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Exam questions · Biology · Inheritance

Inheriting Sex

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

    State the sex chromosomes in (a) a human female body cell, and (b) a human male body cell.

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    Model answer

    (a) XX. (b) XY.

    Mark scheme

    • XX — 1 mark
    • XY — 1 mark
  2. 2 State [2 marks]

    State the sex chromosomes that are in a sperm cell, and in an egg cell.

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    Model answer

    A sperm cell contains either an X or a Y chromosome. An egg cell always contains an X chromosome.

    Mark scheme

    • Sperm: X or Y — 1 mark
    • Egg: X — 1 mark
  3. 3 Explain [3 marks]

    Explain why the sex of a baby is decided by the father's gamete and not the mother's.

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    Model answer

    All of the eggs made by the mother carry an X chromosome. The father makes two types of sperm, one that carries X and one that carries Y. The type of sperm that fertilises the egg decides if the baby is a girl (XX) or a boy (XY).

    Mark scheme

    • All eggs carry X — 1 mark
    • Sperm carry either X or Y — 1 mark
    • Which sperm fertilises decides the sex — 1 mark
  4. 4 Draw [4 marks]

    Draw a genetic diagram to show how the sex of a child is inherited.

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    Model answer

    Parents: XX and XY. Gametes: X and X from the mother, X and Y from the father. Offspring: XX, XY, XX, XY, which is 2 girls and 2 boys, a ratio of 1 : 1.

    Mark scheme

    • Parents XX and XY — 1 mark
    • Correct gametes — 1 mark
    • Correct offspring XX, XY, XX, XY — 1 mark
    • Ratio 1 : 1 / 50% each — 1 mark
  5. 5 Calculate [3 marks]

    A couple have three daughters. Calculate the probability that their fourth child is a girl, and the probability that all four children are girls. Give your answers as fractions.

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    Model answer

    The fourth child has a probability of \(\frac{1}{2}\) of being a girl. The probability of all four being girls is \(\frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} = \frac{1}{16}\).

    Mark scheme

    • Probability of a girl is 1/2 — 1 mark
    • Multiplies four probabilities — 1 mark
    • 1/16 — 1 mark
  6. 6 Evaluate [6 marks]

    A newspaper article claims that the mother decides whether a baby is a boy or a girl. Use your knowledge of genetics to evaluate this claim.

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    Model answer

    The claim is incorrect. A mother is XX, so all of her eggs carry an X chromosome, and she can only pass on an X. The father is XY, so half of his sperm carry an X and half carry a Y. If a sperm with a Y fertilises the egg, the baby is a boy (XY), and if one with an X does, it is a girl (XX). So the sex is decided by the sperm, which is random. The chance of each is 50%, with a ratio of 1 : 1. The mother has no way to decide which sperm fertilises her egg.

    Mark scheme

    • Claim is incorrect — 1 mark
    • All eggs carry X — 1 mark
    • Sperm carry X or Y — 1 mark
    • Sperm that fertilises decides the sex — 1 mark
    • Random / 50% chance each — 1 mark
    • Mother has no control — 1 mark

Quick check

  1. 1

    Which sex chromosomes does a human male have?

    1. AXX
    2. BXY
    3. CYY
    4. DX only
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    B: XY

    Males have one X and one Y chromosome, XY.

  2. 2

    Which sex chromosome do all eggs have?

    1. AX
    2. BY
    3. CX or Y
    4. DNeither
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    A: X

    A woman is XX, so every egg has an X chromosome.

  3. 3

    What is the probability that a baby is a girl?

    1. A25%
    2. B75%
    3. C50%
    4. D100%
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    C: 50%

    The chances of XX and XY are equal, so a girl has a 1 in 2 chance, or 50%.

  4. 4

    Which gamete decides the sex of the baby?

    1. AThe egg
    2. BBoth equally
    3. CNeither
    4. DThe sperm
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    D: The sperm

    The sperm carries either X or Y, so it decides the sex.

  5. 5

    What is the sex ratio of the offspring from XX x XY?

    1. A3 : 1
    2. B1 : 1
    3. C2 : 1
    4. D1 : 3
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    B: 1 : 1

    Two of four boxes are XX and two are XY, so the ratio is 1 : 1.

  6. 6

    A couple have four boys. What is the chance that the fifth child is a boy?

    1. A1 in 2
    2. B1 in 5
    3. C1 in 32
    4. D0
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    A: 1 in 2

    Each child is an independent event, so the chance is still 1 in 2.

  7. 7

    Which genotype is a baby boy?

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

    A boy has one X chromosome and one Y chromosome.

  8. 8

    What does the SRY gene do?

    1. AMakes eggs
    2. BMakes the X chromosome
    3. CMakes a baby a girl
    4. DStarts the development of testes
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    D: Starts the development of testes

    The SRY gene on the Y chromosome starts the development of male features.

  9. 9

    What is the probability that three babies in a row are girls?

    1. A1 in 6
    2. B1 in 8
    3. C1 in 4
    4. D1 in 3
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    B: 1 in 8

    Each has a probability of 1/2, so \(\frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} = \frac{1}{8}\).