1. 1
   A gardener is investigating stem height in pea plants. The allele for tall stems (T) is dominant over the allele for short stems (t). The gardener crosses a heterozygous tall plant (genotype Tt) with a short plant (genotype tt) and grows 40 offspring seeds.

   (a)

   In pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A gardener crosses a heterozygous tall plant (Tt) with a short plant (tt) and grows 40 offspring seeds. Which term correctly describes the genotype Tt?

   [1 mark]

   • AHomozygous dominant
   • BHomozygous recessive
   • CHeterozygous
   • DCodominant

   (b)

   Based on a genetic diagram (Punnett square) for this cross, what is the expected ratio of tall to short offspring?

   [1 mark]

   • A3 tall : 1 short
   • B1 tall : 1 short
   • C1 tall : 3 short
   • DAll offspring tall

   (c)

   Of the 40 offspring seeds grown, the gardener counted 22 tall plants and 18 short plants. Explain why this observed ratio is not exactly 1:1, even though the expected ratio from the cross is 1:1.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A husband and wife are both carriers of the recessive allele for cystic fibrosis. Each parent has the genotype Ff, where F is the dominant normal allele and f is the recessive allele that causes cystic fibrosis when present in two copies. The couple wants to know the chance of their children being affected.

   (a)

   What is the genotype of a child who has cystic fibrosis?

   [1 mark]

   • Aff
   • BFf
   • CFF
   • DfF only

   (b)

   What is the probability that a child of this couple will have cystic fibrosis?

   [1 mark]

   • A0
   • BAll children will be affected
   • C1 in 2
   • D1 in 4

   (c)

   The couple's first child does not have cystic fibrosis. Explain why this child could still be a carrier of the condition, and describe how you could determine the child's genotype with certainty.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A family tree (pedigree chart) shows a family across three generations. The woman's father had haemophilia, a sex-linked recessive condition carried on the X chromosome. The woman's mother did not carry the allele for haemophilia. The woman herself does not show any symptoms of haemophilia. The woman is now planning to have children with a man who does not have haemophilia and does not carry the allele.

   (a)

   A family tree shows that a woman's father had haemophilia (a sex-linked recessive condition) but her mother did not carry the allele. The woman herself does not have haemophilia. Using the sex chromosomes XY (male) and XX (female), state the woman's genotype for this gene and explain your reasoning.

   [3 marks]

   (b)

   The woman from part (a) has children with a man who does not have haemophilia and does not carry the allele. Using a genetic diagram in words, describe the possible genotypes and phenotypes of their children with respect to haemophilia, and explain why sons and daughters are affected differently.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A genetic counsellor is explaining blood group inheritance to a couple before the birth of their child. The ABO blood group system is controlled by a single gene with three alleles: IA, IB and IO, where IA and IB are codominant with each other and both are dominant over IO.

   (a)

   Describe how the ABO blood group system in humans demonstrates codominance and multiple alleles, and explain how genetic diagrams can be used to predict the possible blood groups of children born to parents with known blood group genotypes.

   [6 marks]

   (b)

   The mother in this scenario has blood group A with genotype IAIO, and the father has blood group B with genotype IBIO. Using a genetic diagram in words, determine all the possible blood groups of their children, state the expected proportions, and explain what this shows about how codominance and recessive alleles combine to affect inheritance patterns.

   [6 marks]

   Total for question 4: 12 marks

End of questions