54 5.14 Non-Mendelian Inheritance

Created by: CK-12/Adapted by Christine Miller


Figure 5.14.1 Collage of Diverse Faces.

This collage shows some of the variation in human skin colour, which can range from very light to very dark, with every possible gradation in between.  As you might expect, the skin color trait has a more complex genetic basis than just one gene with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like skin color, many other human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called non-Mendelian inheritance, and they include inheritance of multiple allele traits, traits with codominance or incomplete dominance, and polygenic traits, among others. All of these modes are described below.

Multiple Allele Traits


ABO Blood types
Figure 5.14.2 ABO blood types per genotype.

The majority of human genes are thought to have more than two normal versions, or alleles. Traits controlled by a single gene with more than two alleles are called multiple allele traits. An example is ABO blood type. Your blood type refers to which of certain proteins called antigens are found on your red blood cells. There are three common alleles for this trait, which are represented by the letters A, B, and O.

As shown in the table there are six possible ABO genotypes, because the three alleles, taken two at a time, result in six possible combinations. The A and B alleles are dominant to the O allele. As a result, both AA and AO genotypes have the same phenotype, with the A antigen in their blood (type A blood). Similarly, both BB and BO genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the O allele, so people with the OO genotype have no antigens for ABO blood type in their blood (type O blood).


Look at the genotype AB in the ABO blood group table. Alleles A and B for ABO blood type are neither dominant nor recessive to one another. Instead, they are codominant. Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. In the case of ABO blood type, AB heterozygotes have a unique phenotype, with both A and B antigens in their blood (type AB blood).

Incomplete Dominance

Another relationship that may occur between alleles for the same gene is incomplete dominance. This occurs when the dominant allele is not completely dominant. In this case, an intermediate phenotype results in heterozygotes who inherit both alleles. Generally, this happens when the two alleles for a given gene both produce proteins, but one protein is not functional. As a result, the heterozygote individual produces only half the amount of normal protein as is produced by an individual who is homozygous for the normal allele.

An example of incomplete dominance in humans is Tay Sachs disease. The normal allele for the gene in this case produces an enzyme that is responsible for breaking down lipids. A defective allele for the gene results in the production of a nonfunctional enzyme. Heterozygotes who have one normal and one defective allele produce half as much functional enzyme as the normal homozygote, and this is enough for normal development. Homozygotes who have only defective allele, however, produce only nonfunctional enzyme. This leads to the accumulation of lipids in the brain starting in utero, which causes significant brain damage. Most individuals with Tay Sachs disease die at a young age, typically by the age of five years.


5.14.2 Incomplete dominance of hair.
Figure 5.14.3 Three phenotypes of hair through the incomplete dominance model.

Another good example of incomplete dominance in humans is hair type.  There are genes for straight and curly hair, and if an individual is heterozygous, they will typically have the phenotype of wavy hair.

Polygenic Traits

Like many other polygenic traits, adult height has a bell-shaped distribution.
Figure 5.14.4 Human Adult Height. Like many other polygenic traits, adult height has a bell-shaped distribution.

Many human traits are controlled by more than one gene. These traits are called polygenic traits. The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.

An example of a human polygenic trait is adult height. Several genes, each with more than one allele, contribute to this trait, so there are many possible adult heights. One adult’s height might be 1.655 m (5.430 feet), and another adult’s height might be 1.656 m (5.433 feet). Adult height ranges from less than 5 feet to more than 6 feet, with males, on average, being somewhat taller than females. The majority of people fall near the middle of the range of heights for their sex, as shown in Figure 5.14.4.

Environmental Effects on Phenotype

Image shows a hand with a tan line where a watch had been worn.
Figure 5.14.5 Due to the effects of UV radiation, the skin on the upper part of the arm is much darker in color than the  skin that was protected by a watch strap.

Many traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Adult height, for example, might be negatively impacted by poor diet or childhood illness. Skin color is another polygenic trait. There is a wide range of skin colors in people worldwide. In addition to differences in genes, differences in exposure to ultraviolet (UV) light cause some variation. As shown in Figure 5.14.5, exposure to UV light darkens the skin.



Some genes affect more than one phenotypic trait. This is called pleiotropy. There are numerous examples of pleiotropy in humans. They generally involve important proteins that are needed for the normal development or functioning of more than one organ system. An example of pleiotropy in humans occurs with the gene that codes for the main protein in collagen, a substance that helps form bones. This protein is also important in the ears and eyes. Mutations in the gene result in problems not only in bones, but also in these sensory organs, which is how the gene’s pleiotropic effects were discovered.

Another example of pleiotropy occurs with sickle cell anemia. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell protein called hemoglobin. People with the disorder have two alleles for sickle cell hemoglobin, so named for the sickle shape (pictured in Figure 5.14.6) that their red blood cells take on under certain conditions (like physical exertion). The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.

Image shows the difference in morphology between a sickle cell and a normal red blood cell.
Figure 5.14.6 For comparison, the sickle-shaped red blood cell on the left is shown next to several normal red blood cells.


Some genes affect the expression of other genes. This is called epistasis. Epistasis is similar to dominance, except that it occurs between different genes, rather than between different alleles for the same gene.

Albinism is an example of epistasis. A person with albinism has virtually no pigment in the skin. The condition occurs due to an entirely different gene than the genes that encode skin color. Albinism occurs because a protein called tyrosinase, which is needed for the production of normal skin pigment, is not produced, due to a gene mutation. If an individual has the albinism mutation, he or she will not have any skin pigment, regardless of the skin color genes that were inherited.

Feature: My Human Body

Do you know your ABO blood type? In an emergency, knowing this valuable piece of information could possibly save your life. If you ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your own blood does not contain, antibodies in your blood plasma (the liquid part of your blood) will recognize the antigen as foreign to your body and cause a reaction called agglutination. In this reaction, the transfused red blood cells will clump together. The agglutination reaction is serious and potentially fatal.

Knowing the antigens and antibodies present in each of the ABO blood types will help you understand which type(s) of blood you can safely receive if you ever need a transfusion. This information is shown in Figure 5.14.7 for all of the ABO blood types. If you have blood type A, this means that your red blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which have the B antigen, your anti-B antibodies would attack the transfused red blood cells, causing agglutination.


Image shows a table of each blood type, which antigens and antibodies are present, and acceptable blood donor types.
Figure 5.14.7 Antigens and antibodies in ABO blood types.


You may have heard that people with blood type O are called “universal donors,” and that people with blood type AB are called universal recipients. People with type O blood have neither A nor B antigens in their blood, so if their blood is transfused into someone with a different ABO blood type, it causes no immune reaction, meaning they can donate blood to anyone. On the other hand, people with type AB blood have no anti-A or anti-B antibodies in their blood, so they can receive a transfusion of blood from anyone. Which blood type(s) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?

5.14 Summary

  • Non-Mendelian inheritance refers to the inheritance of traits that have a more complex genetic basis than one gene with two alleles and complete dominance.
  • Multiple allele traits are controlled by a single gene with more than two alleles. An example of a human multiple allele trait is ABO blood type, for which there are three common alleles: A, B, and O.
  • Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. A human example of codominance also occurs in the ABO blood type, in which the A and B alleles are codominant.
  • Incomplete dominance is the case in which the dominant allele for a gene is not completely dominant to a recessive allele for the gene, so an intermediate phenotype occurs in heterozygotes who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional enzyme as normal homozygotes.
  • Polygenic traits are controlled by more than one gene, each of which has a minor additive effect on the phenotype. This results in a whole continuum of phenotypes. Examples of human polygenic traits include skin color and adult height.
  • Many traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Skin color, for example, may be affected by exposure to UV light, and adult stature may be affected by diet or childhood disease.
  • Pleiotropy refers to the situation in which a gene affects more than one phenotypic trait. A human example of pleiotropy occurs with sickle cell anemia. People who inherit two recessive alleles for this disorder have abnormal red blood cells and may exhibit multiple other phenotypic effects, such as stunting of physical growth, kidney failure, and strokes.
  • Epistasis is the situation in which one gene affects the expression of other genes. An example of epistasis is albinism, in which the albinism mutation negates the expression of skin color genes.

5.14 Review Questions

  1. What is non-Mendelian inheritance?
  2. Explain why the human ABO blood group is an example of a multiple allele trait with codominance.
  3. What is incomplete dominance? Give an example of this type of non-Mendelian inheritance in humans.
  4. Explain the genetic basis of human skin color.
  5. How can the human trait of adult height be influenced by the environment?
  6. Define pleiotropy, and give a human example.
  7. Compare and contrast epistasis and dominance.
  8. What is the difference between pleiotropy and epistasis?


5.14 Explore More

Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!,
Amoeba Sisters, 2015.

Non-Mendelian Genetics, Teacher’s Pet, 2015.


Figure 5.14.1

Figure 5.14.2

ABO Blood Types Per Genotype by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.14.3

Three Phenotypes of Hair Based on Inheritance/ Incomplete Dominance Hair by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.14.4

Average height /Human Adult Height by CK-12 Foundation is used under a CC BY 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 5.14.5

Tan lines by katiebordner on Flickr is used under a  CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 5.14.6

Sickle cell anemia by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) ©

Figure 5.14.7

ABO_blood_type.svg by InvictaHOG on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).


Amoeba Sisters. (2015, May 25). Incomplete dominance, codominance, polygenic traits, and epistasis! YouTube. https://www.youtube.com/watch?v=YJHGfbW55l0

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 18.9 Sickle cells [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/18-3-erythrocytes

Brainard, J/ CK-12 Foundation. (2016). Figure 2 Human adult height [digital image]. In CK-12 College Human Biology (Section 5.13) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/5.13/

Mayo Clinic Staff. (n.d.). Tay-Sachs disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/tay-sachs-disease/symptoms-causes/syc-20378190

Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/symptoms-causes/syc-20355876

Teacher’s Pet. (2015, January 25). Non-mendelian genetics. YouTube. https://www.youtube.com/watch?v=-4vsio8TZrU



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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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