Side by side
| Genotype | Phenotype | |
|---|---|---|
| Definition | The organism's underlying genetic makeup, consisting of both expressed and non-expressed alleles | The observable traits the organism expresses |
| How it is written | As allele pairs, such as YY, Yy or yy | As a described trait, such as yellow seeds or green seeds |
| How it is determined | Inferred from crosses, from offspring ratios, or read directly by sequencing | Observed or measured directly |
| Mapping between them | Three genotypes exist for a two-allele gene: homozygous dominant, heterozygous, homozygous recessive | Simple dominance collapses those three into two visible classes |
| Which one you can always see | Not directly visible; a dominant phenotype hides whether the second allele is dominant or recessive | Directly visible, which is why only the recessive phenotype identifies its genotype outright |
| Behavior in a monohybrid F2 | Falls in a 1:2:1 ratio | Falls in a 3:1 ratio under simple dominance |
| Why the distinction matters | It explains how a trait can disappear in F1 and reappear in F2 | It is what a test cross is designed to resolve back into a genotype |
The same trait, two different questions
In a diploid organism two alleles for each gene are expressed and interact to produce physical characteristics. The observable traits that result are the phenotype; the underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is the genotype. Mendel's hybridization experiments are the cleanest demonstration of why the two need separate names. Crossing true-breeding yellow-pod and green-pod plants gave F1 offspring that all had yellow pods, phenotypically identical to one parent. Yet the green allele had not been lost, because green pods reappeared in the F2, which means the F1 plants must have been genotypically different from the parent they resembled.
Where students go wrong
The recurring error is treating a dominant phenotype as if it named a genotype. A yellow-seeded pea can be YY or Yy, and nothing about its appearance distinguishes the two; only the recessive phenotype reports its genotype unambiguously, since it requires two recessive alleles. That asymmetry is exactly why the test cross exists, and it is also why Hardy-Weinberg problems always start from the recessive count. A second error is assuming phenotype is genotype plus nothing else. Phenotype is what is observed, and traits can be influenced by more than one gene and by the environment.
Where this leads next
The Punnett square page is where genotypes become ratios you can count, and it is the natural next step once the distinction here is solid. From there, the mapping from genotype to phenotype is not always two to one: under incomplete dominance and codominance all three genotypes are phenotypically distinct, which is the comparison page for those patterns. And at population scale the same distinction is what makes the Hardy-Weinberg equation work, since only the recessive phenotype count can be converted directly into a genotype frequency.
Common mistakes
- Writing a genotype when the question asked for a phenotype, or the reverse. Genotypes are allele pairs; phenotypes are described traits.
- Assuming an organism showing the dominant trait is homozygous. It may be heterozygous, and only a test cross or the offspring will tell.
- Saying the recessive allele disappears in the F1. It is present but not expressed, which is why it reappears in the F2.
- Treating phenotype as determined by one gene alone. Observable traits can involve several genes and environmental influence.
Related concepts
- Incomplete dominance vs codominance: In incomplete dominance the heterozygote shows an intermediate phenotype, as a red and a white snapdragon parent give pink offspring.
- Hardy-Weinberg equation: The Hardy-Weinberg equation, p² + 2pq + q² = 1, predicts genotype frequencies in a population that is not evolving, with p and q the frequencies of the two alleles.
- Punnett square: A Punnett square applies the rules of probability to predict the outcomes of a genetic cross.