Original practice · 5 questions · 15 marks · about 17 minutes

Codominance and Blood Groups Practice Questions

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These questions test your understanding of codominance and the inheritance of ABO blood groups, which involve multiple alleles.

1. Question 1

   Paper 4 Extended 3 marks Difficulty 3/5

   Define codominance and give one example.

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   1. Codominance occurs when both alleles are equally expressed in the phenotype of a heterozygote.
   2. Neither allele is dominant over the other: both contribute to the phenotype.
   3. Example: in some flowers, a red (C^R) × white (C^W) cross produces pink (C^R C^W) flowers.

2. Question 2

   Paper 4 Extended 4 marks Difficulty 4/5

   ABO blood groups are controlled by three alleles: I^A, I^B, and i. I^A and I^B are codominant; both are dominant to i. A man with blood group A (I^A i) marries a woman with blood group B (I^B i). What are the possible blood groups of their children?

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   1. Parents: I^A i × I^B i.
   2. Offspring genotypes: I^A I^B, I^A i, I^B i, ii.
   3. Phenotypes: AB, A, B, O.
   4. All four blood groups are possible: ratio 1:1:1:1.

3. Question 3

   Paper 4 Extended 3 marks Difficulty 3/5

   Explain why a person with blood group AB has the genotype I^A I^B.

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   1. Both I^A and I^B alleles are codominant: both are expressed.
   2. The person produces both A antigens and B antigens on their red blood cells.
   3. This gives the AB blood group phenotype.

4. Question 4

   Paper 4 Extended 2 marks Difficulty 3/5

   A child has blood group O. What must be true about the parents' genotypes?

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   1. Blood group O is genotype ii (homozygous recessive).
   2. Both parents must carry at least one i allele: each parent contributes one i allele to the child.

5. Question 5

   Paper 4 Extended 3 marks Difficulty 5/5

   In snapdragons, flower colour shows codominance. Red (C^R C^R) × white (C^W C^W) produces pink (C^R C^W). Cross two pink snapdragons and give the phenotype ratio of the offspring.

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   1. Cross: C^R C^W × C^R C^W.
   2. Offspring: C^R C^R (red), C^R C^W (pink), C^R C^W (pink), C^W C^W (white).
   3. Phenotype ratio: 1 red: 2 pink: 1 white.