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
Practice this objective
Coverage 2014–2023 · Updated 16 Jul 2026
The Sperm Sex Chromosome Determines the Typical XX/XY Outcome
In the simplified human model, eggs carry X, whereas sperm carry X or Y; therefore the sperm's sex chromosome determines whether the zygote is typically XX or XY.
| Egg | Sperm | Typical zygote outcome |
|---|---|---|
| X | X | XX, typical female sex characteristics |
| X | Y | XY, typical male sex characteristics |
The X chromosome carries far more genes than the Y chromosome. Sex-chromosome inheritance therefore also affects many genes unrelated to sex determination.
An X-bearing and a Y-bearing sperm are expected in roughly equal proportions, so each fertilization has approximately equal model probabilities of XX and XY.
XX/XY is a simplified model of typical development; chromosome variation and differences in gene function can produce other biological outcomes.
Sex determination
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Explain.
Command terms
Identify / Explain
What earns marks
Build the answer around this relationship: Eggs normally contribute an X chromosome.
Representative question
Question 1
Distinguish between autosomes and sex chromosomes in humans.
Retrieve the Core Inheritance Route
Core D3.2 is secure when the student can move from allele rules into predictions and evidence: gametes form genotypes, genotypes can produce phenotypes, different dominance patterns need different notation, and pedigrees or plots require evidence-based interpretation.
- haploid gametes carry one allele and fertilization restores a diploid genotype
- dominance, codominance, incomplete dominance, environment, and plasticity affect the observed trait
- PKU, ABO, sex determination, and haemophilia use different inheritance rules and notation
- pedigrees infer inheritance patterns and box plots summarize continuous variation
Solve Core Inheritance Questions
Core inheritance exam questions reward disciplined reasoning. First identify the inheritance rule, then write the correct notation or evidence, then state the phenotype, ratio, or conclusion. This prevents the common mistake of writing definitions without solving the genetic problem.
- Use allele and genotype notation correctly for monohybrid, ABO, PKU, haemophilia, and sex-determination contexts.
- Connect genotype, dominance pattern, environment, or plasticity to phenotype.
- Use pedigree or box-plot evidence to justify an inheritance or variation conclusion.
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.