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D3.2.5—Dominant and recessive alleles

Dominant alleles affect phenotype in heterozygotes, while recessive alleles are expressed only when no dominant allele is present in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.5
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2022
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Explain
  • Deduce

Recent exam appearances

May 2022Paper1 ["HL"] · TZ113[ 1 ]D3.2.5—Dominant and recessive alleles
May 2018Paper1 ["HL"] · TZ112[ 1 ]D3.2.5—Dominant and recessive alleles
November 2017Paper1 ["HL"] · TZ012[ 1 ]D3.2.5—Dominant and recessive alleles
May 2016Paper2 ["HL"] · TZ05(c)[ 4 ]D3.2.5—Dominant and recessive alleles
May 2016Paper1 ["HL"] · TZ020[ 1 ]D3.2.5—Dominant and recessive alleles

Coverage 2016–2022 · Updated 16 Jul 2026

Dominant and Recessive Describe What Happens in a Heterozygote

Under complete dominance, a dominant allele determines the heterozygous phenotype. A recessive phenotype appears only when no dominant allele is present.

Genotype Allele state Phenotype when T is dominant
TT homozygous dominant tall
Tt heterozygous tall
tt homozygous recessive dwarf

Dominant does not mean better, stronger or more common. It only describes expression in a heterozygote; a harmful or rare allele can be dominant.

Mendel's P, F1 and F2 Generations Reveal Segregating Alleles

1

P generation: true-breeding tall TT × true-breeding dwarf tt → every gamete carries T or t, respectively.

2

F1 generation: every offspring is Tt and tall. The t allele has not blended away; its phenotypic effect is masked in the heterozygote.

3

F1 self-cross: Tt × Tt → F2 genotypes 1 TT : 2 Tt : 1 tt → phenotypes 3 tall : 1 dwarf in a large sample.

Reappearance of dwarf offspring shows that hereditary factors remain discrete across generations. The observed ratio approaches the probability prediction only when many offspring are counted.

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.

Two Unaffected PKU Carriers Can Have an Affected Child

Genotype PKU status Reproductive meaning
PP unaffected, not a carrier passes P
Pp unaffected carrier can pass P or p
pp affected passes p
Pp × Pp P gamete p gamete
P gamete PP unaffected Pp carrier
p gamete Pp carrier pp affected

For each pregnancy: 1/4 affected, 1/2 unaffected carrier, 1/4 unaffected non-carrier. The probability resets for every fertilization; previous children do not change the next gamete combination.

Carrier and affected are not synonyms. A heterozygous carrier has a working dominant allele and usually does not show PKU, but can transmit the recessive allele.

The Heterozygote Separates Complete Dominance, Codominance and Incomplete Dominance

Allele relationship Heterozygote phenotype Example
complete dominance matches the dominant homozygote Tt pea is tall
codominance both allele products are detectably expressed IᴬIᴮ has A and B antigens
incomplete dominance intermediate between the two homozygotes red × white Mirabilis gives pink F1
Heterozygote × heterozygote Genotype ratio Phenotype ratio
complete dominance 1:2:1 3:1
codominance or incomplete dominance 1:2:1 1:2:1 when all three genotypes are distinguishable

Codominance is not blending: both products remain identifiable. In incomplete dominance, the heterozygote is intermediate rather than showing two separate products side by side.

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?

Dominant and recessive alleles

[ 4 ]

Many genetic diseases are due to recessive alleles of autosomal genes that code for an enzyme. Using a Punnett grid, explain how parents who do not show signs of such a disease can produce a child with the disease.

Concept essentials

  • Dominant alleles are expressed in heterozygotes.
  • Recessive phenotypes usually require two recessive alleles.
  • Dominance describes expression, not allele frequency or value.