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AP Biology · Unit 5 Heredity

Homozygous vs Heterozygous AP Biology: Unit 5 Guide

Homozygous and heterozygous describe allele pairs in a genotype. Homozygous means two matching alleles, such as AA or aa. Heterozygous means two different alleles, such as Aa. In AP Biology Unit 5, these terms help you interpret Punnett squares, genotype ratios, phenotype ratios, and inheritance patterns.

Updated June 3, 2026 · Reviewed by APScore5 Editorial Team

← Unit 5 Hub Review Genotype vs Phenotype → Practice Unit 5
HomozygousHeterozygousAA / Aa / aaAllele pairsDominant phenotypePunnett squares
🎯 Classifier ✅ 8 MCQs 📝 2 FRQs
AP Biology homozygous vs heterozygous infographic showing AA, Aa, aa, allele pairs, genotype names, and phenotype outcomes
Figure - Homozygous Vs Heterozygous AP Biology
Quick answer

What is the difference between homozygous and heterozygous in AP Biology?

Homozygous means an organism has two identical alleles for a gene, such as AA or aa. Heterozygous means an organism has two different alleles, such as Aa. In complete dominance, AA and Aa can show the dominant phenotype, while aa shows the recessive phenotype.

Say it fast

Homozygous = same alleles. Heterozygous = different alleles.

AP exam tip: If the two letters match, the genotype is homozygous. If the letters differ, the genotype is heterozygous.
Homozygous

What Does Homozygous Mean?

Homozygous means both alleles for a gene are the same. A genotype can be homozygous dominant or homozygous recessive.

  • AA = homozygous dominant
  • aa = homozygous recessive
  • BB = homozygous dominant
  • bb = homozygous recessive
Homozygous matching alleles
Figure - Homozygous Means Two Matching Alleles

For how genotype connects to visible traits, see genotype vs phenotype and Mendelian genetics.

Heterozygous

What Does Heterozygous Mean?

Heterozygous means the two alleles for a gene are different. A heterozygous genotype has one dominant allele and one recessive allele in simple Mendelian notation.

  • Aa = heterozygous
  • Bb = heterozygous
  • Tt = heterozygous

Under complete dominance, the dominant allele is expressed in the heterozygote.

Heterozygous different alleles
Figure - Heterozygous Means Two Different Alleles
AA Aa aa

AA, Aa, and aa: Homozygous or Heterozygous?

GenotypeClassificationAllelesComplete-Dominance Phenotype
AAHomozygous dominantTwo dominant allelesDominant phenotype
AaHeterozygousOne dominant, one recessiveDominant phenotype
aaHomozygous recessiveTwo recessive allelesRecessive phenotype
Key idea: AA and Aa can have the same phenotype under complete dominance, but they are not the same genotype.
AA Aa aa genotype names
Figure - Allele Pairs Reveal Genotype Names
Examples

Homozygous vs Heterozygous Examples with Answers

Use these genotype examples to practice naming homozygous dominant, heterozygous, and homozygous recessive allele pairs under complete dominance.

GenotypeClassificationWhyPhenotype Under Complete Dominance
AAHomozygous dominantTwo matching dominant allelesDominant phenotype
AaHeterozygousTwo different allelesDominant phenotype
aaHomozygous recessiveTwo matching recessive allelesRecessive phenotype
BBHomozygous dominantTwo matching dominant allelesDominant phenotype
BbHeterozygousOne dominant and one recessive alleleDominant phenotype
bbHomozygous recessiveTwo matching recessive allelesRecessive phenotype
AEO answer: AA and Aa can show the same phenotype under complete dominance, but they are different genotypes.
Interactive

Allele Pair Classifier

Choose an allele pair to classify it as homozygous dominant, heterozygous, or homozygous recessive.

Dominant vs recessive

Homozygous Dominant vs Homozygous Recessive

Both homozygous dominant and homozygous recessive genotypes have matching alleles. The difference is whether the matching alleles are dominant or recessive.

TypeExampleMeaningPhenotype Under Complete Dominance
Homozygous dominantAATwo dominant allelesDominant phenotype
Homozygous recessiveaaTwo recessive allelesRecessive phenotype
AP warning: Do not say “homozygous” automatically means dominant. Homozygous can be dominant or recessive.
Punnett squares

Homozygous and Heterozygous in Punnett Squares

Punnett squares produce genotype combinations. For Aa × Aa, the offspring genotypes are AA, Aa, Aa, and aa. That means one homozygous dominant, two heterozygous, and one homozygous recessive genotype.

Genotype ratio: 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive

Review Punnett Squares · Review Monohybrid Crosses

Aa cross Punnett square
Figure - Punnett Squares Create Genotype Combinations
Phenotype

How Homozygous and Heterozygous Affect Phenotype

In complete dominance, AA and Aa show the dominant phenotype because one dominant allele is enough for expression. The aa genotype shows the recessive phenotype because it has no dominant allele. In incomplete dominance or codominance, the heterozygous phenotype can look different.

Review Genotype vs Phenotype · Review Non-Mendelian Genetics

Carrier

Is a Carrier Homozygous or Heterozygous?

A carrier is usually heterozygous for a recessive allele. For an autosomal recessive trait, a carrier might be Aa: one normal dominant allele and one recessive allele. The carrier may not show the recessive phenotype but can pass the recessive allele to offspring.

For X-linked traits, a carrier female may be written as XNXn.

Sex-Linked Traits

Exam clues

AP Bio Exam Clues for Homozygous vs Heterozygous

"Two same alleles"

→ homozygous

"Two different alleles"

→ heterozygous

"AA"

→ homozygous dominant

"aa"

→ homozygous recessive

"Aa"

→ heterozygous

"Carrier"

→ often heterozygous

"Dominant phenotype, unknown genotype"

→ could be AA or Aa

"Recessive phenotype"

→ usually aa in complete dominance

"1:2:1 genotype ratio"

→ one homozygous dominant, two heterozygous, one homozygous recessive

Unknown genotype: If a dominant phenotype could be AA or Aa, a test cross with a homozygous recessive parent can help reveal the unknown genotype.

Mistakes

Common Homozygous vs Heterozygous Mistakes

Thinking homozygous means dominant

Fix: Homozygous means matching alleles. It can be AA or aa.

Thinking heterozygous means recessive

Fix: Heterozygous means different alleles. Under complete dominance, Aa shows the dominant phenotype.

Confusing genotype name with phenotype

Fix: Homozygous and heterozygous describe genotypes, not directly visible traits.

Saying AA and Aa are the same

Fix: They can have the same phenotype under complete dominance, but they are different genotypes.

Forgetting carriers are often heterozygous

Fix: A carrier usually has one recessive allele and one dominant allele.

Ignoring non-Mendelian patterns

Fix: In incomplete dominance and codominance, heterozygotes can have different phenotypes.

Non-Mendelian reminder: When heterozygotes look intermediate or show both alleles, review non-Mendelian genetics instead of assuming complete dominance.
MCQ practice

Homozygous vs Heterozygous Practice Questions

Answer all eight questions. Choices shuffle on reload.

Question 1 of 8 Start
Correct: 0 Answered: 0 Accuracy: 0%

More drills: Unit 5 practice questions.

FRQ practice

Homozygous vs Heterozygous FRQ Practice

FRQ genotype phenotype flow
Figure - Name Genotypes Before Explaining Phenotypes

Open each card, draft your response, then reveal the rubric and sample answer.

0 of 2 FRQs opened
Prompt

A student says homozygous means dominant and heterozygous means recessive. Explain why this statement is incorrect using AA, Aa, and aa.

Status: Draft your answer first—then open the rubric or sample.

Prompt

In a cross between two heterozygous parents, explain the expected genotype ratio and identify which offspring are homozygous or heterozygous.

Status: Draft your answer first—then open the rubric or sample.

Unit 5 path

Continue the Unit 5 Heredity Path

HubUnit 5 Heredity HubReturn to the full Unit 5 map and journey.FoundationsGenotype vs PhenotypeSeparate genetic makeup from expressed traits.FoundationsMendelian GeneticsReview alleles, dominance, and Mendel's laws.PracticePunnett SquaresBuild squares that produce AA, Aa, and aa offspring.PracticeMonohybrid CrossesPractice 1:2:1 genotype ratios from Aa × Aa.ExtensionsNon-Mendelian GeneticsWhen heterozygotes do not match classic dominance.ExtensionsPedigreesTrack carriers and inheritance in families.PracticeUnit 5 Practice QuestionsMixed MCQs and FRQs across heredity topics.
FAQ

Homozygous vs Heterozygous FAQs

What is the difference between homozygous and heterozygous in AP Biology?

Homozygous means an organism has two identical alleles for a gene, such as AA or aa. Heterozygous means an organism has two different alleles, such as Aa. In complete dominance, AA and Aa can show the dominant phenotype, while aa shows the recessive phenotype.

What does homozygous mean?

Homozygous means both alleles for a gene are the same. A genotype can be homozygous dominant (AA) or homozygous recessive (aa).

What does heterozygous mean?

Heterozygous means the two alleles for a gene are different, such as Aa. Under complete dominance, the dominant allele is usually expressed.

Is AA homozygous or heterozygous?

AA is homozygous because both alleles are the same. It is homozygous dominant when A is the dominant allele.

Is Aa homozygous or heterozygous?

Aa is heterozygous because the two alleles are different.

Is aa homozygous or heterozygous?

aa is homozygous because both alleles are the same. It is homozygous recessive when a is the recessive allele.

What is homozygous dominant?

Homozygous dominant means two identical dominant alleles, such as AA. Under complete dominance, this genotype shows the dominant phenotype.

What is homozygous recessive?

Homozygous recessive means two identical recessive alleles, such as aa. Under complete dominance, this genotype shows the recessive phenotype.

Why does a heterozygote often show the dominant phenotype?

Under complete dominance, one dominant allele is enough to mask the recessive allele in a heterozygote such as Aa.

Can homozygous mean recessive?

Yes. Homozygous means matching alleles, not dominant. aa is homozygous recessive.

Is a carrier homozygous or heterozygous?

A carrier is usually heterozygous for a recessive allele, such as Aa for an autosomal recessive trait.

How should I answer homozygous vs heterozygous FRQs?

Name allele pairs and genotypes first (homozygous dominant, heterozygous, homozygous recessive), then explain phenotype under the dominance pattern given in the prompt.

Can AA and Aa have the same phenotype?

Yes. Under complete dominance, AA and Aa can both show the dominant phenotype because one dominant allele is enough to mask the recessive allele. They have the same phenotype but different genotypes.

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