8.5 Environmental Factors

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)

Or:

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

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%).

Ovals of various shades of black, grey and white, used to demonstrate penetrance
Figure 8.5.1 Relationship Between Penetrance and Expressivity in Eight Individuals That All Have a Mutant Genotype. Penetrance can be complete (all eight have the mutant phenotype) or incomplete (only some have the mutant phenotype). Amongst those individuals with the mutant phenotype the expressivity can be narrow (very little variation) to broad (lots of variation).

Expressivity

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).

Mutations in wings of Drosophila melanogaster showing weak to strong expressivity.
Figure 8.5.2 Mutations in Wings of Drosophila Melanogaster Showing Weak to Strong Expressivity

The video, Penetrance vs. Expressivity, by The Excel Cycle (2020) on YouTube, discusses the difference between expressivity and penetrance.

Media Attributions

Reference

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

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Open Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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