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Exam questions · Biology · Inheritance

Single Gene Inheritance and Punnett Squares

  • 6 exam questions
  • 19 marks
  • 9 quick checks
  1. 1 State [1 mark]

    State the genotype of a female human, in terms of sex chromosomes.

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    Model answer

    XX.

    Mark scheme

    • XX — 1 mark
  2. 2 Give [2 marks]

    A plant that is heterozygous for height is crossed with a plant of the same genotype. Give the ratio of phenotypes expected in the offspring if tall (T) is dominant to dwarf (t).

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    Model answer

    3 tall : 1 dwarf.

    Mark scheme

    • Tt x Tt gives TT, Tt, Tt, tt — 1 mark
    • 3 tall : 1 dwarf — 1 mark
  3. 3 Explain [3 marks]

    Explain why a woman who is a carrier of cystic fibrosis (Ff) and a man who is a carrier (Ff) can have a child who does not carry the allele.

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    Model answer

    Each parent makes F and f gametes. If both of them pass on the F allele, the child is FF, which does not carry the f allele. There is a 1 in 4 chance of this.

    Mark scheme

    • Each makes F and f gametes — 1 mark
    • Both pass on F — 1 mark
    • Child is FF, with a 1 in 4 chance — 1 mark
  4. 4 Calculate [3 marks]

    Two heterozygous black rabbits (Bb) have 80 offspring. Black is dominant to white. Calculate the number of white rabbits that you would expect.

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    Model answer

    Bb x Bb gives 1 in 4 bb, so \(\frac{1}{4} \times 80 = 20\) white rabbits.

    Mark scheme

    • 1 in 4 are white — 1 mark
    • 80 ÷ 4 — 1 mark
    • 20 — 1 mark
  5. 5 Describe [4 marks]

    Use a genetic cross to show that the probability that a baby is a girl is 50%.

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    Model answer

    The mother is XX, so every egg has an X. The father is XY, so half of the sperm carry X and half carry Y. A Punnett square gives XX, XY, XX and XY. Two of the four are XX, which is a girl, so the probability is 2 in 4, which is 50%.

    Mark scheme

    • Mother XX, eggs all X — 1 mark
    • Father XY, sperm X or Y — 1 mark
    • Punnett square XX, XY, XX, XY — 1 mark
    • 2 in 4, which is 50% girls — 1 mark
  6. 6 Evaluate [6 marks]

    A couple already have three daughters. They think that their fourth child is much more likely to be a boy. Evaluate this idea, using a genetic cross.

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    Model answer

    The mother is XX, so all of her eggs carry an X. The father is XY, so half of his sperm carry an X and half carry a Y. The Punnett square gives two XX and two XY, so the chance of a boy is 1 in 2, or 50%. Each fertilisation is a separate event, and the sperm that fertilises the egg is random. The earlier babies do not change the chance, because the gametes have no memory. So the idea is wrong, and the chance of a fourth child being a boy is still 50%, the same as for each of the earlier children. The chance of four girls in a row is 1 in 16, but this was only a matter of chance.

    Mark scheme

    • Mother XX, all eggs carry X — 1 mark
    • Father XY, half of the sperm carry X and half Y — 1 mark
    • Chance of a boy is 50% — 1 mark
    • Fertilisation is random — 1 mark
    • Earlier children do not change the probability — 1 mark
    • A reasoned conclusion — 1 mark

Quick check

  1. 1

    What are the gametes of an individual with the genotype Tt?

    1. ATT only
    2. Btt only
    3. CT and t
    4. DTt only
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    C: T and t

    T and t.

  2. 2

    What is the genotype ratio of Tt x Tt?

    1. A1 TT : 2 Tt : 1 tt
    2. B1 Tt : 1 tt
    3. CAll Tt
    4. D3 TT : 1 tt
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    A: 1 TT : 2 Tt : 1 tt

    1 TT : 2 Tt : 1 tt.

  3. 3

    What is the probability of a recessive phenotype in Tt x Tt?

    1. A50%
    2. B75%
    3. C100%
    4. D25%
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    D: 25%

    1 in 4, which is 25%.

  4. 4

    What are all of the offspring of TT x tt?

    1. ATT
    2. BTt
    3. Ctt
    4. DA mixture
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    B: Tt

    All are Tt.

  5. 5

    What are the sex chromosomes of a male?

    1. AXX
    2. BYY
    3. CXY
    4. DX
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    C: XY

    XY.

  6. 6

    What proportion of sperm carry a Y chromosome?

    1. AHalf
    2. BAll
    3. CNone
    4. DA quarter
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    A: Half

    Half.

  7. 7

    Two carriers (Ff) have 40 children. How many would you expect to have the disorder (ff)?

    1. A0
    2. B4
    3. C20
    4. D10
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    D: 10

    1 in 4 of 40 is 10.

  8. 8

    What is a Punnett square used for?

    1. ATo count chromosomes
    2. BTo predict the results of a cross
    3. CTo measure growth
    4. DTo find the size of a gene
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    B: To predict the results of a cross

    To predict the results of a genetic cross.

  9. 9

    A tall pea plant is crossed with a dwarf plant, and about half of the offspring are dwarf. What is the genotype of the tall plant?

    1. ATT
    2. Btt
    3. CTt
    4. DImpossible to tell
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    C: Tt

    Half dwarf means the tall plant must be heterozygous, Tt.