The phenotypes described thus far, have a nearly perfect correlation with their associated genotypes; in other words, an individual with a particular genotype always has the expected phenotype. However, most phenotypes are not determined entirely by genotype alone. Instead, they are determined by an interaction between genotype and environmental factors and can be conceptualized in the following relationship:
Genotype + Environment
⇒ Phenotype (G + E ⇒ P)
Genotype + Environment + InteractionGE
⇒ Phenotype (G + E + IGE ⇒ P)
*GE = Genetics and Environment
This interaction is especially relevant in the study of economically important phenotypes, such as human diseases or agricultural productivity. For example, a particular genotype may pre-dispose an individual to cancer, but cancer may only develop if the individual is exposed to certain DNA-damaging chemicals or carcinogens. Therefore, not all individuals with the particular genotype will develop the cancer phenotype, only those who experience a particular environment. The terms penetrance and expressivity are also useful to describe the relationship between certain genotypes and their phenotypes.
Penetrance is the proportion of individuals with a particular genotype that display a corresponding phenotype (Figure 8.5.1). It is usually expressed as a percentage of the population. Because all pea plants are homozygous for the allele for white flowers, this genotype is completely (100%) penetrant. In contrast, many human genetic diseases are incompletely penetrant; since not all individuals with the disease genotype develop symptoms associated with the disease (less than 100%).
Expressivity describes the variability in mutant phenotypes observed in individuals with a particular phenotype (Figure 8.5.1 and Figure 8.5.2). Many human genetic diseases provide examples of broad expressivity, since individuals with the same genotypes may vary greatly in the severity of their symptoms. Incomplete penetrance and broad expressivity are due to random chance, non-genetic (environmental), and genetic factors (mutations in other genes).
The video, Penetrance vs. Expressivity, by The Excel Cycle (2020) on YouTube, discusses the difference between expressivity and penetrance.
- Figure 8.5.1 Original by Locke (2017), CC BY-NC 3.0, Open Genetics Lectures
- Figure 8.5.2 Original by Locke (2017), CC BY-SA, Open Genetics Lectures
Locke, J. (2017). Figures: 18. Relationship between penetrance and expressivity; and 19. Mutations in wings of Drosophila melanogaster… [digital images]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 26, p. 11). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf
The Excel Cycle. (2020, June 5). Penetrance vs. expressivity (video file). YouTube. https://www.youtube.com/watch?v=nurrFUIDBHc