4.27: Sex determination

LEARNING GOALS

By the end of this lesson

Construct an XX × XY inheritance diagram.
Explain which gamete determines XX or XY.
Interpret the approximately 1:1 probability.

BIG QUESTION

Why is the probability of XX or XY approximately equal at each fertilisation?

Eggs normally contribute X, while meiosis produces similar numbers of X- and Y-bearing sperm.

The egg contributes X

In the standard model, meiosis in an XX parent produces eggs carrying one X chromosome.

Meiosis in an XY parent produces X-bearing and in approximately equal proportions.

The fertilising sperm determines the combination

An plus an X-bearing egg produces an XX zygote. A Y-bearing sperm plus an X-bearing egg produces an XY zygote.

Therefore the sperm contribution determines whether the zygote is XX or XY; it is not controlled by parental behaviour.

The ratio is probabilistic

gives an approximately 1/2 probability of each combination for each conception in the simple model.

Previous children do not change the next conception’s probability, and real population ratios can depart slightly from 1:1 for biological and sampling reasons.

Sex-chromosome cross

Combine the single egg type with two sperm types.

XX/XY determination
Gametes	Zygote	Common developmental association
X egg + X sperm	XX.	Typically female.
X egg + Y sperm	XY.	Typically male.
Probability per conception	About 1/2 each.	Independent event.
Source of difference	Sperm X or Y.	Paternal gamete contribution.

KEY TERMS

Language that earns precision

X-bearing sperm: Sperm containing an X chromosome.
Y-bearing sperm: Sperm containing a Y chromosome.
sex determination: Biological processes establishing pathways of sexual development.
random fertilisation: Fusion in which any compatible gamete may combine by chance.

Common misconceptions

Claim: The egg determines whether a zygote is XX or XY.

Correction: Eggs normally carry X; sperm contribute X or Y.

Claim: After several sons, a daughter becomes due.

Correction: Each conception is an independent event.

Claim: A parent can control the chromosome carried by the fertilising sperm through ordinary behaviour.

Correction: Fertilisation is probabilistic and not a matter of blame or choice.

EXAM LANGUAGE

Match the answer to the command word

Draw the cross

XX parent produces X eggs; XY parent produces X and Y sperm.

Interpret

State “approximately 50% for each conception”, not a guaranteed family ratio.

CHECK YOUR UNDERSTANDING

Recall, apply, then justify

01Explain4 marks

Use a genetic cross to explain the approximately 1:1 probability of XX and XY zygotes.

Hint

Gamete types and two combinations.

Suggested marking points

The XX parent produces eggs carrying X.
The XY parent produces approximately equal numbers of X- and Y-bearing sperm.
X sperm produce XX and Y sperm produce XY at fertilisation.
The two combinations therefore each have about a 1/2 probability.

02Explain2 marks

Explain why the sex-chromosome combination of one child does not change the probability for the next child.

Hint

Independent meiosis and fertilisation.

Suggested marking points

New gametes are produced and combine independently for each conception.
Previous fertilisation outcomes do not alter the X:Y sperm probability.

STRETCH AND CHALLENGE

A family has four children with XY chromosome combinations. Evaluate the claim that this proves Y-bearing sperm are produced more often.

Planning hint

Small samples and probability.

What a strong design includes

Each conception has an approximate 1/2 XY probability in the simple model.
Four XY outcomes can occur by chance.
The probability is 1/2 raised to the fourth power.
This equals 1/16 or 6.25%, so it is possible.
A family of four is a very small sample.
Large, representative sperm or birth data would be needed to test a systematic bias.
The observation alone does not prove unequal sperm production.

END OF THE LESSON

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