Exam questions · Biology · Genetic Inheritance
Sex Determination
- 8 exam questions
- 22 marks
- 8 quick checks
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1 State [2 marks]
State the sex chromosomes found in an ordinary human body cell from a female and from a male.
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Model answer
A female has two X chromosomes (XX). A male has one X and one Y chromosome (XY).
Mark scheme
- Female: XX — 1 mark
- Male: XY — 1 mark
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2 Describe [2 marks]
Ordinary human body cells contain 23 pairs of chromosomes. Describe the job of the 23 pairs of chromosomes.
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Model answer
22 pairs control characteristics only. One pair carries the genes that determine sex.
Mark scheme
- 22 pairs control characteristics only — 1 mark
- One pair determines sex — 1 mark
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3 Name [2 marks]
The diagram shows how the sex of a baby is inherited. State the sex chromosomes carried by every egg, and by the sperm.
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Model answer
Every egg carries an X chromosome. Half of the sperm carry an X chromosome and half carry a Y chromosome.
Mark scheme
- Every egg carries X — 1 mark
- Sperm carry X or Y — 1 mark
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4 Explain [3 marks]
Explain why the father, and not the mother, determines the sex of a baby.
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Model answer
The mother is XX, so every egg she makes carries an X chromosome. The father is XY, so half of his sperm carry an X and half carry a Y. The sperm that fertilises the egg decides whether the baby is XX (a girl) or XY (a boy).
Mark scheme
- Mother's eggs all carry X — 1 mark
- Sperm carry X or Y — 1 mark
- The sperm that fertilises the egg decides the sex — 1 mark
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5 Calculate [2 marks]
In one year 2 400 babies were born in a town. Use the sex ratio from a genetic cross to estimate how many of the babies were boys.
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Model answer
1 200 boys. The ratio of girls to boys is 1 : 1, so half of the babies are boys, and \(\frac{1}{2} \times 2400 = 1200\).
Mark scheme
- Ratio 1 : 1, so half are boys — 1 mark
- 1 200 — 1 mark
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6 Explain [3 marks]
A couple have five daughters and are expecting a sixth child. The father says, "It must be a boy this time." Explain why he is wrong.
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Model answer
Each baby is the result of a separate fertilisation. The chance that the sperm which fertilises the egg carries a Y chromosome is the same every time, 50%. So the sixth child still has a 50% chance of being a girl.
Mark scheme
- Each fertilisation is a separate event — 1 mark
- The chance is 50% every time — 1 mark
- Earlier children do not change the chance — 1 mark
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7 Describe [4 marks]
Use a genetic cross to show how sex is inherited in humans.
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Model answer
The mother is XX, so she makes only X eggs. The father is XY, so he makes X sperm and Y sperm. The Punnett square gives XX, XY, XX and XY. XX is female and XY is male, so the ratio of girls to boys is 1 : 1, or a 50% chance of each.
Mark scheme
- Mother XX and father XY — 1 mark
- Gametes X, X and X, Y — 1 mark
- Offspring XX, XY, XX, XY — 1 mark
- Ratio 1 : 1 (50% girls and 50% boys) — 1 mark
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8 Explain [4 marks]
In a family with four children, the ratio of girls to boys was 3 : 1. In a country with 20 million births, the ratio was almost exactly 1 : 1. Explain these results.
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Model answer
The ratio from a genetic cross is 1 : 1, but each fertilisation is a separate chance event with a 50% chance of a girl. In a family with only four children, chance variation can easily give a different ratio such as 3 : 1. In a population of 20 million births the chance variation is much smaller relative to the total, so the ratio is very close to the expected 1 : 1.
Mark scheme
- Expected ratio is 1 : 1 — 1 mark
- Each fertilisation is a chance event — 1 mark
- With small numbers, chance can give a different ratio — 1 mark
- With very large numbers the ratio is close to the expected one — 1 mark
Quick check
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1
How many pairs of chromosomes in a human body cell carry the genes that determine sex?
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A: One
One of the 23 pairs carries the genes that determine sex. The other 22 pairs control characteristics only.
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2
Which pair of sex chromosomes does a human male have?
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C: XY
Males have one X and one Y chromosome.
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3
Which sex chromosome is carried by every egg cell?
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B: X
The mother is XX, so every egg carries an X chromosome.
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4
A sperm cell carrying a Y chromosome fertilises an egg. What is the sex of the baby?
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D: A boy
The baby is XY, so a boy.
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5
What is the expected ratio of girls to boys from a genetic cross?
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A: 1 : 1
The Punnett square gives two XX to two XY, which simplifies to 1 : 1.
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6
A couple have had three sons. What is the chance that their next child is a daughter?
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C: 50%
Each fertilisation is independent, so the chance is 50% every time.
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7
Why can a very large population have almost equal numbers of males and females even though a single family may not?
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B: Because chance variation averages out in large numbers
Each birth is a chance event with a 50% probability, and the effect of chance averages out over large numbers.
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8
Which statement about human chromosomes is correct?
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D: 22 pairs control characteristics only
22 of the 23 pairs control characteristics only, and the last pair determines sex.