How sex is determined

Male or female?

Sexual reproduction needs two kinds of individuals, males and females. So something must decide whether a new individual becomes male or female. Different species use different ways.

The environment decides, in some animals

• In some reptiles, the temperature at which the fertilised eggs are kept decides the sex. In some turtles, warmer nests give more females.
• Some snails can change their sex. So in them, sex is not fixed by the genes at all.

In these animals, the sex of a young one is not set at fertilisation.

Genes decide, in human beings

In human beings the sex of a child is decided genetically. Look at the $23$ pairs of chromosomes in a body cell.

• 22 pairs are the same in men and women. Both chromosomes of each pair match.
• The last pair is the sex chromosomes.
• Women have a matched pair, XX.
• Men have a mismatched pair: one normal-sized X and one short Y.

The sperm decides

Now think about the gametes. Each gamete carries one sex chromosome.

• A woman is XX, so every egg carries an X.
• A man is XY, so half his sperm carry an X, and half carry a Y.

• An egg fertilised by an X sperm gives XX, a girl.
• An egg fertilised by a Y sperm gives XY, a boy.

So a child always gets an X from the mother. The sex is decided by what it gets from the father. The mother cannot decide the sex of the child, because all her eggs are the same. Blaming a mother for the birth of a daughter is unscientific and unfair.

An equal chance every time

The X sperm and the Y sperm are made in equal numbers. So at every birth the chance of a girl is one half, and the chance of a boy is one half.

The chance does not change with the children already born. A couple with three daughters still has an even chance of a son next time. In a large town, about half the babies are girls and half are boys.

Switch the sperm between X and Y and the child changes from XX to XY. With $100$ births, about $50$ girls and $50$ boys are expected. A real count is rarely exact, as the program below shows.

Try it in Python

The program picks a sperm at random for each of a thousand births.

You should see $511$ girls and $489$ boys: close to one half, but not exact. That is what chance looks like.

Practise with new numbers

Common mistakes

• Thinking the mother decides the sex. All her eggs carry an X; the sperm decides.
• Thinking a couple with daughters is due for a son. Each birth has an even chance, whatever came before.
• Thinking all animals decide sex by genes. In some reptiles the temperature decides it, and some snails change sex.
• Thinking the Y chromosome is the same size as the X. The Y is much shorter.

Remember these

• Sex may be decided by the environment (some reptiles, snails) or by genes (human beings).
• Humans: $22$ matched pairs and one pair of sex chromosomes.
• Women are XX; men are XY.
• Every egg has an X; a sperm has an X or a Y. An X sperm gives a girl, a Y sperm a boy.
• Each birth has an equal chance of a girl or a boy.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which sex chromosomes does a man have?

2. Why can the mother not decide the sex of her child?

3. A sperm carrying a Y fertilises an egg. Will the child be a girl or a boy?

4. Name one animal in which the environment decides the sex.

5. A couple has two sons. What is the chance that their third child is a daughter?