AP Biology · Unit 5 Heredity
Genotype and phenotype are two of the most important vocabulary pairs in AP Biology Unit 5. A genotype is the allele combination an organism has, such as AA, Aa, or aa. A phenotype is the expressed trait that results from the genotype and the inheritance pattern.
A genotype is an organism's allele combination, such as AA, Aa, or aa. A phenotype is the observable or expressed trait. In complete dominance, AA and Aa can have the same dominant phenotype because the dominant allele masks the recessive allele.
Genotype is the allele code. Phenotype is the trait you see.
Unit 5 heredity questions often stack vocabulary: alleles → genotype → dominance pattern → phenotype. Mastering this chain early makes Punnett square and monohybrid cross problems much faster because you already know which label the rubric wants.
A genotype is the allele combination an organism carries for a gene. For a simple Mendelian trait, genotypes may be written as AA, Aa, or aa. The letters represent alleles, not the trait itself. On AP Biology exams, genotype answers use allele notation—never describe a visible flower color when the rubric asks for genotype.
Genotypes can be homozygous (two matching alleles) or heterozygous (two different alleles). The same gene may have many allele versions in a population, but a diploid organism carries only two alleles for that gene at a given locus. When a pedigree or Punnett square lists parent genotypes, that information tells you which allele combinations offspring can inherit.
For broader allele and dominance vocabulary, review Mendelian genetics. To classify AA, Aa, and aa by allele pairing, see homozygous vs heterozygous.
A phenotype is the expressed trait or observable result. A phenotype can be visible, such as flower color, or measurable, such as enzyme activity. Phenotype depends on genotype, dominance pattern, and sometimes environment. AP Biology FRQs often award points when you connect a stated genotype to the correct phenotype using the inheritance pattern in the prompt.
Phenotypes appear at the organism level: purple petals, white petals, type A blood, or reduced enzyme function. Two organisms with the same genotype can sometimes show different phenotypes if environmental conditions differ, but the genotype itself does not change unless a mutation occurs.
| Genotype | Genotype Name | Complete-Dominance Phenotype | AP Meaning |
|---|---|---|---|
| AA | Homozygous dominant | Dominant phenotype | Two dominant alleles |
| Aa | Heterozygous | Dominant phenotype | One dominant allele masks recessive allele |
| aa | Homozygous recessive | Recessive phenotype | No dominant allele present |
These three genotypes are the foundation of most introductory AP Biology genetics problems. When a question says “homozygous dominant,” write AA. When it says “heterozygous,” write Aa. When it says “homozygous recessive,” write aa—unless the prompt assigns different allele letters.
Suppose purple (P) is dominant over white (p). Genotypes PP and Pp both produce purple flowers, while pp produces white flowers.
Result: Genotypes PP and Pp → purple phenotype. Genotype pp → white phenotype.
AP clue: AP clue: Two different genotypes (PP and Pp) can share the same phenotype under complete dominance.
Blood type is a phenotype you can test in a lab even though you cannot see it without a test. The genotype includes the allele combination at the blood-type locus.
Result: Genotype determines which antigens appear on red blood cells.
AP clue: AP clue: Phenotype is not always visible to the naked eye—it can be measurable.
A monohybrid cross produces genotypes AA, Aa, Aa, and aa. Under complete dominance, three of four genotypes show the dominant phenotype.
Result: Genotype ratio: 1:2:1. Phenotype ratio: 3:1.
AP clue: AP clue: Genotype and phenotype ratios differ because phenotype groups genotypes with the same expression.
A pedigree may show an affected individual with unaffected parents. That pattern can reveal recessive inheritance, but a dominant phenotype in one generation does not prove AA rather than Aa.
Result: Dominant phenotype could be AA or Aa under complete dominance.
AP clue: AP clue: Link pedigree symbols to phenotype first, then infer possible genotypes.
| Genotype | Genotype Meaning | Phenotype Under Complete Dominance | Why |
|---|---|---|---|
| AA | Homozygous dominant | Dominant phenotype | Two dominant alleles |
| Aa | Heterozygous | Dominant phenotype | One dominant allele masks the recessive allele |
| aa | Homozygous recessive | Recessive phenotype | No dominant allele is present |
| XNXn | Carrier female for X-linked recessive trait | Usually unaffected carrier | One normal X-linked allele masks the recessive allele |
| XnY | Affected male for X-linked recessive trait | Recessive X-linked phenotype | The male has one X chromosome carrying the recessive allele |
Always use the inheritance pattern in the prompt before translating genotype into phenotype.
For X-linked recessive traits, review sex-linked traits when genotypes use X and Y notation instead of AA, Aa, or aa.
Choose a genotype and inheritance pattern to see how the phenotype changes. The decoder shows why AP Biology treats Aa differently under complete dominance, incomplete dominance, and codominance. Try switching patterns while keeping Aa selected—that is the fastest way to see why “heterozygous” alone does not tell you what the organism looks like.
Genotype
Inheritance pattern
Remember: Same genotype, different inheritance pattern, different phenotype.
A Punnett square gives genotypes first. Then you translate those genotypes into phenotypes using the inheritance pattern. For example, Aa × Aa produces AA, Aa, Aa, and aa. Under complete dominance, AA and Aa share the dominant phenotype, so the phenotype ratio is 3 dominant : 1 recessive.
Step-by-step for Aa × Aa:
The genotype ratio and phenotype ratio are both valid answers—but they answer different questions. Read the prompt carefully before you respond.
Review Punnett Squares for square setup. For ratio practice, see Monohybrid Crosses.
Yes. In complete dominance, AA and Aa can both show the dominant phenotype. This is why an organism with a dominant phenotype may be homozygous dominant or heterozygous. A test cross can help identify the unknown genotype.
Example: A purple-flower plant could be AA or Aa if purple is dominant. Without a cross or pedigree evidence, you cannot tell which genotype produced the dominant phenotype. That is a common source of FRQ reasoning points—students must explain that dominant appearance does not prove homozygosity.
Test crosses are covered in depth on the monohybrid crosses guide. Pedigrees show phenotypes across generations; use the pedigrees guide when family trees replace Punnett squares in the prompt.
The simple AA/Aa/aa pattern assumes complete dominance. In non-Mendelian inheritance, the heterozygote may have a different phenotype. In incomplete dominance, the heterozygote can show an intermediate phenotype. In codominance, both alleles can be expressed.
Use the Genotype-to-Phenotype Decoder above to compare how the same Aa genotype changes under each pattern. On the exam, keywords like “blended,” “intermediate,” or “both traits visible” signal that complete-dominance logic may not apply.
Review Non-Mendelian Genetics when heterozygotes do not look fully dominant.
Yes. Phenotype can be influenced by genotype and environment. For example, nutrition, temperature, light, or other conditions can affect how a trait appears. AP Biology often asks students to separate genetic causes from environmental effects.
A classic example is coat color in some mammals where temperature affects which pigment is produced—the underlying genotype sets the potential, but environment shapes the final phenotype. When an FRQ mentions conditions outside inheritance rules, explain how environment modifies expression without changing allele letters.
| Change | Genotype Changed? | Phenotype Changed? |
|---|---|---|
| DNA allele changes from A to a | Yes | Maybe |
| Plant grows taller with more light | No | Yes |
| AA and Aa both look dominant | Yes | No visible difference under complete dominance |
| Enzyme activity changes due to temperature | No | Yes |
| aa shows recessive trait | Yes compared with AA/Aa | Yes |
→ genotype
→ phenotype
→ genotype
→ phenotype
→ could be AA or Aa
→ usually aa in complete dominance
→ incomplete dominance
→ codominance
→ genotypes first, phenotypes second
Underline the exact wording. If the question asks for allele letters or combinations, report genotype. If it asks for the expressed trait or appearance, report phenotype.
Complete dominance, incomplete dominance, and codominance change how Aa looks. Use the Genotype-to-Phenotype Decoder on this page to compare patterns quickly.
First fill genotypes in each box. Then group genotypes into phenotypes using the dominance rule stated in the prompt.
AP Biology expects standard allele notation. Homozygous dominant is AA, heterozygous is Aa, and homozygous recessive is aa unless the prompt uses different letters.
Fix: Genotype is the allele combination. Phenotype is the expressed trait.
Fix: Phenotype can be visible or measurable, such as enzyme activity.
Fix: Aa shows the dominant phenotype only in complete dominance.
Fix: Genotype ratio counts allele combinations. Phenotype ratio counts expressed traits.
Fix: Dominant means expressed in a heterozygote, not necessarily more frequent.
Fix: Phenotype can be affected by both genes and environment.
When you miss a practice MCQ, reread the question stem for the words “allele combination” versus “expressed trait.” That single check fixes a large share of genotype vs phenotype errors on timed exams.
Answer all eight questions. Choices shuffle on reload.
More drills: Unit 5 practice questions, pedigree analysis, or sex-linked traits when inheritance depends on X and Y chromosomes.
Open each card, draft your response, then reveal the rubric and sample answer. Strong FRQ answers name genotype and phenotype separately, cite AA/Aa/aa when relevant, and explain how dominance connects allele combination to expressed trait.
A student says that genotype and phenotype are the same because both describe traits. Explain why this statement is incorrect using AA, Aa, and aa examples.
The statement is incorrect because genotype and phenotype describe different things. A genotype is the allele combination, such as AA, Aa, or aa. A phenotype is the expressed trait. Under complete dominance, AA and Aa can both show the dominant phenotype even though they have different genotypes. The genotype aa usually shows the recessive phenotype. This shows that genotype influences phenotype, but they are not the same.
Status: Draft your answer first—then open the rubric or sample.
A Punnett square for Aa × Aa gives AA, Aa, Aa, and aa. Explain the genotype ratio and phenotype ratio under complete dominance.
The Punnett square produces one AA, two Aa, and one aa, so the genotype ratio is 1:2:1. Under complete dominance, AA and Aa both show the dominant phenotype because one dominant allele is enough for expression. The aa genotype shows the recessive phenotype. Therefore, the phenotype ratio is 3 dominant : 1 recessive. The ratios differ because genotype counts allele combinations, while phenotype counts expressed traits.
Status: Draft your answer first—then open the rubric or sample.
These twelve questions cover the genotype vs phenotype vocabulary AP Biology Unit 5 expects on MCQs and FRQs. If a ratio question still feels confusing, return to the AA/Aa/aa table and decoder sections above before practicing.
A genotype is an organism's allele combination, such as AA, Aa, or aa. A phenotype is the observable or expressed trait. In complete dominance, AA and Aa can have the same dominant phenotype because the dominant allele masks the recessive allele.
A genotype is the allele combination an organism carries for a gene. For a simple Mendelian trait, genotypes may be written as AA, Aa, or aa.
A phenotype is the expressed trait or observable result. It can be visible, such as flower color, or measurable, such as enzyme activity.
Examples include AA (homozygous dominant), Aa (heterozygous), and aa (homozygous recessive). The letters represent alleles, not the trait itself.
Examples include purple flower color, white flower color, blood type, tall plant height, and enzyme activity level.
Yes. In complete dominance, AA and Aa can both show the dominant phenotype even though they have different genotypes.
The dominant allele A masks the recessive allele a in the heterozygote. One dominant allele is enough to show the dominant phenotype.
Under complete dominance, aa usually shows the recessive phenotype because no dominant allele is present.
A Punnett square gives offspring genotypes first. You then translate those genotypes into phenotypes using the inheritance pattern, such as complete dominance.
Yes. Phenotype can be influenced by genotype and environment. Nutrition, temperature, light, or other conditions can affect how a trait appears.
Yes. Different genotypes can sometimes produce the same phenotype. For example, under complete dominance, AA and Aa both show the dominant phenotype even though the genotypes are different.
Define genotype as allele combination and phenotype as expressed trait. Use AA, Aa, and aa examples. Explain how dominance pattern connects genotype to phenotype. Separate genotype ratios from phenotype ratios when Punnett squares are involved.