Dihybrid inheritance, multiple alleles and sex linkageAQA A-Level Biology: Subtopic test

1. 1
   In a species of garden pea, round seeds are produced by the dominant allele $R$ and wrinkled seeds by the recessive allele $r$. Yellow seeds are produced by the dominant allele $Y$ and green seeds by the recessive allele $y$. The two genes are on different chromosomes. A plant breeder crosses plants that are heterozygous for both genes ($RrYy$).

   (a)

   How many genetically different types of gamete can a plant with the genotype $RrYy$ produce?

   [1 mark]

   • A2
   • B4
   • C8
   • D16

   (b)

   Two $RrYy$ plants are crossed. What phenotypic ratio of round yellow : round green : wrinkled yellow : wrinkled green is expected in the offspring?

   [1 mark]

   • A9 : 3 : 3 : 1
   • B1 : 1 : 1 : 1
   • C3 : 1 : 3 : 1
   • D1 : 2 : 1 : 2

   (c)

   An $RrYy$ plant is crossed with a plant that has wrinkled green seeds. State the expected phenotypic ratio in the offspring and explain how it arises.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The ABO blood group in humans is controlled by one gene, $I$, with three alleles. The alleles $I^A$ and $I^B$ are codominant, and the allele $I^O$ is recessive to both. A woman with blood group A (genotype $I^AI^O$) and a man with blood group B (genotype $I^BI^O$) have children. In a separate paternity enquiry, a woman of blood group A has a child of blood group O.

   (a)

   Which genotype gives blood group AB?

   [1 mark]

   • A$I^AI^A$
   • B$I^AI^O$
   • C$I^BI^O$
   • D$I^AI^B$

   (b)

   Which phenotypes could the children of the woman of group A and the man of group B have?

   [1 mark]

   • AGroup A and group B only
   • BGroup AB only
   • CGroups A, B, AB and O
   • DGroups A, B and O only

   (c)

   A man with blood group AB is named as the father of the child of blood group O. Explain why he cannot be the father.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Haemophilia A is a blood-clotting disorder caused by a recessive allele, $h$, on the X chromosome. The dominant allele, $H$, codes for a working clotting factor. There is no corresponding gene on the Y chromosome. A woman who is a carrier and a man with normal clotting plan to have children.

   (a)

   Explain why haemophilia is much more common in males than in females.

   [3 marks]

   (b)

   Complete a fully labelled genetic diagram for this cross. State the probability that a child of this couple is a boy with haemophilia.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Fruit flies (Drosophila) are used to study inheritance. Body colour is controlled by a gene with alleles $G$ (grey, dominant) and $g$ (ebony). Wing length is controlled by a gene with alleles $N$ (normal, dominant) and $n$ (vestigial). These genes are on different autosomes. Eye colour is controlled by a gene on the X chromosome, with alleles $X^R$ (red, dominant) and $X^r$ (white).

   (a)

   Two flies heterozygous for body colour and wing length ($GgNn$) are crossed. Use a fully labelled genetic diagram to show the expected phenotypic ratio, and calculate the number of flies expected in each phenotypic class among 640 offspring.

   [6 marks]

   (b)

   A white-eyed female is crossed with a red-eyed male. Their offspring (F1) are interbred to give an F2. Using genetic diagrams, explain the phenotypes of the F1 and the expected phenotypes in the F2.

   [6 marks]

   Total for question 4: 12 marks

End of questions