There are other factors that affect an organism’s phenotype and thus appear to alter Mendelian inheritance.
- Genetic heterogeneity: There is more than one gene or genetic mechanism that can produce the same phenotype.
- Polygenic determination: One phenotypic trait is controlled by multiple genes.
- 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.
- Incomplete penetrance: even though an organism possesses the genotype for trait A, it might not be expressed with 100% effect.
- 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