References only, not question text. Open Official paper ↗ where Cambridge publishes the paper,
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school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Monohybrid crosses & pedigrees · Core · Paper 1 style · 1 mark
(a)In pea plants, the allele for tall (T) is dominant to the allele for short (t). What is the genotype of a short plant?[1]
Show answer
(a) C tt
A recessive phenotype shows only when two recessive alleles are present (homozygous recessive). Tt is tall because T is dominant, so B is wrong.
Q2Monohybrid crosses & pedigrees · Core · Paper 2 style · 4 marks
(a)Define the terms:(i) allele(ii) homozygous(iii) phenotype[3]
(b)State what is meant by a dominant allele.[1]
Show answer
(a) (i) a version (alternative form) of a gene; (ii) having two identical alleles of a gene; (iii) the observable features of an organism
B1 each
(b) an allele that is expressed (shows in the phenotype) when present, even if only one copy
B1
Q3Monohybrid crosses & pedigrees · Core · Paper 2 style · 5 marks
(a)In guinea pigs, black fur (B) is dominant to white fur (b). Two black guinea pigs, both Bb, are crossed. Use a genetic diagram to show the genotypes and phenotypes of the offspring.[4]
(b)State the probability that one offspring is white.[1]
Show answer
(a) parents Bb × Bb; gametes B, b and B, b; offspring BB, Bb, Bb, bb; phenotypes 3 black : 1 white
B1 parental genotypes; B1 gametes (one allele each); B1 offspring genotypes; B1 phenotypes with ratio 3 black : 1 white
(b) 0.25 (1 in 4, 25 %)
B1
Q4Monohybrid crosses & pedigrees · Standard · Paper 1 style · 1 mark
(a)Two heterozygous plants are crossed. What proportion of the offspring is expected to be homozygous?[1]
Show answer
(a) B 50 %
Aa × Aa gives 1 AA : 2 Aa : 1 aa, so AA + aa = 2 out of 4 = 50 %. 75 % is the proportion showing the dominant phenotype, so C is the main distractor.
Q5Monohybrid crosses & pedigrees · Standard · Paper 2 style · 5 marks
In a plant, purple flowers (P) are dominant to white flowers (p). Two heterozygous plants are crossed and 840 seeds are grown.
(a)Calculate the expected number of plants with white flowers.[1]
(b)The actual numbers were 618 purple and 222 white. Suggest why these differ from the expected numbers.[1]
(c)Describe how a gardener could find out whether one purple-flowered plant is homozygous or heterozygous.[3]
Show answer
(a) 840 ÷ 4 = 210
B1 210
(b) fertilisation is random, so ratios are only probabilities
B1 random fusion of gametes / chance
(c) cross it with a white (homozygous recessive) plant (test cross); if all offspring purple, it is homozygous; if about half white (1 : 1), it is heterozygous
B1 cross with white/recessive; B1 all purple → homozygous; B1 some/half white → heterozygous
Q6Monohybrid crosses & pedigrees · Standard · Paper 2 style · 5 marks
A family tree for an inherited condition (described in words):Generation 1: person 1 (unaffected male) and person 2 (affected female).Generation 2: their children are person 3 (unaffected male) and person 5 (unaffected female). Person 3 marries person 4 (unaffected female from another family).Generation 3: persons 3 and 4 have three children: person 6 (unaffected female), person 7 (affected male) and person 8 (unaffected male).
(a)Explain how you can tell from the family tree that the allele for the condition is recessive.[2]
(b)State the genotypes of persons 3, 4 and 7, using D for the normal allele and d for the condition.[2]
(c)Calculate the probability that the next child of persons 3 and 4 is affected.[1]
Show answer
(a) persons 3 and 4 are unaffected but have an affected child (person 7); so both parents must carry a hidden (recessive) allele
B1 unaffected parents (3 and 4) with affected child (7); B1 parents are carriers/heterozygous
(b) 3: Dd; 4: Dd; 7: dd
B1 3 and 4; B1 7
(c) Dd × Dd: 1/4 = 0.25
B1
Q7Monohybrid crosses & pedigrees · Standard · Paper 2 style · 4 marks
(a)Explain why the ratio 3 : 1 is seen in the offspring of a cross between two heterozygous parents.[3]
(b)State the meaning of 'genotype'.[1]
Show answer
(a) each parent produces two types of gamete (A and a) in equal numbers; fertilisation is random; giving 1 AA : 2 Aa : 1 aa; AA and Aa both show the dominant feature, so 3 : 1
B1 gametes A and a in equal proportions (meiosis); B1 random fertilisation; B1 AA and Aa same phenotype → 3 : 1
(b) the alleles (genetic make-up) an organism has, e.g. Aa
B1
Q8Monohybrid crosses & pedigrees · Challenge · Paper 2 style · 5 marks
A family tree for an inherited condition caused by a recessive allele d (normal allele D), described in words:Generation 1: person 1 (unaffected male) and person 2 (unaffected female).Generation 2: their children are person 3 (affected female) and person 4 (unaffected male). Person 4 marries person 5 (affected female).Generation 3: persons 4 and 5 have two children: person 6 (unaffected male) and person 7 (affected female).
(a)Explain how person 7 shows that person 4 is heterozygous.[2]
(b)Before persons 4 and 5 had any children, what was the probability that person 4 was a carrier? Explain your answer.[2]
(c)State the genotype of person 6 and explain how you know.[1]
Show answer
(a) person 7 is dd, so she received one d allele from each parent, including person 4; person 4 is unaffected, so he also has a D allele: Dd
B1 person 7 (dd) received d from father/person 4; B1 person 4 unaffected so has D → Dd
(b) 2/3: persons 1 and 2 must both be Dd (they have an affected child, person 3); their offspring are DD, Dd, Dd, dd; person 4 is unaffected, so he is one of DD, Dd, Dd, and 2 of these 3 are carriers
M1 persons 1 and 2 both Dd, dd excluded as person 4 is unaffected; A1 2/3
(c) Dd: he is unaffected (has D) but his mother, person 5, is dd and must have passed a d allele to him
B1 Dd with reason (d from affected mother)
Q9Monohybrid crosses & pedigrees · Challenge · Paper 2 style · 5 marks
(a)A breeder of goats in Balochistan finds that horns (h) are recessive to hornless (H). She crosses a hornless male with several horned females. Out of 24 kids, 11 have horns. Deduce the genotype of the male, with a reason.[2]
(b)Calculate the expected numbers of horned and hornless kids from this cross, out of 24.[1]
(c)Explain why the breeder would prefer a homozygous hornless male if she wants only hornless kids.[2]
Show answer
(a) Hh; the horned kids are hh, so they must have received an h allele from the male as well as from the mother
B1 Hh; B1 horned kids need h from father
(b) Hh × hh gives 1 : 1, so 12 horned and 12 hornless
B1
(c) HH × hh gives all Hh, all hornless; every gamete from the male carries H
B1 all offspring Hh; B1 all hornless/dominant allele always passed on
Q10Chromosomes, genes & DNA · Core · Paper 1 style · 1 mark
(a)Which base always pairs with adenine in a DNA molecule?[1]
Show answer
(a) C thymine
A pairs with T and C pairs with G in DNA. Uracil is not found in DNA, so D is wrong.
Q11Chromosomes, genes & DNA · Core · Paper 2 style · 5 marks
(a)State what is meant by a gene.[1]
(b)Put these in order of size, smallest first: chromosome, nucleus, gene, base.[1]
(c)Describe the structure of a DNA molecule.[3]
Show answer
(a) a length of DNA that codes for a protein
B1
(b) base, gene, chromosome, nucleus
B1
(c) two strands coiled into a double helix; each strand has a sequence of bases; bases pair across the strands (A with T, C with G)
Q15Chromosomes, genes & DNA · Challenge · Paper 2 style · 4 marks
(a)A goat body cell has 60 chromosomes. State the number of chromosomes in a goat egg cell and in a goat zygote.[2]
(b)Explain why it is important that gametes are haploid.[2]
Show answer
(a) egg: 30; zygote: 60
B1 each
(b) when two gametes fuse at fertilisation, the diploid number is restored; otherwise the chromosome number would double each generation
B1 fertilisation restores diploid number; B1 prevents doubling each generation
Q16Chromosomes, genes & DNA · Challenge · Paper 2 style · 4 marks
(a)Explain the difference between a gene and an allele, using an example.[2]
(b)Explain why identical twins have the same alleles but non-identical twins do not.[2]
Show answer
(a) a gene is a length of DNA coding for a characteristic/protein, e.g. the gene for flower colour; alleles are different versions of that gene, e.g. purple allele and white allele
B1 gene definition; B1 allele as version of gene with example
(b) identical twins come from one zygote that splits (mitosis gives identical cells); non-identical twins come from two different eggs and sperm, which were made by meiosis and differ genetically
B1 identical from one zygote/mitosis; B1 non-identical from two different gametes produced by meiosis
Q17Mitosis & meiosis · Core · Paper 1 style · 1 mark
(a)Which type of cell division produces gametes in humans?[1]
Show answer
(a) B meiosis
Meiosis halves the chromosome number to make haploid gametes. Mitosis produces genetically identical diploid cells, so A is wrong.
Q18Mitosis & meiosis · Core · Paper 2 style · 4 marks
(a)State two uses of mitosis in the human body.[2]
(b)State the number of cells produced when one cell divides by mitosis, and how they compare with the parent cell.[2]
Show answer
(a) growth; repair of damaged tissue; replacement of cells (e.g. skin, blood cells)
1 mark each for two
(b) two cells; genetically identical to the parent (same number of chromosomes)
B1 two; B1 genetically identical
Q19Mitosis & meiosis · Standard · Paper 2 style · 4 marks
(a)Compare mitosis and meiosis. Give three differences.[3]
(b)State where meiosis takes place in a flowering plant.[1]
Show answer
(a) mitosis gives 2 cells, meiosis 4; mitosis keeps the chromosome number (diploid), meiosis halves it (haploid); mitosis gives genetically identical cells, meiosis genetically different cells; mitosis for growth/repair, meiosis for gametes
1 mark each for any three paired differences
(b) anthers (to make pollen) and ovules/ovary
B1
Q20Mitosis & meiosis · Challenge · Paper 2 style · 4 marks
Some cells used in medical research are grown in a flask. In good conditions each cell divides by mitosis every 20 minutes.
(a)Starting from one cell, calculate the number of cells after 3 hours, assuming no cells die.[2]
(b)Explain why all the cells produced are genetically identical.[2]
(b) DNA (chromosomes) is copied exactly before division; each new cell gets an identical set of chromosomes
B1 chromosomes replicated/copied exactly; B1 each daughter cell receives identical set
Q21Mitosis & meiosis · Challenge · Paper 2 style · 4 marks
(a)Explain how meiosis and fertilisation together lead to genetic variation among offspring of the same parents.[3]
(b)Explain why a banana plant grown from a cutting is genetically identical to its parent.[1]
Show answer
(a) meiosis produces gametes with different combinations of alleles/chromosomes; each gamete is genetically different; fertilisation is random, so any sperm can fuse with any egg
B1 meiosis gives genetically different gametes; B1 different combinations of chromosomes/alleles; B1 random fertilisation
(b) it is produced by asexual reproduction, using mitosis only
B1
Q22Protein synthesis · Core · Paper 2 style · 3 marks
(a)Name the molecule that carries a copy of a gene from the nucleus to the cytoplasm.[1]
(b)State what determines the order of amino acids in a protein.[1]
(c)Explain why DNA does not leave the nucleus during protein synthesis.[1]
Show answer
(a) mRNA (messenger RNA)
B1
(b) the sequence of bases in the gene (DNA)
B1
(c) DNA is too large to pass through the nuclear membrane / it is kept safe; a copy (mRNA) is made instead
B1
Q23Protein synthesis · Standard · Paper 1 style · 1 mark
(a)Where in a cell are proteins assembled from amino acids?[1]
Show answer
(a) B ribosome
Ribosomes join amino acids in the order coded by mRNA. The nucleus holds the DNA and makes mRNA but does not assemble the protein, so A is wrong.
Q24Protein synthesis · Standard · Paper 2 style · 4 marks
(a)Describe how the information in a gene is used to make a protein.[4]
Show answer
(a) the gene's base sequence is copied into mRNA in the nucleus; mRNA moves to the cytoplasm; to a ribosome; the ribosome reads the base sequence and joins amino acids in the correct order to make the protein
1 mark each for four of: mRNA made from gene in nucleus; mRNA moves out to cytoplasm; ribosome; base sequence read (three bases per amino acid); amino acids joined in specific order
Q25Protein synthesis · Challenge · Paper 2 style · 4 marks
(a)A gene contains 1 260 bases that code for a protein. Each amino acid is coded for by three bases. Calculate the number of amino acids in the protein.[1]
(b)One base in this gene is changed. Explain how this could stop the protein working.[3]
Show answer
(a) 1 260 ÷ 3 = 420
B1 420
(b) the change alters the base sequence (gene mutation); so a different amino acid may be inserted; the protein folds into a different shape, e.g. an enzyme's active site changes
B1 base sequence changed; B1 different amino acid sequence; B1 different shape/active site so protein does not function
Q26Protein synthesis · Challenge · Paper 2 style · 4 marks
(a)All body cells of a person contain the same genes, but insulin is made only in certain cells of the pancreas. Explain this.[2]
(b)Explain why bacteria given a human insulin gene can make human insulin.[2]
Show answer
(a) only some genes are expressed (switched on) in each cell type; the insulin gene is expressed only in those pancreas cells
B1 genes switched on/off (expressed) differently; B1 insulin gene only expressed in pancreas cells
(b) the base code is read in the same way in bacteria (universal code); the bacteria's ribosomes join amino acids in the order coded for by the gene, making the same protein
B1 genetic code is the same/universal; B1 bacterial ribosomes make the same amino acid sequence
Q27Codominance & blood groups · Core · Paper 2 style · 4 marks
(a)State what is meant by codominance.[2]
(b)State the possible genotypes of a person with blood group A.[2]
Show answer
(a) both alleles in a heterozygote are expressed in the phenotype; neither is dominant
B1 both alleles expressed; B1 in heterozygote
(b) Iᴬ Iᴬ or Iᴬ i
B1 each
Q28Codominance & blood groups · Standard · Paper 1 style · 1 mark
(a)[1]
Show answer
(a) A A or B only
Father Iᴬ Iᴮ × mother i i gives Iᴬ i (group A) or Iᴮ i (group B). AB needs both Iᴬ and Iᴮ but the mother gives only i, so B is wrong.
Q29Codominance & blood groups · Standard · Paper 2 style · 5 marks
(a)A woman with blood group A (genotype Iᴬ i) and a man with blood group B (genotype Iᴮ i) have children. Use a genetic diagram to show all the possible blood groups of their children.[4]
(b)Calculate the probability that their first child has blood group O.[1]
Show answer
(a) parents Iᴬ i × Iᴮ i; gametes Iᴬ, i and Iᴮ, i; offspring Iᴬ Iᴮ (AB), Iᴬ i (A), Iᴮ i (B), i i (O)
B1 gametes; B1 offspring genotypes; B1 phenotypes; B1 all four groups identified
(b) 1/4 = 0.25
B1
Q30Codominance & blood groups · Challenge · Paper 2 style · 4 marks
(a)In a flower, the allele for red petals (Cᴿ) and the allele for white petals (Cᵂ) are codominant. Heterozygous plants have red-and-white streaked petals. Two streaked plants are crossed and 200 seeds are grown. Calculate the expected numbers of red, streaked and white plants.[2]
(b)Explain why a gardener who wants only streaked plants cannot get them by crossing streaked plants, but can by crossing red with white.[2]
Show answer
(a) Cᴿ Cᵂ × Cᴿ Cᵂ gives 1 : 2 : 1; red 50, streaked 100, white 50
M1 1 : 2 : 1 ratio; A1 50, 100, 50
(b) streaked × streaked gives three phenotypes (homozygotes appear); red × white (Cᴿ Cᴿ × Cᵂ Cᵂ) gives all Cᴿ Cᵂ, all streaked
B1 streaked × streaked gives red and white too; B1 red × white gives all heterozygous/streaked
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
