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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

Coverage 2013–2013 · Updated 16 Jul 2026

Gene, Allele, Locus and Genotype Describe Different Levels

Term Meaning Example
gene DNA sequence with a functional product or role a gene affecting pea height
locus the gene's position on a chromosome the height-gene position
allele one sequence version at that locus T or t
genotype allele combination carried TT, Tt or tt
  • homozygous: two matching alleles, such as TT or tt
  • heterozygous: two different alleles, such as Tt

Use genotype for the allele combination, not for the visible trait. A genotype can be written for one locus, several loci or the whole genome, so state the scope when it matters.

A Punnett Grid Combines Gamete Probabilities, Not Whole Parental Genotypes

  1. define allele symbols and the dominance relationship
  2. write both parental genotypes
  3. derive the allele carried by each possible gamete
  4. place one parent's gametes across the grid and the other's down the side
  5. combine alleles in each cell, then convert genotypes to phenotypes
Tt × Tt T gamete t gamete
T gamete TT, tall Tt, tall
t gamete Tt, tall tt, dwarf

Each cell has probability 1/4 because each parent produces T and t gametes with probability 1/2. The grid predicts probabilities for each fertilization, not a guarantee that every four offspring will contain one of each cell.

A Test Cross Exposes a Hidden Dominant Genotype

A tall plant could be TT or Tt because both contain the dominant T allele. Cross the unknown plant with a homozygous recessive tester, tt, whose gametes reveal what the unknown parent contributes.

Unknown parent Cross with tt Expected offspring evidence
TT TT × tt all Tt, all tall
Tt Tt × tt about 1 Tt tall : 1 tt dwarf

A small all-tall sample does not prove the parent is TT because a heterozygote can produce tall offspring repeatedly by chance. Confidence increases as more offspring are scored.

Inheritance Predictions Move from Chromosomes to Alleles to Phenotypes

homologous chromosomes carry two alleles in a diploid parent → meiosis separates one allele into each gamete → fertilization restores a two-allele genotype → allele relationship and environment shape phenotype → offspring data test the prediction

For any genetic cross:

  • define allele symbols and their relationship
  • write parental genotypes, not only phenotypes
  • derive gametes before drawing the grid
  • combine gametes, then translate genotype to phenotype
  • state probabilities and the assumptions behind them
If the simple 3:1 expectation fails First question to ask
heterozygote has a third phenotype incomplete dominance?
both products appear codominance?
phenotype changes with conditions environmental or plastic response?
more than two alleles occur in the population multiple-allele locus such as ABO?

Genotype exam focus

[ 1 ]

Define the term genotype.

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.