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D3.2.11—Sex determination

Sex determination in humans depends on X and Y chromosomes, with sperm determining whether the zygote is XX or XY.

Syllabus
First assessment 2025
Objective
D3.2.11
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Explain

Recent exam appearances

May 2023Paper1 ["HL"] · TZ124[ 1 ]D3.2.11—Sex determination
May 2015Paper1 ["HL"] · TZ28[ 1 ]D3.2.11—Sex determination
May 2014Paper1 ["HL"] · TZ123[ 1 ]D3.2.11—Sex determination

Coverage 2014–2023 · Updated 16 Jul 2026

The Sperm Chromosome Sets the Typical XX or XY Starting Point

Gamete contribution X-bearing sperm Y-bearing sperm
X-bearing egg XX zygote XY zygote
expected probability about 1/2 about 1/2

Y chromosome with functional SRY → testis-determining factor (TDF) is produced → embryonic gonads develop as testes → testicular hormones direct typical male reproductive development

Without a functional SRY signal, the undifferentiated gonads normally follow ovarian development and typical female reproductive structures develop.

XX/XY is the core human chromosomal model, but chromosome number, SRY location or function, hormone synthesis and tissue response can vary. Chromosomal pattern alone does not describe every aspect of sex or gender.

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.

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.

Sex determination

[ 4 ]

Distinguish between autosomes and sex chromosomes in humans.

Concept essentials

  • Eggs normally contribute an X chromosome.
  • Sperm may contribute either an X or a Y chromosome.
  • The sperm chromosome determines whether the zygote is XX or XY.