Monohybrid inheritance and sex linkageEdexcel International A Level Biology: Subtopic test

1. 1
   Cystic fibrosis is caused by a recessive allele, f, of a single gene on an autosome. The dominant allele is F. Two parents who do not have cystic fibrosis are expecting a child, and the couple already have one child who has cystic fibrosis.

   (a)

   What is the genotype of each of the two parents?

   [1 mark]

   • AFF
   • BFf
   • Cff
   • DOne is FF and one is Ff

   (b)

   What is the probability that the next child of these parents will be a carrier of the cystic fibrosis allele without having the condition?

   [1 mark]

   • A25%
   • B75%
   • C50%
   • D0%

   (c)

   Explain how two parents who do not have cystic fibrosis can have a child who has the condition.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a breed of cattle, coat colour is controlled by one gene with two alleles, $C^{R}$ (red hairs) and $C^{W}$ (white hairs). Neither allele is dominant. Cattle with the genotype $C^{R}C^{W}$ have a roan coat, with both red and white hairs.

   (a)

   Which term describes the relationship between the two alleles of this gene?

   [1 mark]

   • AComplete dominance
   • BRecessiveness
   • CSex linkage
   • DCodominance

   (b)

   Two roan cattle are crossed. What percentage of their calves is expected to be roan?

   [1 mark]

   • A50%
   • B25%
   • C75%
   • D100%

   (c)

   A roan bull is crossed with a white cow. Give the genotypes of the possible calves and the expected ratio of their phenotypes.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A rare inherited condition is studied in a family. Neither individual 1 (a man) nor individual 2 (a woman) has the condition. They have three children: individual 3, a daughter who has the condition, and individuals 4 and 5, two sons who do not have the condition.

   (a)

   Deduce the mode of inheritance of this condition, using evidence from the family.

   [3 marks]

   (b)

   Individuals 1 and 2 have another child. Using A for the dominant allele and a for the recessive allele, calculate the probability that this child is a daughter with the condition.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A woman with normal colour vision has a father who is red-green colour blind. She has children with a man who has normal colour vision. Red-green colour blindness is caused by a recessive allele on the X chromosome.

   (a)

   Using genetic notation, explain the genotypes of the woman and her partner, and predict the probability of colour blindness in the sons and in the daughters of this couple.

   [6 marks]

   (b)

   Explain why red-green colour blindness is more common in males than in females, and why a colour-blind man cannot pass the condition to his sons.

   [6 marks]

   Total for question 4: 12 marks

End of questions