7 min readPathway sheetSection A MCQ (1 mark); Section B short answer (3 marks) or long answer (5 marks) with a line diagram

Sex determination in humans

Women have 44 + XX chromosomes and men 44 + XY. Every egg carries X; a sperm carries X or Y, so chance alone decides a girl or a boy.

Female body cell44 autosomes + XX

Male body cell44 autosomes + XY

GametesEggs: one kind, 22 + X. Sperms: two kinds, 22 + X or 22 + Y

The law against sex testsThe Pre-natal Diagnostic Techniques (Regulation and Prevention of Misuse) Act, 1994, in force from 1 January 1996, bans finding out or telling the sex of a foetus

How the sex of a baby is decided

1
Human body cells have 46 chromosomes: 44 autosomes and 2 sex chromosomes.
2
A woman's body cells are 44 + XX; a man's are 44 + XY.
3★
Gametes are haploid (they have n, half the number). Eggs are of one kind only: 22 + X.
4★
Sperms are of two kinds: 22 + X and 22 + Y.
5
Egg (22 + X) + X sperm (22 + X) → zygote 44 + XX: girl. Egg (22 + X) + Y sperm (22 + Y) → zygote 44 + XY: boy. The zygote is diploid (2n).
6
Which kind of sperm reaches the egg first is purely a matter of chance, so a boy and a girl are equally likely in every birth.

★ key step

Sex determination chart

Egg	Sperm	Zygote	Baby
22 + X	22 + X	44 + XX	Girl
22 + X	22 + Y	44 + XY	Boy

X chromosome vs Y chromosome

X chromosomeCarries many genes, some of them needed for survival. Women have two, men have one. Every egg and half the sperms carry it.

Y chromosomeCarries few genes, mainly those for maleness, such as the testes determining factor. Only men have it. A baby who gets it is a boy.

No one is to blame

In some families a woman is wrongly blamed for not having a son. Science shows that the sex of a child depends on which kind of sperm joins the egg, and that is pure chance. Neither parent can choose it, and neither can be blamed. Science expects about equal numbers of girls and boys. A sex ratio with far fewer girls points to female foeticide, the killing of girls before birth. To stop it, India passed the Pre-natal Diagnostic Techniques (Regulation and Prevention of Misuse) Act, 1994, renamed in 2003 as the Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act. It bans finding out or telling the sex of a foetus, and any advertisement for such tests. Those who break it face jail and a fine.

How to draw the line diagram of sex determination

Write Parents on the first line: mother 44 + XX × father 44 + XY.
Below, draw the gametes in circles: 22 + X from the mother; 22 + X and 22 + Y from the father.
Draw lines joining the egg to each kind of sperm, and write the zygotes: 44 + XX and 44 + XY.
Label 44 + XX as girl and 44 + XY as boy, and write 1 : 1 under them.

In real life

Doctors in India are not allowed to tell parents the sex of an unborn baby. Knowing that chance decides the sex helps families stop blaming mothers and value daughters and sons equally.

Exam angle

1 marks: “What chromosomes does a human egg carry?” 22 autosomes and one X chromosome (22 + X).

2 marks: “A family blames the mother because their three children are all girls. Using what you know about eggs and sperms, explain why this blame is unfair.” Every egg carries X. The sex depends on whether an X or a Y sperm joins the egg, which is chance, so the mother cannot decide it.

3 marks: “Using a labelled chart of gametes and zygotes, show why a couple has an equal chance of having a son or a daughter.” Mother 44 + XX gives only 22 + X eggs; father 44 + XY gives 22 + X and 22 + Y sperms. Egg + X sperm = 44 + XX girl; egg + Y sperm = 44 + XY boy. Two equal outcomes: 1 : 1.

Remember it

Mother always gives X; chance picks the sperm: X + X = girl, X + Y = boy.