Sex determination, sex linkage and haemophilia

Organisms (Continuity and change) · Inheritance · note 5 of 10

Spec D3.2.11, D3.2.12Download all Inheritance notes (PDF)Download all notes (PDF)13 pages

Sex determination, sex linkage and haemophiliaSpec D3.2.11, D3.2.12

In short

In humans the sex chromosome in the sperm, X or Y, determines whether a zygote develops male-typical or female-typical characteristics. The X chromosome carries far more genes than the Y, so males have only one copy of most X-linked genes. Haemophilia is a sex-linked recessive disorder, so it is much more common in males.

Human body cells have 22 pairs of autosomes and one pair of sex chromosomes: XX in female-typical and XY in male-typical individuals. Every egg carries an X. Half the sperm carry an X and half a Y, so the sex chromosome in the sperm determines the development of the zygote.

A gene on the Y chromosome (SRY) causes the embryonic gonads to develop into testes. Testosterone from the testes then leads to male-typical characteristics. Without it, ovaries and female-typical characteristics develop.
The X chromosome is much larger and carries far more genes than the Y chromosome, most of them nothing to do with sex.
Genes carried on the X chromosome are sex-linked. A male has only one X, so whichever allele he has on it is expressed, even if recessive.

Genetic diagram: mother XX and father XY; eggs all X, sperm X or Y; offspring XX, XX, XY, XY giving 2 girls to 2 boys. — Inheritance of sex: the sperm carries X or Y, so each child has a 50% chance of XX (female-typical) or XY (male-typical).

Haemophilia

In haemophilia the blood clots very slowly because a clotting protein (factor VIII in the commonest type) is missing. The gene is on the X chromosome. The normal allele is Xᴴ and the recessive haemophilia allele Xʰ.

Haemophilia genotypes
Genotype	Phenotype
XᴴXᴴ	Unaffected female
XᴴXʰ	Unaffected carrier female
XʰXʰ	Affected female (rare)
XᴴY	Unaffected male
XʰY	Affected male

Carrier mother and unaffected father

A carrier woman (XᴴXʰ) and an unaffected man (XᴴY) have children. Deduce the probabilities of the possible outcomes.

Mother's gametes: Xᴴ or Xʰ. Father's gametes: Xᴴ or Y.
Punnett grid: XᴴXᴴ, XʰXᴴ, XᴴY, XʰY.
Daughters: half unaffected, half carriers. Sons: half unaffected, half affected.

Answer: Probability a child is an affected son = 0.25; probability that a son is affected = 0.5; no daughters affected.

Common mistake:

A father cannot pass an X-linked allele to his sons, because sons receive his Y. Always write sex-linked alleles as superscripts on an uppercase X.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between genotype and phenotype?

Genotype is the combination of alleles an organism inherits, for example Aa. Phenotype is the observable traits of the organism, which result from its genotype and environmental factors. Some traits depend on genotype only, such as ABO blood group; others, such as height, depend on genes interacting with the environment.

What is the difference between codominance and incomplete dominance?

In codominance the heterozygote has a dual phenotype, with both alleles fully expressed, as in blood group AB with A and B antigens. In incomplete dominance the heterozygote has an intermediate phenotype, as in pink four o'clock flowers from red and white parents. Both give a 1:2:1 phenotype ratio in the F2.

Why are sex-linked disorders more common in males?

Sex-linked disorders such as haemophilia are caused by recessive alleles on the X chromosome. Males have only one X chromosome, so a single recessive allele is expressed. Females have two X chromosomes and are affected only if both carry the recessive allele, so they are usually unaffected carriers.

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