Sex Chromosomes

In every body cell you have 23 pairs of chromosomes — 46 chromosomes in total. One of those 23 pairs is special: it is called the pair of sex chromosomes, and it controls whether you are female or male.

There are two kinds of sex chromosome, named after their shapes:

• the X chromosome, and
• the Y chromosome.

The rule you must know is simple:

Sex	Sex chromosomes
Female	XX (two X chromosomes)
Male	XY (one X and one Y chromosome)

So a female has two X chromosomes and a male has one X and one Y chromosome. The other 22 pairs are the same in males and females — it is only this one pair that determines sex.

Exam tip. Memorise it as a short phrase: "females XX, males XY". A common one-mark question simply asks for the sex chromosomes of a male or a female — write the two letters, e.g. XY for a male.

When the body makes gametes (sex cells — eggs in the female and sperm in the male), each gamete gets only one chromosome from each pair. So each gamete gets one sex chromosome.

Think about what this means for the X and Y:

• The mother is XX. Both of her sex chromosomes are X, so every egg she makes carries an X chromosome.
• The father is XY. He has one X and one Y, so half of his sperm carry an X and half carry a Y.

This is the key idea: the egg is always X, but the sperm can be X or Y. Because of this, it is the sperm that decides the sex of the baby:

• If an X sperm fertilises the egg → X + X = XX → girl.
• If a Y sperm fertilises the egg → X + Y = XY → boy.

Exam tip. If a question asks "which parent determines the sex of the child?", the answer is the father (the sperm), because only the father can pass on a Y chromosome.

To show how sex is decided at fertilisation, we draw a genetic diagram called a Punnett square. It puts the mother's gametes along one side and the father's gametes along the other, then fills in the combinations.

The cross is: mother XX × father XY.

• Mother's gametes (eggs): X and X.
• Father's gametes (sperm): X and Y.

Here is the Punnett square:

	X (egg)	X (egg)
X (sperm)	XX (girl)	XX (girl)
Y (sperm)	XY (boy)	XY (boy)

Reading the four boxes:

• Two boxes are XX → female (girl).
• Two boxes are XY → male (boy).

So the ratio is 2 XX : 2 XY, which simplifies to 1 girl : 1 boy — a 50% (½) chance of each sex at every fertilisation.

Here is the same cross drawn as a diagram:

Exam tip. When a question says "use a genetic diagram", you must draw the boxes — write the parents' chromosomes, the gametes, and the four offspring. State the final result clearly as 50% female (XX) and 50% male (XY), or a 1:1 ratio.

The Punnett square gives a prediction, not a guarantee. It tells you the probability (chance) of each sex, not exactly how many girls and boys a family will have.

Key points to get right:

• At every fertilisation there is a 50% (1 in 2, or ½) chance of a girl and a 50% chance of a boy.
• Each pregnancy is independent — the result of one fertilisation does not change the next one.
• That is why a family can have, for example, four boys in a row. Each time, the chance was still 50:50; it just happened that the Y sperm reached the egg each time.

Because the chances are equal, across the whole human population the numbers of males and females born are roughly equal (about 1:1). The large numbers smooth out the randomness.

Exam tip. If a couple already has three girls and a question asks for the chance the next baby is a boy, the answer is still 50% (½) — the previous children make no difference. Examiners love this "memory" trap.