8.6 Factors Causing Deviation from Mendelian Phenotypic Ratios

There are other factors that affect an organism’s phenotype and thus appear to alter Mendelian inheritance.

  1. Genetic heterogeneity: There is more than one gene or genetic mechanism that can produce the same phenotype.
  2. Polygenic determination: One phenotypic trait is controlled by multiple genes.
  3. Phenocopy: Organisms that do not have the genotype for trait A can also express trait A due to environmental conditions; they do not have the same genotype but the environment simply “copies” the genetic phenotype.
  4. Incomplete penetrance: even though an organism possesses the genotype for trait A, it might not be expressed with 100% effect.
  5. Certain genotypes show a survival rate that is less than 100%. For example, genotypes that cause death, recessive lethal mutations, at the embryo or larval stage will be underrepresented when adult flies are counted.

The video, Extension of Mendelism – Phenocopy, Incomplete Penetrance, Expressivity (BI_08), by Biology Insights (2020) on YouTube, discusses various extensions of Mendelism, including phenocopy and incomplete penetrance.


Biology Insights. (2020, July 29). Extension of Mendelism – Phenocopy, incomplete penetrance, expressivity (BI_08) (video file). YouTube. https://www.youtube.com/watch?v=m9Dv3gS73d8


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Introduction to Genetics Copyright © 2023 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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