Sex Determination

The sex chromosomes

Humans have 23 pairs of chromosomes; 22 pairs are autosomes and one pair is the sex chromosomes. Females have two X chromosomes (XX).

Males have one X and one Y chromosome (XY). The Y chromosome carries the genes that trigger male development.

Which parent determines the sex of the offspring

A female (XX) can only produce eggs carrying an X chromosome, since both her sex chromosomes are X. A male (XY) produces two types of sperm in equal numbers: half carry an X chromosome and half carry a Y chromosome.

It is therefore the father's sperm that determines whether the offspring is male or female.

Worked Punnett square

Reading the result

The Punnett square shows a genotype and phenotype ratio of 1:1 between male and female offspring. This is because the mother always contributes an X chromosome, while the father contributes an X or a Y with equal likelihood, so each fertilisation has an equal chance of producing a boy or a girl.

Worked problem 1, easy: probability of a boy

Question. State the two types of sperm a man produces and give the probability that a given child is a boy.

Model answer. A man (XY) produces X-carrying and Y-carrying sperm in equal numbers. The mother's egg always carries an X, so a Y sperm makes a boy (XY) and an X sperm makes a girl (XX).

The probability that a given child is a boy is therefore 1/2, one in two.

Worked problem 2, medium: two children

Question. A couple plan to have two children. Using a grid of equally likely outcomes, work out the probability that they have one boy and one girl in either order.

Model answer. Each fertilisation is independent, so two children give four equally likely combinations, shown below. Two of the four combinations (girl then boy, boy then girl) give one of each, so the probability of one boy and one girl is 2/4 = 1/2.

The probability of two girls is 1/4 and of two boys is 1/4.

Worked problem 3, SPM-level: a family of four daughters

Question. A couple has four children, all daughters. (a) Draw a genetic diagram (genotypes, gametes and offspring) to show how the sex of a child is determined.

(b) Explain why the couple can still have a fifth child who is a daughter.

Model answer. (a) Parents: mother XX x father XY. Gametes: the mother forms only X eggs; the father forms X and Y sperm in equal numbers.

The offspring are XX (daughter) and XY (son) in a 1 : 1 ratio, as the grid shows. (b) The 1 : 1 ratio is a probability for each pregnancy, not a fixed outcome for one family.

Every fertilisation is independent of the ones before it, so the chance the fifth child is a daughter is still 1/2 regardless of the four daughters already born.

Practice problems

• 1. A woman is expecting one child. State the probability that the child is a girl and explain your answer in one sentence.
• 2. Using the idea of gametes, explain why the number of boys and girls born in a large population is close to equal.
• 3. A family has two children. List the possible combinations of sexes and state the probability that both children are the same sex.

Practice answers

How it is examined

SPM questions may ask you to state the sex chromosome combinations for males and females, draw a Punnett square to show sex determination, or explain why the father's gamete determines the offspring's sex.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions