Genetic crosses and inheritanceEdexcel A-Level Biology B: Subtopic test

1. 1
   A mother with blood group A and a father with blood group B have a first child with blood group O. The ABO blood group gene has three alleles: $I^{A}$ and $I^{B}$, which are codominant, and $I^{O}$, which is recessive to both.

   (a)

   Which term describes the relationship between the alleles $I^{A}$ and $I^{B}$?

   [1 mark]

   • A$I^{A}$ is recessive to $I^{B}$
   • BLinkage
   • CCodominance
   • D$I^{B}$ is dominant to $I^{A}$

   (b)

   What is the probability that the next child of these parents has blood group AB?

   [1 mark]

   • A$\frac{1}{4}$
   • B$\frac{1}{2}$
   • C$\frac{3}{4}$
   • D0

   (c)

   Explain how two parents with blood groups A and B can have a child with blood group O.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In pea plants, round seeds (R) are dominant to wrinkled seeds (r) and yellow seeds (Y) are dominant to green seeds (y). The two genes are on different chromosomes. A plant heterozygous for both genes has the genotype RrYy.

   (a)

   A plant of genotype RrYy is crossed with a plant of genotype rryy. What is the expected phenotypic ratio of the offspring?

   [1 mark]

   • A3:1
   • B1:1:1:1
   • C9:3:3:1
   • D1:2:1

   (b)

   Two plants of genotype RrYy are crossed. What fraction of the offspring are expected to be round and green?

   [1 mark]

   • A$\frac{1}{16}$
   • B$\frac{9}{16}$
   • C$\frac{1}{4}$
   • D$\frac{3}{16}$

   (c)

   State the genotypes of the four types of gamete made by a plant of genotype RrYy and explain why they are produced in equal numbers.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Haemophilia A is caused by a recessive allele, $X^{h}$, on the X chromosome. The dominant allele, $X^{H}$, gives normal blood clotting. A woman who is a carrier of haemophilia ($X^{H}X^{h}$) has a child with a man who does not have haemophilia.

   (a)

   Construct a genetic cross to show the genotypes of the possible children and state the probability that the next child will have haemophilia.

   [3 marks]

   (b)

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

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In Drosophila, grey body (G) is dominant to black body (g) and long wing (L) is dominant to vestigial wing (l). A female that inherited G and L together on one chromosome and g and l together on the homologous chromosome was test crossed with a black-bodied, vestigial-winged male. The 1000 offspring were: 415 grey body long wing, 405 black body vestigial wing, 90 grey body vestigial wing and 90 black body long wing.

   (a)

   Explain these results.

   [6 marks]

   (b)

   A chi-squared test was used to test the null hypothesis that the genes assort independently. Calculate the value of chi-squared, using $\chi^{2} = \sum \frac{(O-E)^{2}}{E}$, and state and justify a conclusion. The critical value at $p = 0.05$ with 3 degrees of freedom is 7.82.

   [6 marks]

   Total for question 4: 12 marks

End of questions