D3.2.3—Genotype
A genotype is the allele combination an organism carries for a gene or set of genes in inheritance problems in inheritance problems.
- Syllabus
- First assessment 2025
- Objective
- D3.2.3
- Level
- SL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1
Common command terms
- Identify
- Distinguish
- Define
Recent exam appearances
May 2025Paper2 ["SL"] · TZ34(a)[ 1 ]D3.2.3—Genotype
November 2015Paper2 ["SL"] · TZ03(c)(i)[ 1 ]D3.2.3—Genotype
Practice this objective
Coverage 2015–2025 · Updated 16 Jul 2026
Genotype Records Alleles at a Locus
A gene is a DNA sequence affecting a characteristic; an allele is one version of that gene; a genotype is the allele combination carried at one or more loci.
| Term | Meaning | Example at an A/a locus |
|---|---|---|
| Homozygous | Two identical alleles | AA or aa |
| Heterozygous | Two different alleles | Aa |
Writing Aa identifies the genotype at one locus; it does not by itself name the visible phenotype until the allele relationship is known.
Do not use gene, allele, genotype and phenotype as synonyms. An individual has two alleles at an autosomal locus, while a population may contain more than two.
Genotype exam focus
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Distinguish / Define.
Command terms
Identify / Distinguish / Define
What earns marks
Build the answer around this relationship: A genotype records the alleles an organism carries.
Representative question
Question 1
Define the term genotype.
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
- A genotype records the alleles an organism carries.
- Homozygous and heterozygous genotypes differ in whether the two alleles match.
- Genotype can be inferred from crosses, pedigrees or offspring patterns.