right direction, without giving the answer away. Once you’ve checked your answer, I’ll show you how the marks are awarded.