ScienceCBSE Class 10 · Heredity

Sex Determination in Human Beings

Updated 14 October 2026 · Fact-checked

In humans, sex is largely genetic. Of our 23 chromosome pairs, 22 are matched and one pair is the sex chromosomes: women are XX and men are XY. Every egg carries an X; a sperm carries either X or Y. A child who receives X from the father is a girl, and one who receives Y is a boy.

Understand Sex Determination in Humans

Different strategies in different animals. Species use very different ways to decide the sex of a newborn.

In some reptiles, the temperature at which fertilised eggs are kept decides whether the young will be male or female, an environmental cue.
In animals such as snails, individuals can change sex, which shows that sex is not genetically fixed in them.
In human beings, sex is largely genetically determined: whether a baby is a boy or a girl depends on the chromosomes it receives from its parents.

The puzzle. So far in heredity we assumed similar gene sets come from both parents. If so, how can genes make one child a boy and another a girl? The answer is that not all human chromosomes form perfect pairs.

Human chromosomes. Most human chromosomes come as a maternal and a paternal copy, and there are 22 such pairs. One more pair, the sex chromosomes, is not always a perfect pair:

women have a perfect pair, both called X: XX;
men have a mismatched pair, one normal-sized X and one short Y: XY.

Inheritance of X and Y. Germ cells receive one chromosome from each pair.

A mother (XX) can only pass an X to every egg.
A father (XY) passes X to half of his sperms and Y to the other half.
Egg (X) + sperm (X) → XX, girl; egg (X) + sperm (Y) → XY, boy.

So all children get an X from their mother, and the sex of the child is decided by what it inherits from the father. Since X-carrying and Y-carrying sperms are equally likely, on average half the children will be boys and half girls. Each birth is an independent event, so the chance of a boy (or a girl) is $\frac{1}{2}$ every time, whatever the sex of earlier children.

Why this matters socially. Because the father's chromosome decides the child's sex, blaming a mother for the sex of a child has no scientific basis. NCERT also points out, in the previous chapter, that a healthy society needs the female-male sex ratio to be maintained, and that prenatal sex determination is prohibited by law.

The syllabus asks only for a brief introduction to sex determination: the XX/XY system, the role of the father's chromosome, the 1 : 1 expectation, and examples of other strategies (temperature in some reptiles, sex change in snails).

Sex Determination in Humans: formulas and key facts

Human sex chromosomes: $female: X X; male: X Y$; Plus 22 matched pairs.
Gametes: $eggs: all X; sperms: \frac{1}{2} X, \frac{1}{2} Y$; Each germ cell gets one sex chromosome.
Cross: $X X \times X Y \to X X : X Y = 1 : 1$; Girls : boys = 1 : 1 on average.
Probability per birth: $P (boy) = P (girl) = \frac{1}{2}$; Independent of previous births.

How to solve Sex Determination in Humans questions

For sex determination questions, draw the cross.

1Write the parents: mother XX, father XY.
2List gametes: mother gives X only; father gives X or Y.
3Combine: X + X = XX (girl); X + Y = XY (boy).
4State the ratio 1 : 1 and that the father's chromosome decides the sex.
5For probability questions, remember each birth is independent: the chance is $\frac{1}{2}$ each time.
6For 'other animals' questions, give the reptile temperature example and the snail sex-change example.

Sex Determination in Humans in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Father's chromosome decides sex; each birth independent

Suppose a family has two daughters, and a relative claims that the mother 'can only have girls' and that the next child is now 'certain' to be a boy.

Use the genetics of sex determination to evaluate both claims, and give the probability that the third child is a boy.

Try it first, then show the solutionSolution

The mother is XX and passes an X to every egg, so she cannot decide the child's sex.
The father is XY; half his sperms carry X and half carry Y. The child's sex depends on which kind of sperm fertilises the egg.
So the claim that the mother 'can only have girls' has no scientific basis.
Each birth is an independent event: the chance of a Y-carrying sperm fertilising the egg is $\frac{1}{2}$ every time, regardless of earlier children.
So a boy is not 'certain'; the probability is $\frac{1}{2}$ .

Answer: Both claims are wrong. The father's chromosome decides the sex, and the chance of a boy is 1/2 for each birth, including the third.

Memory hook: Mum always gives X; Dad's X or Y decides; every birth is a fresh coin toss.

Tricks to remember Sex Determination in Humans

Short memory aids. Say each one aloud once, then use it on a question the same day.

Y for 'whY a boy'
Only a Y from the father makes a boy.
Use it when: Explaining who decides the sex.
22 + 1
22 matched pairs + 1 sex pair = 23 pairs.
Use it when: Chromosome count questions.
Mum = XX = only X
Two of the same letter can only give that letter.
Use it when: Listing gametes.

Sex Determination in Humans: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: How sex is decided · 3 marks · Short answerBoard level

How is the sex of a child determined in human beings? Show with a cross.

Try it first, then show the solutionSolution

Women have XX and men have XY sex chromosomes (1 mark).
Mother's eggs all carry X; father's sperms carry X or Y in equal numbers (1 mark).
X egg + X sperm = XX (girl); X egg + Y sperm = XY (boy). So the father's chromosome decides the sex; ratio 1 : 1 (1 mark).

Answer: XX × XY gives XX (girl) and XY (boy) in 1 : 1; the father's X or Y decides.

Example 2: Not always genetic · 2 marks · Very short answerBoard level

Give two examples to show that sex is not determined genetically in all animals. How is it different in human beings?

Try it first, then show the solutionSolution

In some reptiles, the temperature at which fertilised eggs are kept decides the sex; snails can change sex (1 mark).
In humans, sex is largely determined genetically by the sex chromosomes inherited (XX girl, XY boy) (1 mark).

Answer: Temperature in some reptiles; sex change in snails. Humans: genetic, by X and Y chromosomes.

Example 3: Counting chromosomes · 3 marks · Short answerHarder board level

A human body cell has 23 pairs of chromosomes. (a) How many are sex chromosomes? (b) How many chromosomes, and which sex chromosome(s), are in (i) an egg, (ii) a sperm? (c) Explain why the number in a zygote is the same as in the parents.

Try it first, then show the solutionSolution

(a) One pair (2 chromosomes); the other 22 pairs are matched (½ mark).
(b)(i) An egg has 23 chromosomes including one X. (ii) A sperm has 23 chromosomes including either an X or a Y (1½ marks).
(c) Each germ cell gets one chromosome from each pair, so 23 + 23 = 46 (23 pairs) in the zygote, the same as in the parents (1 mark).

Answer: (a) 1 pair. (b) Egg: 23 with X; sperm: 23 with X or Y. (c) 23 + 23 restores 23 pairs.

Example 4: Probability in a family · 3 marks · Short answerHOTS / competency

A couple already has three sons. (a) What is the probability that their fourth child will be a daughter? (b) Explain why. (c) A neighbour blames the mother for having only sons. Evaluate this using genetics.

Try it first, then show the solutionSolution

(a) $\frac{1}{2}$ (1 mark).
(b) Each birth is independent: the egg always carries X, and the chance that an X-carrying rather than a Y-carrying sperm fertilises it is $\frac{1}{2}$ every time (1 mark).
(c) The mother only contributes X; the child's sex is decided by whether the father's sperm carries X or Y, so blaming the mother has no scientific basis (1 mark).

Answer: (a) 1/2. (b) Independent events with equal X and Y sperms. (c) The mother does not decide the sex.

Example 5: Why boys and girls in roughly equal numbers · 3 marks · Short answerHarder board level

Explain, with a cross diagram, why the numbers of boys and girls born are expected to be roughly equal. What would be expected if a father produced twice as many X-carrying as Y-carrying sperms (hypothetically)?

Try it first, then show the solutionSolution

XX × XY: eggs X; sperms X and Y in equal numbers (1 mark).
X + X = XX girls, X + Y = XY boys, each half of the offspring: 1 : 1 (1 mark).
If X sperms were twice as many as Y sperms, then $\frac{2}{3}$ of children would be girls and $\frac{1}{3}$ boys, a 2 : 1 ratio, again showing the father's sperms decide the sex (1 mark).

Answer: Equal X and Y sperms give 1 : 1; with twice as many X sperms the expectation would be 2 girls : 1 boy.

Where students lose marks in Sex Determination in Humans

The slips examiners see most often, and what to write instead.

Mistake: Saying the mother determines the sex
A social myth carried into the answer.
Wrong working
The mother's egg decides whether the child is a boy or a girl.
Right working
All eggs carry X; the father's sperm (X or Y) decides.
Do this: Write the gametes of both parents before concluding.
Usually costs: Full marks for the question.
Mistake: Wrong gametes
Students write XY in a single sperm.
Wrong working
Father's sperm: XY.
Right working
Father's sperms: X or Y (one sex chromosome each).
Do this: Each gamete gets one chromosome from each pair.
Usually costs: 1 mark.
Mistake: Assuming earlier births change the chances
Gambler's-fallacy thinking.
Wrong working
After three girls, the next child must be a boy.
Right working
The probability is still 1/2.
Do this: Treat each fertilisation as independent.
Usually costs: 1 mark in HOTS questions.
Mistake: Over-detailed answers beyond the syllabus
Students add unrelated material.
Wrong working
Long discussion of genetic disorders.
Right working
Brief introduction: XX/XY, father's role, 1 : 1, other strategies.
Do this: The syllabus asks only for a brief introduction.
Usually costs: Time, and marks lost elsewhere.

Exam tips

Draw the XX × XY cross with gametes; it is quick and earns most of the marks.
Mention that sex in humans is 'largely genetically determined'.
Give the reptile (temperature) and snail (sex change) examples for 'other strategies'.
For probability, write 1/2 and the reason (independent events).
Keep the answer brief; the syllabus asks only for an introduction.

Practice questions on Sex Determination in Humans

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
A human male has which sex chromosomes in his body cells?
(a) XX (b) XY (c) YY (d) only X
Hint
Men have a mismatched pair.
Answer
(b) XY.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The sex of a child is determined by the father.
Reason (R): Half the sperms carry an X chromosome and half carry a Y chromosome, while all eggs carry X.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Which parent's gametes differ?
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Why is the Y chromosome described as 'short' and the pair in men as 'mismatched'? Which parent's chromosomes always form a perfect sex-chromosome pair?
Hint
Compare X and Y.
Answer
In men, one sex chromosome is a normal-sized X and the other is a short Y, so the two do not match. Women have two X chromosomes, a perfect pair.
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a wildlife centre incubates the eggs of a turtle species at different temperatures and finds that one temperature range produces mostly females and another mostly males. Meanwhile a biology teacher explains that in humans 'it is all in the chromosomes'.
(a) What determines sex in this turtle species? (1 mark)
(b) Name an animal that can change its sex. (1 mark)
(c) Contrast human sex determination with the turtle case, and explain why changing the temperature of a human embryo cannot change its sex. (2 marks)
Hint
Environmental cue vs genetic determination.
Answer
(a) The temperature at which the fertilised eggs are kept (an environmental cue). (b) Snail. (c) In humans sex is largely determined genetically: XX gives a girl and XY a boy, fixed at fertilisation by the sperm's X or Y. Temperature does not change the chromosomes the embryo has inherited, so it cannot change the sex.
Q5 · 2 marks · Very short answerHarder board level
A student says: 'A girl gets one X from her mother and one X from her father, so her father passed on his X. A boy also gets an X from his father.' Find and correct the error.
Hint
Which chromosome does a boy get from his father?
Answer
The first sentence is correct. The error is in the second: a boy gets his X from his mother and his Y from his father; if he received an X from his father he would be XX, a girl.
Q6 · 3 marks · Short answerHOTS / competency
In a town survey, suppose 1000 babies were born in a year. Using the genetics of sex determination, how many boys would you expect? Give two reasons why the actual number might differ slightly.
Hint
Expectation is 1/2; chance makes real counts vary.
Answer
Expected boys = $\frac{1}{2} \times 1000 = 500$ . Reasons for small differences: (1) which sperm fertilises an egg is a matter of chance, so real counts vary around the expected value, just as tossing a coin 1000 times rarely gives exactly 500 heads; (2) factors after fertilisation, such as the survival of embryos, can affect the numbers actually born. (A large and persistent gap could point to social causes such as illegal sex selection, which is prohibited by law.)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Sex chromosomes of a human female?Show answer
XX.
2.How many matched pairs of chromosomes (besides sex chromosomes) do humans have?Show answer
22.
3.Which parent's chromosome decides the child's sex?Show answer
The father's (X or Y in the sperm).
4.In which animals does temperature decide sex?Show answer
In some reptiles.
5.Which animal can change sex?Show answer
Snail.
6.Expected ratio of girls to boys?Show answer
1 : 1.

Sex Determination in Humans: frequently asked questions

How is sex determined in humans?

By sex chromosomes. Women are XX and men XY. The egg always carries X; if the sperm carries X the child is a girl (XX), and if it carries Y the child is a boy (XY).

Who decides the sex of the baby, the mother or the father?

The father's sperm, which can carry either X or Y. The mother always contributes an X.

Is sex genetically determined in all animals?

No. In some reptiles the incubation temperature of eggs decides the sex, and snails can change sex.

Why are boys and girls born in roughly equal numbers?

Half of a father's sperms carry X and half carry Y, so each is equally likely to fertilise the egg.

How much of sex determination is in the syllabus?

A brief introduction: the XX/XY system, the father's role, and examples of other strategies in animals.