1. 1
   In pea plants, purple flower colour (allele P) is dominant over white flower colour (allele p). A breeder crosses two heterozygous purple-flowered pea plants (genotype Pp) and grows the resulting seeds to observe the flower colours of the offspring.

   (a)

   What is the expected phenotypic ratio of the offspring?

   [1 mark]

   • A3 purple : 1 white
   • B1 purple : 1 white
   • C1 purple : 2 mixed : 1 white
   • D9 purple : 3 mixed : 3 white : 1 unknown

   (b)

   A pea plant in the offspring generation has white flowers. What is its genotype?

   [1 mark]

   • APP
   • BPp
   • Cpp
   • DEither PP or Pp

   (c)

   Explain why a purple-flowered pea plant could have either the genotype PP or the genotype Pp, and describe how a test cross could be used to determine which genotype it has.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In guinea pigs, black fur (allele B) is dominant over brown fur (allele b). A breeder crosses a black guinea pig of unknown genotype with a brown guinea pig (bb) to determine its genotype, and the black guinea pig turns out to be heterozygous (Bb).

   (a)

   What is this type of cross called?

   [1 mark]

   • AMonohybrid dihybrid cross
   • BTest cross
   • CSelf-pollination
   • DCodominant cross

   (b)

   What phenotypic ratio is expected among the offspring of the cross Bb x bb?

   [1 mark]

   • A3 black : 1 brown
   • BAll brown
   • CAll black
   • D1 black : 1 brown

   (c)

   Explain the purpose of carrying out a test cross such as this one, and state what result would show that the unknown black guinea pig used in the breeding programme is homozygous (BB).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cystic fibrosis is an inherited condition caused by a recessive allele (f); the dominant allele (F) results in an unaffected individual. A family pedigree shows two unaffected parents who already have one child with cystic fibrosis. The couple are expecting a second child and want to know the probability that this child will also be affected.

   (a)

   Cystic fibrosis is caused by a recessive allele (f); the dominant allele (F) does not cause the disease. In a family pedigree, two unaffected parents have one child with cystic fibrosis. Using a genetic diagram, determine the genotypes of both parents and explain your reasoning.

   [3 marks]

   (b)

   Using a genetic diagram, calculate the probability that the couple's next child will have cystic fibrosis, and explain your working.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A genetic counsellor is explaining two examples of inheritance to a family. The first example concerns ABO blood groups, controlled by three alleles: I^A and I^B are codominant with each other, and I^O is recessive to both. A woman of genotype I^A I^O has children with a man of genotype I^B I^O. The second example concerns red-green colour blindness, a sex-linked recessive condition carried on the X chromosome, affecting a different couple in the same family.

   (a)

   Human ABO blood groups are controlled by three alleles: I^A and I^B are codominant with each other, and I^O is recessive to both. A woman of genotype I^A I^O has children with a man of genotype I^B I^O. Explain, using a genetic diagram, the possible genotypes and phenotypes (blood groups) of their children, and explain what is meant by codominance in this context.

   [6 marks]

   (b)

   Red-green colour blindness is caused by a recessive allele carried on the X chromosome. A woman who is a carrier for colour blindness (unaffected, but carrying one copy of the recessive allele) has children with a man who has normal colour vision. Using a genetic diagram with sex chromosomes, explain why sons and daughters have different probabilities of being colour blind, and explain what is meant by a sex-linked characteristic.

   [6 marks]

   Total for question 4: 12 marks

End of questions