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
- HL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2013
Most common paperPaper2
Typical marks1
Common command terms
- Identify
- Distinguish
- Define
Recent exam appearances
May 2013Paper2 ["HL"] · TZ22(a)(i)[ 1 ]D3.2.3—Genotype
Practice this objective
Coverage 2013–2013 · Updated 16 Jul 2026
Genotype Is an Allele Combination
A genotype is the allele combination an individual carries at one or more loci.
Alleles arrive in gametes; genotype records inherited information before environment and gene interactions shape the phenotype. Trace the allele combination through the stated biological mechanism before predicting the result.
Name the locus; then distinguish homozygous from heterozygous combinations.; compare the stated alleles and outcome
At a locus with A and a, AA and aa are homozygous while Aa is heterozygous. This gives a concrete prediction from the stated parental information.
Genotype is not the visible trait; different genotypes can share a phenotype. Interpret the result within the stated inheritance model and its sample or environmental limits.
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.