Sex Determination

This lesson covers how biological sex is determined in humans by the sex chromosomes (XX and XY), as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the role of the sex chromosomes, use a Punnett square to predict the sex of offspring and understand the probability of having a boy or a girl.

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Chromosomes and Sex

Human body cells contain 46 chromosomes arranged in 23 pairs. Of these 23 pairs:

• 22 pairs are autosomes — they carry genes for general body characteristics.
• 1 pair are the sex chromosomes — they determine the biological sex of the individual.

The Sex Chromosomes

Sex	Chromosomes	Gametes produced
Female	XX	All eggs carry an X chromosome
Male	XY	Sperm carry either an X or a Y chromosome

Key points:

• Females are XX — they have two X chromosomes.
• Males are XY — they have one X and one Y chromosome.
• The Y chromosome is much smaller than the X chromosome and carries fewer genes.
• The Y chromosome carries the SRY gene (sex-determining region Y), which triggers the development of male characteristics.

Exam Tip: Remember that it is the sperm that determines the sex of the baby, because only sperm can carry a Y chromosome. The egg always contributes an X.

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Sex Determination Using a Punnett Square

Since the mother is XX and the father is XY, we can use a Punnett square to predict the sex of any offspring:

	X (from mother)	X (from mother)
X (from father)	XX (girl)	XX (girl)
Y (from father)	XY (boy)	XY (boy)

Results

Outcome	Genotype	Ratio
Girl	XX	2 in 4
Boy	XY	2 in 4

• Probability of a girl: 50% (1 in 2)
• Probability of a boy: 50% (1 in 2)
• Ratio: 1 girl : 1 boy

graph TD
    A["Mother (XX)"] -->|"All eggs carry X"| C["Fertilisation"]
    B["Father (XY)"] -->|"Sperm carry X or Y"| C
    C -->|"X + X"| D["XX = Female"]
    C -->|"X + Y"| E["XY = Male"]

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Why There Is a 50:50 Ratio

The 50:50 ratio of boys to girls occurs because:

1. The mother can only contribute an X chromosome (she is XX).
2. The father can contribute either an X or a Y chromosome (he is XY).
3. There is an equal chance that a sperm carrying X or a sperm carrying Y will fertilise the egg.
4. Therefore, each pregnancy has a 50% chance of producing a boy and a 50% chance of producing a girl.

Exam Tip: The 50:50 ratio is a theoretical probability. In a real family, having two boys does not make a girl more likely next time — each event is independent. This is a common misconception tested in exams.

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Sex-Linked Inheritance (Higher Tier Context)

Some genes are carried on the X chromosome but not on the smaller Y chromosome. These are called sex-linked genes. Because males have only one X chromosome, they are more likely to be affected by recessive X-linked conditions.

Example: Colour Blindness

• The gene for colour vision is on the X chromosome.
• The allele for colour blindness is recessive (X^b).
• The allele for normal colour vision is dominant (X^B).

Genotype	Phenotype
X^B X^B	Normal female
X^B X^b	Carrier female (normal vision)
X^b X^b	Colour-blind female (rare)
X^B Y	Normal male
X^b Y	Colour-blind male

Males only need one copy of the recessive allele to be affected because the Y chromosome does not carry a corresponding allele. This is why colour blindness is much more common in males.

Punnett Square: Carrier Female x Normal Male

	X^B	X^b
X^B	X^B X^B (normal girl)	X^B X^b (carrier girl)
Y	X^B Y (normal boy)	X^b Y (colour-blind boy)

Results:

• 25% normal girl
• 25% carrier girl (unaffected)
• 25% normal boy
• 25% colour-blind boy

Exam Tip: When drawing a Punnett square for sex-linked conditions, always write the full genotype including X and Y (e.g. X^B Y, not just Y). This shows the examiner that you understand where the allele is located.

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Summary

• Biological sex is determined by the sex chromosomes: XX = female, XY = male.
• The father's sperm determines the sex of the child (carries either X or Y).
• A Punnett square shows a 50:50 (1:1) ratio of boys to girls.
• Each pregnancy is an independent event — the probability is always 50%.
• Sex-linked conditions (e.g. colour blindness) are carried on the X chromosome and affect males more often.
• Males only need one copy of a recessive X-linked allele to be affected because the Y chromosome is smaller.

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Worked Example: Sex Ratio Calculation

A couple have three children, all girls. What is the probability that a fourth child will be a boy?

• Each pregnancy is an independent event.
• The father's sperm determines the sex — half carry X, half carry Y.
• The probability of a boy on any single pregnancy is always 0.5 (50%).
• The three previous girls do not alter this probability.

Common mistake: Writing that the fourth child is "more likely to be a boy to balance things out." This is an example of the gambler's fallacy — it is not supported by biology. Each fertilisation is independent.

Probability of specific family combinations

The probability of having four girls in a row is 0.5 × 0.5 × 0.5 × 0.5 = 0.0625 = 6.25% (about 1 in 16). Rare, but certainly possible — and the probability of the next child being a girl is still 50%.

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Comparison Table: Autosomal vs Sex-Linked Inheritance