Edexcel IGCSE Biology · Spec 3.26-3.27
Sex Determination
How the X and Y sex chromosomes determine whether offspring are male or female.
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Sex Determination
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Explained
Sex determination, and drawing the diagram that earns the marks
A human body cell contains 46 chromosomes, arranged as 23 pairs. Twenty two of those pairs are the same in everyone. The twenty third pair is the sex chromosomes, and it is the one that differs.
A female has two X chromosomes. A male has one X and one Y. So the full description of a female cell is 22 pairs of other chromosomes plus two X chromosomes, and that longer wording is often what a question is after.
Why it is always the father
Gametes are made by meiosis and carry one chromosome from each pair.
A woman has XX, so every egg she produces carries an X. There is no other option available.
A man has XY, so half his sperm carry an X and half carry a Y.
The egg therefore always supplies an X, and whether the offspring is XX or XY depends entirely on which sperm arrives first. The sex of a child is decided by the father, and by chance.
The fifty fifty ratio
Because half the sperm carry X and half carry Y, the probability of each outcome is one half, which is 50 per cent, or 1 in 2.
Every fertilisation is independent of every other. A couple with three daughters are no more likely to have a son next time than they were the first time. The probability is one half for each pregnancy separately, and expecting it to even out is the gambler's error rather than biology.
To find the chance of a particular run, multiply the separate probabilities. Four girls in a row is one half multiplied by itself four times, which is one sixteenth, or 6.25 per cent.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme sets a multiple choice question on the chromosomes in a female cell, and the correct answer is two X chromosomes and 22 pairs of other chromosomes. The three wrong options are wrong in instructive ways: one gives a single X, one gives a single Y, and one gives 22 pairs of other chromosomes but two Y chromosomes.
Two of those are testing whether you noticed the word pairs, and one is testing whether you know which chromosome a female has two of. A quick answer of the X chromosome would not distinguish between them.
The genetic diagram is then worth three marks: the parent genotypes as XX and XY, the gametes as X, and X or Y, and the offspring as XX and XY. The mark scheme allows all three to come from a Punnett square, so the grid earns as much as a written cross.
Three instructions sit beside it, and each closes a different route to losing a mark.
No credit for other letters. Sex determination uses X and Y specifically. Substituting the B and b of an ordinary monohybrid cross gives the right ratio for the wrong reason, and scores nothing.
Gametes must be either circled or clearly separated. Writing XXY in a row does not show which gamete is which.
And no error carried forward. On most calculations a mistake early on is forgiven later; here it is not, because each line of the diagram is a separate claim about a fixed biological fact rather than a step in a calculation.
Setting the diagram out
Label the parents, giving the mother XX and the father XY.
Show the gametes on the line below: X and X from the mother, X and Y from the father, each circled or clearly spaced.
Combine them, either by drawing lines to each of the four combinations or by filling a two by two Punnett square, and write the four offspring genotypes: XX, XX, XY, XY.
Then state the ratio in words. Two out of four, so a 50 per cent chance of a female and a 50 per cent chance of a male, or a ratio of 1 to 1.
That final sentence is often a mark of its own. The diagram shows the outcome; the sentence says what it means, and questions ask for the probability rather than for the grid.
Spec 3.26-3.27
What you need to know
- Know that sex is controlled by the sex chromosomes
- Recall that females are X X and males are X Y
- Use a genetic diagram to show the fifty-fifty ratio
Active recall
Quick check
Answer each question before opening the answer.
What are the sex chromosomes of a human male and female?
Male: XY. Female: XX.
What is the probability that a baby is a boy?
50% (1 in 2).
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