2.3 Common Misconceptions in Non-Mendelian Inheritance

In Lesson 2.1, we explored the various forms of Non-Mendelian inheritance. Due to the complexity of genetics and heredity, we will not delve into some prevalent misconceptions and clarify their distinctions.

Pleiotropy vs. Polygenic Inheritance

Eye color as an example of polygenic inheritance

It is easy to confuse pleiotropy with polygenic inheritance since both explanations examine the number of genes that controls the phenotypic traits. However, both terms have distinct meanings in genetics. Pleiotropy exhibits how one gene is able to control multiple phenotypic traits. While polygenic inheritance, from the word ‘poly’ meaning many and ‘genic’ relating to genes, means that multiple genes control a single phenotypic trait of an individual.

Marfan syndrome resulting from pleiotropy

Eye color is a prime example of a trait influenced by polygenic inheritance. It has been documented that eight genes regulate the quantity of melanin present in the iris, ultimately determining the individual’s eye color1. Conversely, Marfan syndrome is characterized by the impairment of various structures, including the skeletal system, lungs, eyes, heart, and blood vessels caused by one gene. This disorder is associated with a mutation in the FBN1 gene located on chromosome 15. The FBN1 gene is responsible for encoding fibrillin, a vital component in the formation of elastic fibers within connective tissue2.

Codominance vs. Incomplete Dominance

A speckled chicken showing codominant alleles for black and white feathers

Codominance and incomplete dominance are two different types of inheritance. People sometimes confuse codominance and incomplete dominance but the name itself should tell what type of inheritance it is. Incomplete dominance refers to the blending of phenotypic traits which results in the production of an intermediate heterozygote. While codominance is where two distinct phenotypic traits are expressed in a heterozygote, it is a case where neither of the alleles displays complete dominance over the other.

Wavy hair as an example of incomplete dominance

People with semi-curly or wavy hair serves as an illustration of incomplete dominance, wherein the combination of alleles from parents with straight and curly hair results in offspring with intermediate hair textures. Other examples of traits exhibiting incomplete dominance in humans include height and skin color3. Meanwhile, a speckled color of both black and white in chickens illustrates an example of codominance wherein the allele for black feathers is codominant with the allele for white feathers resulting in the phenotypic expression of both4.

  1. The Genetics of Eye Color. (n.d.). https://www.hudsonalpha.org/the-genetics-of-eye-color/
  2. National Center for Biotechnology Information (US). (1998). Marfan syndrome. Genes and Disease – NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK22203/
  3. Biology Online. (2021, August 18). Incomplete dominance – Definition and Examples – Biology Online Dictionary. Biology Articles, Tutorials & Dictionary Online. https://www.biologyonline.com/dictionary/incomplete-dominance
  4. The Editors of Encyclopaedia Britannica. (2020, June 17). Codominance | Definition & Examples. Encyclopedia Britannica. https://www.britannica.com/science/codominance
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