In genetics, the genotype and phenotype are two ways of describing an organism’s traits. The genotype is the genetic code, while the phenotype is the physical expression of a trait. Here is a closer look at what genotype and phenotype are, with examples.
Mnemonic For Genotype vs Phenotype
One easy way of remembering the difference between genotype and phenotype is looking at the first part of each word.
- Genotype starts with “G” or “gen-” for genes.
- Phenotype starts with “P” or “ph-” for physical traits.
Genotype
The genotype is the genetic code or “blueprint.” A genotype is heritable, meaning parents pass genes on to their offspring. A gene is a set of instructions in DNA or RNA that codes for proteins that produce a trait, such as eye color or height. Alleles are different forms of a trait. For example, consider two alleles of flower color: purple and white. In this example, purple is a dominant trait and the genotypes BB (homozygous dominant) and Bb (heterozygous) code for purple. The genotype bb (homozygous recessive) does not produce the protein that makes the purple color, so this genotype codes for a white flower. The flower colors are the phenotype.
In this example, you can predict the phenotype of the flowers from the genotype. However, you can’t necessarily tell the genotype of the flower from its phenotype. While a white flower most likely has the homozygous recessive genotype, a purple flower could have either the homozygous dominant or heterozygous genotype.
Phenotype
Phenotype is the physical expression of the genotype, but environmental factors also affect it. For example, many flamingos are pink, while some are not, even if they have the same genotypes for feather color. This is because feather color depends on the flamingo’s diet. Birds that eat food rich in carotenoid pigments have colorful feathers. If they switch to a different diet, their feathers lose their color.
As another example, consider two genetically identical plants. These plants have the same genotype, but a plant grown with plenty of sunlight, water, and nutrients is green and healthy. Meanwhile, a plant deprived of these things is pale and spindly.
The same situation arises with identical twins. Twins share the exact same genotype, yet their phenotypes often differ in many ways, such as height, weight, and fingerprints. This is because external factors, such as nutrition and exercise, affect gene expression.
But, under ideal conditions, you can often predict phenotype if you know genotype. For example, consider the alleles for eye color. Brown is the dominant allele, while blue is recessive. A person with blue eyes (a phenotype) has the homozygous recessive genotype for this allele. If a person has brown eyes, you can’t tell the genotype, because it could be either homozygous dominant or else heterozygous.
Genotype vs Phenotype Examples
Still confused about the difference between genotype and phenotype? The genetics indicate genotype. A physical characteristic is a phenotype.
| Genotype | Phenotype | |
|---|---|---|
| Definition | genetic code responsible for a trait | observable traits and characteristics |
| Example | genes for eye color | brown, green, blue |
| Heritable? | yes | not necessarily |
| How to Identify | DNA analysis, family history, or by test crossing | simple observation |
| Depends on? | genetic code | genetic code plus environmental factors and epigenetics |
Genotypic and Phenotypic Ratios
In a cross between two individuals, the genotypic ratio is the ratio of the different genotypes, while the phenotypic ratio is the ratio of observable phenotypes. The two ratios are not necessarily the same.
For example, consider a cross between two plants and a gene that controls the plant height allele. In this example, T is the dominant allele, for tall and t is the recessive allele, for short. So, plants with the genotype TT or Tt are tall. Plants with the genotype tt are short. The phenotypes are tall and short.
Assuming independent assortment, offspring of a cross between a TT parent and a tt parent produce the following offspring: 1/4 TT, 1/2 Tt, 1/4 tt. The genotypic ratio is 1:2:1. Since both TT and Tt are tall, 3/4 of the offspring are tall, while 1/4 of the offspring are short. The phenotypic ratio is 3:1.
References
- Johannsen, W. (1911). “The Genotype Conception of Heredity”. The American Naturalist. 45 (531): 129–159. doi:10.1086/279202
- Lewontin, Richard C. (1974). The Genetic Basis of Evolutionary Change. New York: Columbia University Press. ISBN 978-0231083188.
- Pigliucci, Massimo (2010). “Genotype–phenotype mapping and the end of the ‘genes as blueprint’ metaphor”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 365 (1540): 557–566. doi:10.1098/rstb.2009.0241
- Washington, Nicole L.; Haendel, Melissa A.; Mungall, Christopher J.; Ashburner, Michael; Westerfield, Monte; Lewis, Suzanna E. (2009). Buetow, Kenneth H. (ed.). “Linking Human Diseases to Animal Models Using Ontology-Based Phenotype Annotation”. PLOS Biology. 7 (11): e1000247. doi:10.1371/journal.pbio.1000247