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Exam questions · Biology · Inheritance

Codominance and ABO Blood Groups

  • 6 exam questions
  • 20 marks
  • 9 quick checks
  1. 1 State [2 marks]

    State the genotypes that result in (a) blood group AB, and (b) blood group O.

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    Model answer

    (a) \(I^A I^B\). (b) \(ii\).

    Mark scheme

    • AB: \(I^A I^B\) — 1 mark
    • O: \(ii\) — 1 mark
  2. 2 Explain [2 marks]

    Explain what is meant by multiple alleles.

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    Model answer

    A gene has more than two alleles in a population. For the ABO gene there are three alleles, but each person has only two.

    Mark scheme

    • More than two alleles — 1 mark
    • Example with each person only having two — 1 mark
  3. 3 Explain [3 marks]

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

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    Model answer

    The alleles \(I^A\) and \(I^B\) are codominant. This means that both alleles show their effect, so the red blood cells have both A antigens and B antigens, making the blood group AB.

    Mark scheme

    • The alleles are codominant — 1 mark
    • Both alleles are expressed — 1 mark
    • Both antigens are present — 1 mark
  4. 4 Draw [4 marks]

    A man has blood group AB, and a woman has blood group O. Use a genetic diagram to find the possible blood groups of their children.

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    Model answer

    The man is \(I^A I^B\), and the woman is \(ii\). The man's gametes are \(I^A\) and \(I^B\), and the woman's are all \(i\). The children are \(I^A i\) (group A) or \(I^B i\) (group B), half of each.

    Mark scheme

    • Correct parent genotypes — 1 mark
    • Correct gametes — 1 mark
    • Correct offspring genotypes — 1 mark
    • Groups A and B in a 1 : 1 ratio — 1 mark
  5. 5 Explain [3 marks]

    A couple, one with group A blood and one with group B blood, have a child with group O. Explain how this is possible.

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    Model answer

    Both parents must be heterozygous, with the genotypes \(I^A i\) and \(I^B i\). Each parent passes on the recessive \(i\) allele, so the child has the genotype \(ii\), which is blood group O.

    Mark scheme

    • Both parents heterozygous — 1 mark
    • Each passes on the allele i — 1 mark
    • Child is ii, so group O — 1 mark
  6. 6 Evaluate [6 marks]

    Compare dominant and recessive inheritance with codominance, using the ABO blood group system as an example, and say why the ABO system is described as showing multiple alleles.

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    Model answer

    In complete dominance one allele hides the effect of the other in a heterozygous person, so the heterozygote looks like the homozygous dominant. An example is the \(I^A\) allele with the recessive \(i\): a person with \(I^A i\) has group A and not O. In codominance both alleles show their effect, so the heterozygote has a different phenotype from either homozygote. The genotype \(I^A I^B\) gives group AB, with both A and B antigens. The ABO system shows multiple alleles because the gene has three alleles, \(I^A\), \(I^B\) and \(i\), in the population, even though each person has only two of them.

    Mark scheme

    • Dominance: one allele hides the other — 1 mark
    • Example of dominance, such as \(I^A i\) is A — 1 mark
    • Codominance: both alleles shown — 1 mark
    • \(I^A I^B\) is AB — 1 mark
    • Three alleles in the population — 1 mark
    • Each person has only two — 1 mark

Quick check

  1. 1

    How many alleles does the ABO blood group gene have in the population?

    1. ATwo
    2. BFour
    3. CThree
    4. DOne
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    C: Three

    There are three alleles: \(I^A\), \(I^B\) and \(i\).

  2. 2

    Which genotype gives blood group AB?

    1. A\(I^A I^A\)
    2. B\(I^A I^B\)
    3. C\(I^B i\)
    4. D\(ii\)
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    B: \(I^A I^B\)

    The codominant alleles \(I^A\) and \(I^B\) together give group AB.

  3. 3

    Which genotype gives blood group O?

    1. A\(I^A i\)
    2. B\(I^B i\)
    3. C\(I^A I^B\)
    4. D\(ii\)
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    D: \(ii\)

    Group O needs two copies of the recessive allele, \(ii\).

  4. 4

    What does codominance mean?

    1. ABoth alleles show their effect
    2. BOne allele hides the other
    3. CNeither allele has an effect
    4. DThere are three chromosomes
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    A: Both alleles show their effect

    In codominance both alleles show their effect in the heterozygous person.

  5. 5

    Which blood groups can a child of an AB parent and an O parent have?

    1. AAB or O
    2. BA or B
    3. CA, B, AB or O
    4. DO only
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    B: A or B

    The AB parent passes on \(I^A\) or \(I^B\), and the O parent passes on \(i\), so the child is A or B.

  6. 6

    A person has the genotype \(I^A i\). What is their blood group?

    1. AA
    2. BB
    3. CAB
    4. DO
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    A: A

    \(I^A\) is dominant to \(i\), so the person is group A.

  7. 7

    A group A parent and a group B parent have a group O child. What are the parents' genotypes?

    1. A\(I^A I^A\) and \(I^B I^B\)
    2. B\(I^A I^B\) and \(ii\)
    3. C\(I^A i\) and \(I^B i\)
    4. D\(I^A I^A\) and \(I^B i\)
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    C: \(I^A i\) and \(I^B i\)

    The child is \(ii\), so each parent has an \(i\) allele: \(I^A i\) and \(I^B i\).

  8. 8

    A red cow and a white bull produce roan calves. What is this an example of?

    1. AComplete dominance
    2. BMutation
    3. CSex linkage
    4. DCodominance
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    D: Codominance

    The calf shows both red and white hairs, so both alleles show their effect: codominance.

  9. 9

    Why can a group AB person not have a group O child?

    1. AAB parents always have AB children
    2. BAn AB parent has no i allele to pass on
    3. CGroup O is dominant
    4. DBlood groups cannot be inherited
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    B: An AB parent has no i allele to pass on

    A group AB person is \(I^A I^B\), so can only pass on \(I^A\) or \(I^B\), and the child cannot be \(ii\).