CourseCourseQuestionQuestionDocumentDocument

D3.2.12—Haemophilia

Haemophilia is an X-linked recessive disorder, so carrier females can have affected sons and affected males pass alleles through daughters.

Syllabus
First assessment 2025
Objective
D3.2.12
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2021
Most common paperPaper2
Typical marks1–8

Common command terms

  • Identify
  • Deduce
  • Explain
  • Outline
  • State
  • Predict

Recent exam appearances

November 2021Paper2 ["HL"] · TZ02(a)(ii)[ 1 ]D3.2.12—Haemophilia
November 2019Paper1 ["HL"] · TZ012[ 1 ]D3.2.12—Haemophilia
November 2013Paper2 ["HL"] · TZ02(b)[ 2 ]D3.2.12—Haemophilia
May 2013Paper2 ["HL"] · TZ16(c)[ 8 ]D3.2.12—Haemophilia

Coverage 2013–2021 · Updated 16 Jul 2026

X-Linked Recessive Alleles Are Unmasked in Most XY Genotypes

Most genes in the non-homologous region of X have no matching allele on Y. An XY individual is therefore hemizygous for those X-linked genes: the single allele on X is expressed even if it is recessive.

Genotype Typical status for an X-linked recessive allele h
XᴴXᴴ unaffected XX
XᴴXʰ unaffected carrier XX
XʰXʰ affected XX
XᴴY unaffected XY
XʰY affected XY
  • a son receives his X chromosome from his mother and Y from his father
  • a father passes his X chromosome to every daughter and to no son
  • an affected father cannot pass his X-linked allele directly to a son

Write the allele as a superscript on X, such as Xᴴ or Xʰ. Do not place the X-linked allele on Y when the gene is absent from the relevant Y region.

A Haemophilia Cross Gives Different Risks to Sons and Daughters

XᴴXʰ × XᴴY Xᴴ sperm Y sperm
Xᴴ egg XᴴXᴴ unaffected daughter XᴴY unaffected son
Xʰ egg XᴴXʰ carrier daughter XʰY affected son

Among daughters: 1/2 carriers and 1/2 non-carriers; none affected in this cross. Among sons: 1/2 affected and 1/2 unaffected. Across all offspring, each grid outcome has probability 1/4.

Haemophilia A and B result from deficient clotting factors. The inheritance pattern reflects an X-linked recessive allele; treatment supplies or supports the missing clotting function but does not alter the inherited allele.

A carrier female XH Xh crossed with an unaffected male XH Y produces unaffected daughters, carrier daughters, unaffected sons and sons with haemophilia.

Pedigree Inference Eliminates Models That Cannot Produce the Family

Observation Strong inference
two unaffected parents have an affected child supports recessive inheritance; contradicts simple complete-dominant inheritance
affected person has an affected parent in every generation supports dominance, but does not prove it alone
affected father has an affected son contradicts X-linked transmission from father to son
affected sons arise from unaffected mothers consistent with X-linked recessive carrier mothers
males and females affected similarly supports autosomal inheritance

Start with forced genotypes. For an autosomal recessive trait, affected people are aa; each unaffected parent of an affected child must carry a. For an X-linked recessive trait, an affected son XʰY proves that his mother supplied Xʰ.

Treat each inheritance mode as a hypothesis. Predict transmissions that must or cannot occur, compare them with the pedigree, and reject a model only when a relationship contradicts it.

Skipping a generation suggests recessiveness but is not a proof. Chance, incomplete records, late onset and variable expression can obscure the apparent pattern.

Human Inheritance Evidence Runs from Chromosomes to Families to Populations

X and Y contributions establish the usual chromosomal starting point → unequal X-linked gene dosage changes inheritance risk → pedigrees reveal transmission across families → polygenes and environment create continuous population variation → box plots summarize centre and spread

Evidence source Reliable question
X-linked cross which parent supplies the allele to sons or daughters?
pedigree which inheritance hypotheses are contradicted by family relationships?
continuous measurements where is the median and how wide is the middle 50%?
apparent outlier biological extreme, data error or different process?

Match the claim to the evidence scale. One Punnett grid predicts a fertilization probability, a pedigree constrains family genotypes, and a box plot describes a sample distribution; none alone proves a universal biological rule.

Haemophilia exam focus

[ 8 ]

Explain how males inherit hemophilia and how females can become carriers for the condition.

Concept essentials

  • Males express an X-linked recessive allele if it is on their single X chromosome.
  • Heterozygous females are usually carriers for haemophilia.
  • Affected fathers pass their X chromosome to daughters, not sons.