BiologyICSE Class 10 · Genetics

Sex Determination in Humans and X-linked Inheritance of Haemophilia and Colour Blindness (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Women are XX and make only X-carrying eggs; men are XY and make X- or Y-carrying sperms in equal numbers, so the father's sperm decides a child's sex. Haemophilia and colour blindness are caused by recessive genes on the X chromosome, so a son is affected with one copy but a daughter needs two.

All eggs carry X, so the father's sperm (X or Y) decides the sex, with a 50% chance of a girl or a boy.

Illustration drawn for QuestionBuddy.

Understand Sex Determination and X-linked Diseases

Autosomes and sex chromosomes. A human body cell has 46 chromosomes in 23 pairs: 22 pairs of autosomes, the same in both sexes, and 1 pair of sex chromosomes. A female has two similar sex chromosomes, XX; a male has two unlike ones, XY. The Y is shorter and carries the genes that make the embryo develop as a male.

Female body cell: 44 + XX; every egg: 22 + X.
Male body cell: 44 + XY; sperms: 22 + X or 22 + Y, in equal numbers.

Sex determination at fertilisation.

Egg (down), sperm (across)	22 + X	22 + Y
22 + X	44 + XX girl	44 + XY boy

So the sex of the child is decided by the sperm, that is, by the father: an X-bearing sperm gives a girl and a Y-bearing sperm gives a boy. The chance of a boy or a girl is 1 : 1 (50% each) at every birth, and each birth is independent of the previous ones. Blaming a mother for the sex of her children has no scientific basis.

X-linked (sex-linked) inheritance. The X chromosome carries many genes that have no matching gene on the small Y. A recessive allele on the X therefore behaves differently in the two sexes:

A male has one X. If it carries the recessive allele, he shows the disease (XʰY). There is no second X to mask it.
A female has two X chromosomes. With one recessive allele she is normal but a carrier (XᴴXʰ); she is affected only with two (XʰXʰ).
A son always receives his X from his mother (and Y from his father). A daughter receives one X from each parent. So an affected son's allele came from his mother, and an affected father passes his allele to all his daughters (who become carriers) and to none of his sons.

Haemophilia ('bleeder's disease'). Caused by a recessive allele on the X. A protein needed for blood clotting is not formed properly, so blood clots very slowly and even a small cut or tooth extraction can lead to prolonged bleeding. Much more common in males.

Colour blindness (red–green). Caused by a recessive allele on the X. The person cannot distinguish red from green, because the cones that respond to these colours do not work normally. Much more common in males.

The four crosses to know (H normal, h affected; the same pattern for colour blindness):

Mother × Father	Daughters	Sons
carrier XᴴXʰ × normal XᴴY	all normal; half carriers	half affected, half normal
normal XᴴXᴴ × affected XʰY	all carriers	all normal
carrier XᴴXʰ × affected XʰY	half affected, half carriers	half affected, half normal
affected XʰXʰ × normal XᴴY	all carriers	all affected

Flow chart: how a grandfather's allele reaches a grandson. Affected grandfather (XʰY) → gives Xʰ to every daughter → daughter is a carrier (XᴴXʰ) → passes Xʰ to half her sons → affected grandson. The trait seems to skip a generation and passes through a carrier woman.

Draw-and-label checklist: sex determination chart. Parents: 'Father 44 + XY' and 'Mother 44 + XX'; arrow 'meiosis' to gametes: sperms 22 + X and 22 + Y, eggs 22 + X (two circles of the same kind); a 2 × 2 grid or crossing lines to four zygotes: 44 + XX (girl), 44 + XY (boy), 44 + XX (girl), 44 + XY (boy); final line 'girls : boys = 1 : 1'.

Sex Determination and X-linked Diseases: key facts

Human chromosome sets: Female body cell 44 + XX
Male body cell 44 + XY
Egg 22 + X only
Sperm 22 + X or 22 + Y, equal numbers; So the sperm decides the sex.
Genotypes for an X-linked recessive disease: Normal female XᴴXᴴ
Carrier female XᴴXʰ
Affected female XʰXʰ
Normal male XᴴY
Affected male XʰY; Never write Hh for an X-linked gene.
Carrier mother × normal father: $X^{H} X^{h} \times X^{H} Y \to X^{H} X^{H}, X^{H} X^{h}, X^{H} Y, X^{h} Y$; Half the sons affected; half the daughters carriers; 1/4 of all children affected.
Two X-linked diseases in the syllabus: Haemophilia: blood fails to clot normally, prolonged bleeding | Colour blindness: cannot distinguish red and green; Both recessive, both on the X, both commoner in males.

Female body cell	44 + XX
Male body cell	44 + XY
Egg	22 + X only
Sperm	22 + X or 22 + Y, equal numbers

Normal female	XᴴXᴴ
Carrier female	XᴴXʰ
Affected female	XʰXʰ
Normal male	XᴴY
Affected male	XʰY

How to answer Sex Determination and X-linked Diseases questions

ICSE asks who decides sex, what a zygote with Y becomes, why these diseases are commoner in men, and simple crosses with probabilities for sons and daughters.

1Write each person's genotype with the allele as a superscript on X; the Y carries no allele.
2Find each parent's gametes: a woman gives one of her two X's; a man gives X or Y.
3Make a 2 × 2 Punnett square and label each box girl or boy and normal, carrier or affected.
4Read the question: 'chance that a son is affected' counts among sons only; 'chance that a child is an affected son' counts among all four children.
5For family stories, work out unknown genotypes first: an affected son means his mother has at least one Xʰ; an affected father means every daughter is at least a carrier.

Sex Determination and X-linked Diseases in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Who decides the sex of a baby

Suppose a relative tells Mrs Das, who has two daughters, that 'she cannot give a son'.

Use chromosomes to explain whose gamete decides the sex of a child and the chance that her third child is a boy.

Try it first, then show the solutionSolution

Mrs Das (44 + XX) can make only one kind of egg: 22 + X.
Her husband (44 + XY) makes two kinds of sperm, 22 + X and 22 + Y, in equal numbers.
An egg fertilised by a Y-bearing sperm develops into a boy; by an X-bearing sperm, into a girl.
Each fertilisation is independent, so the chance of a boy in the next pregnancy is still 1/2.

Answer: The father's sperm decides the sex; the chance of a boy is 1/2 each time, regardless of earlier children.

Memory hook: Mother always gives X; father tosses the coin.

A carrier in the family

Suppose Neha's father is red–green colour blind, but Neha and her mother have normal vision. Neha marries Sameer, who is also colour blind.

What is the chance that their son will be colour blind, and that their daughter will?

Try it first, then show the solutionSolution

Neha's father (XⁿY) gave his only X, Xⁿ, to Neha; her mother gave Xᴺ, so Neha is XᴺXⁿ, a carrier.
Sameer is XⁿY.
Sons get X from Neha (Xᴺ or Xⁿ) and Y from Sameer: half XᴺY normal, half XⁿY colour blind.
Daughters get Xⁿ from Sameer and Xᴺ or Xⁿ from Neha: half XᴺXⁿ (carriers with normal vision), half XⁿXⁿ colour blind.

Answer: Each son has a 1/2 chance of colour blindness, and so does each daughter, because the father gives every daughter an Xⁿ.

Memory hook: Carrier mother plus affected father: daughters can be affected too.

Tricks to remember Sex Determination and X-linked Diseases

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

Y makes the boy
No Y, no boy: a zygote with a Y from the father is always male.
Use it when: 1-mark questions on the Y chromosome.
Sons from Mum, daughters from both
A son's X is always his mother's.
Use it when: Tracing X-linked diseases.
One X, no backup
Males have one X and no spare to cover a faulty gene.
Use it when: Why X-linked diseases are commoner in males.

Sex Determination and X-linked Diseases: solved examples

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

Example 1: The Y chromosome · 2 marks · Short answerBoard level

A zygote has received a Y chromosome from the father. (a) Into what will it develop? (b) What is the sex-chromosome make-up of the egg that formed it? (A question of this type was asked in 2024.)

Try it first, then show the solutionSolution

(a) A boy (male), since the zygote is 44 + XY (1 mark).
(b) Eggs always carry 22 + X, so the egg had an X (1 mark).

Answer: (a) A boy (b) 22 + X.

Example 2: Why commoner in males · 2 marks · Short answerBoard level

Give a reason why haemophilia is much more common in men than in women.

Try it first, then show the solutionSolution

The haemophilia allele is recessive and lies on the X chromosome; men have only one X, so a single allele makes them affected (1 mark).
Women have two X chromosomes and need the allele on both; with one they are only carriers (1 mark).

Answer: Men have one X, so one recessive allele shows; women need two.

Example 3: Carrier × normal man · 3 marks · Short answerHarder board level

A woman who is a carrier for colour blindness marries a man with normal vision. Show the cross and give the expected percentage of (a) colour-blind daughters, (b) colour-blind sons, (c) carrier daughters.

Try it first, then show the solutionSolution

Let N = normal vision and n = colour blind: mother XᴺXⁿ, father XᴺY (½ mark).
Gametes: Xᴺ, Xⁿ and Xᴺ, Y; offspring XᴺXᴺ, XᴺXⁿ, XᴺY, XⁿY (1 mark).
(a) Daughters: every daughter gets Xᴺ from the father, so 0% are colour blind (½ mark). (b) Sons: half XⁿY, 50% (½ mark). (c) Half the daughters XᴺXⁿ carriers, 50% (½ mark).

Answer: (a) 0% (b) 50% (c) 50%.

Example 4: Sex determination chart · 3 marks · Long answerBoard level

With the help of a chart, explain how sex is determined in human beings.

Try it first, then show the solutionSolution

Parents: mother 44 + XX, father 44 + XY; gametes: eggs all 22 + X, sperms 22 + X and 22 + Y in equal numbers (1 mark).
Fertilisation: 22 + X egg + 22 + X sperm → 44 + XX girl; 22 + X egg + 22 + Y sperm → 44 + XY boy (1 mark).
Conclusion: the type of sperm decides the sex; ratio of girls to boys 1 : 1 (1 mark).

Answer: Chart from 44 + XX and 44 + XY to gametes to 44 + XX (girl) and 44 + XY (boy); the sperm decides; 1 : 1.

Example 5: Working backwards in a family · 3 marks · Case-basedHOTS / competency

Suppose a couple with normal blood clotting have a son with haemophilia. (a) Which parent passed on the allele? Give genotypes of both parents. (b) What is the chance that their next child will be a haemophilic son? (c) Could their daughter have haemophilia? Explain.

Try it first, then show the solutionSolution

(a) A son's X comes from his mother, so the mother is a carrier XᴴXʰ; the father is normal XᴴY (1 mark).
(b) Children: XᴴXᴴ, XᴴXʰ, XᴴY, XʰY; one of four is a haemophilic son: 1/4 (1 mark).
(c) No: every daughter receives Xᴴ from the normal father, so she is at most a carrier (1 mark).

Answer: (a) Mother; XᴴXʰ and XᴴY. (b) 1/4. (c) No, the father's Xᴴ protects every daughter.

Example 6: Three girls in a row · 2 marks · Short answerHOTS / competency

Suppose a couple's first three children are girls. (a) What is the probability that a family of three children are all girls? (b) What is the probability that the fourth child is a boy? Explain the difference.

Try it first, then show the solutionSolution

(a) Each birth has P(girl) = 1/2, independently: 1/2 × 1/2 × 1/2 = 1/8 (1 mark).
(b) 1/2, because the next sperm is X- or Y-bearing with equal chance whatever happened before; (a) asks about a whole sequence, (b) about one new event (1 mark).

Answer: (a) 1/8 (b) 1/2.

Where students lose marks in Sex Determination and X-linked Diseases

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

Mistake: Answering 'boy or girl' for a zygote with a Y
CISCE examiners' comments (2024): for a zygote with a Y chromosome inherited from the father, candidates answered either boy or girl, ignoring that the Y chromosome determines a boy.
Do this: Any zygote with a Y is male (44 + XY).
Usually costs: 1 mark.
Mistake: Writing X-linked genotypes as Hh
Autosomal notation hides the fact that males have only one allele, so crosses come out wrong.
Wrong working
Haemophilic man hh × carrier woman Hh.
Right working
Haemophilic man XʰY × carrier woman XᴴXʰ.
Do this: Always put the allele on the X and leave the Y bare.
Usually costs: 1-2 marks.
Mistake: Confusing 'among sons' with 'among all children'
Students give 1/4 when the question asks for the chance that a son is affected.
Do this: Underline 'son', 'daughter' or 'child' in the question before answering.
Usually costs: 1 mark.
Mistake: Not recognising genetic disorders
CISCE examiners' comments (2024): examiners advised giving examples of genetic and non-genetic disorders after a question asking which of four quoted examples were genetic disorders.
Do this: Haemophilia and colour blindness are genetic (inherited); diseases caused by germs or diet are not.
Usually costs: 1 mark.

Exam tips

Show sex determination as a chart with chromosome numbers (44 + XX).
Use the word carrier for XᴴXʰ women.
Give probabilities as fractions or percentages and say for whom.
Keep to X-linked examples; the syllabus covers only haemophilia and colour blindness.

Practice questions on Sex Determination and X-linked Diseases

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
The sex chromosomes of a human male are:
(a) XX (b) XY (c) XO (d) YY
Hint
Heteromorphic pair.
Answer
(b) XY.
Q2 · 1 mark · Very short answerBoard level
Name two X-linked diseases of human beings.
Hint
Both in the syllabus.
Answer
Haemophilia and colour blindness.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A haemophilic father cannot pass haemophilia to his son.
Reason (R): A son receives the Y chromosome, not the X, from his father.
(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
Where is the allele?
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
A normal woman (not a carrier) marries a haemophilic man. What will be the condition of (a) their sons, (b) their daughters?
Hint
XᴴXᴴ × XʰY.
Answer
(a) All sons normal (XᴴY), since they get the father's Y. (b) All daughters carriers (XᴴXʰ), normal themselves.
Q5 · 2 marks · Short answerHarder board level
How many autosomes and how many sex chromosomes are present in (a) a human sperm, (b) a human egg? What differs between sperms?
Hint
Halve 44 + XY.
Answer
(a) 22 autosomes + 1 sex chromosome. (b) 22 autosomes + 1 sex chromosome (always X). Sperms differ in carrying X or Y.
Q6 · 2 marks · Short answerHarder board level
Suppose a colour-blind woman marries a man with normal vision. Predict the vision of their sons and daughters.
Hint
Every son gets one of her X's.
Answer
XⁿXⁿ × XᴺY: all sons XⁿY, colour blind; all daughters XᴺXⁿ, normal vision but carriers.
Q7 · 2 marks · Short answerHOTS / competency
Suppose a woman's brother has haemophilia but both her parents are healthy. She marries a healthy man. What is the probability that her first son will have haemophilia?
Hint
Is she a carrier? Chance 1/2.
Answer
Her brother got Xʰ from their mother, so the mother is a carrier; the woman is a carrier with probability 1/2. If she is, each son has a 1/2 chance. Overall 1/2 × 1/2 = 1/4.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose in a family, Arun (colour blind) marries Priya (normal vision, not a carrier). Their daughter Isha marries Vikram (normal vision).
(a) Give the genotypes of Isha and Vikram. (1)
(b) What fraction of Isha's sons are expected to be colour blind? (1)
(c) Why is none of Arun's own sons colour blind? (1)
Hint
Isha received Arun's X.
Answer
(a) Isha XᴺXⁿ (carrier), Vikram XᴺY. (b) 1/2. (c) Arun's sons receive his Y, not his X; their X comes from Priya, who has normal alleles.

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.How many pairs of autosomes do humans have?Show answer
22.
2.Which parent determines the sex of the child?Show answer
The father (through the sperm).
3.What is the genotype of a carrier woman for haemophilia?Show answer
XᴴXʰ.
4.What is the probability that a couple's child is a girl?Show answer
1/2.
5.From whom does a colour-blind boy inherit the allele?Show answer
His mother.

Sex Determination and X-linked Diseases: frequently asked questions

Who decides the sex of the baby?

The father. Eggs all carry X; sperms carry X or Y. A Y-bearing sperm gives a boy, an X-bearing sperm a girl.

Why are haemophilia and colour blindness more common in males?

Their genes are recessive and on the X chromosome; males have one X, so a single faulty allele shows, while females need two.

Can a woman have haemophilia?

Yes, but only if she receives the allele from both parents (a haemophilic father and a carrier or affected mother), which is very rare.

What is a carrier?

A woman with one normal and one recessive allele on her X chromosomes; she is healthy but can pass the allele to her children.