6 min readConcept sheet1-mark MCQ and assertion-reason; a 2 or 3-mark question with a flow chart; often a 4-mark case-based question

Sex determination in human beings

Human sex is set by genes: women are XX, men XY. The father's sperm, carrying X or Y, decides whether the child is a girl or boy.

Human chromosomes23 pairs: 22 matching pairs + 1 pair of sex chromosomes

FemaleXX: a perfect pair

MaleXY: one X and a shorter Y

Chance of a boy or a girlEqual (50% each) in every birth

Different ways of deciding sex

Animals use different ways to decide sex. In some reptiles, the temperature at which the fertilised eggs are kept decides whether the young will be male or female. In some animals, such as snails, an individual can change its sex, so sex there is not fixed by genes. In human beings, sex is largely decided by genes inherited from the parents.

How sex is determined in humans

1
Human body cells have 23 pairs of chromosomes: 22 pairs match, and 1 pair is the sex chromosomes.
2
A woman has a perfect pair, both called X: she is XX.
3
A man has a mismatched pair: one normal-sized X and a short Y: he is XY.
4★
Every egg carries an X. Half the sperms carry X, and half carry Y.
5★
Egg (X) + sperm (X) → XX: girl. Egg (X) + sperm (Y) → XY: boy.
6
So the sex of the child depends on the father's sperm, and in every birth a boy or a girl is equally likely.

★ key step

Sex determination chart

Mother's egg	Father's sperm	Zygote	Child
X	X	XX	Girl
X	Y	XY	Boy

X chromosome vs Y chromosome

X chromosomeNormal-sized. Women have two, men have one. Every egg and half the sperms carry it.

Y chromosomeShort. Only men have it. Half the sperms carry it, and a child who gets it is a boy.

How to draw the sex determination chart

Write Parents: mother (XX) × father (XY).
Below, draw the gametes in circles: X from the mother; X and Y from the father.
Draw arrows joining the gametes to the zygotes: XX and XY.
Label XX as girl and XY as boy, and write the ratio 1 : 1 (50% each).

In real life

Some people wrongly blame a mother for not having a son. Science shows that the father's sperm decides the sex of the child, no parent can choose it, and every birth has an equal chance of a girl or a boy.

Exam angle

2 marks: “How is the sex of a child determined in human beings?” Mothers give X in every egg; fathers give X or Y. X from the father gives a girl (XX); Y gives a boy (XY).

3 marks: “Show with a flow chart that the sex of a child depends on the father.” XX × XY; gametes X and X, Y; zygotes XX (girl) and XY (boy) in a 1 : 1 ratio; only the father's gametes differ.

1 marks: “Name the sex chromosomes of a human male.” X and Y.

Remember it

Mother always gives X; father decides: X + X = girl, X + Y = boy.