Difference Between Pleiotropy and Polygenic Inheritance

Main Difference – Pleiotropy vs Polygenic Inheritance

Both pleiotropy and polygenic inheritance are two terms used to describe the relationship between genes and their phenotypes or traits. Pleiotropy follows the Mendelian inheritance patterns while polygenic inheritance is a pattern of non-mendelian inheritance. In Mendelian inheritance, a single gene is involved in determining a single trait. The main difference between pleiotropy and polygenic inheritance is that in pleiotropy, one gene affects many traits whereas, in polygenic inheritance, many genes affect one trait. Both pleiotropy and polygenic inheritance occur in all living organisms. Albinism, phenylketonuria, autism, schizophrenia, sickle cell anemia, and Marfan syndrome are examples of pleiotropy. Height, weight, body shape, eye color, skin color, and hair color of humans are controlled by the polygenic inheritance.

Key Areas Covered

1. What is Pleiotropy
      – Definition, Features, Examples
2. What is Polygenic Inheritance
      – Definition, Features, Examples
3. What are the Similarities Between Pleiotropy and Polygenic Inheritance
      – Outline of Common Features
4. What is the Difference Between Pleiotropy and Polygenic Inheritance
      – Comparison of Key Differences

Key Terms: Genotype, Incomplete Dominance, Mendelian Inheritance, Offspring, Pleiotropy, Polygenic Inheritance, Trait

What is Pleiotropy

In pleiotropy, a single gene affects many traits. That means the gene product is used in many types of cells in different tissues. Sometimes, the gene product may act as a signaling molecule, affecting the functions of many tissues. The gene, which is responsible for the coat color of mice is pleiotropic. The dominant allele Y produces the yellow coat color and the recessive allele y produces the agouti color in the mice. The genotype yy produces agouti color mice. The genotype Yy produces yellow color mice. The genotype YY produces two traits, the coat color and lethality. Therefore, the mice embryo with the genotype YY will terminate prematurely. Since one gene is involved in the determination of the trait in pleiotropy, only three different genotypic outcomes can be observed in the offspring. The mechanism of pleiotropy is shown in figure 1.

When a mutation occurs in this gene, many symptoms may arise since the gene affects many traits. Phenylketonuria is a disease caused by a mutation in the gene which is coded for the enzyme, phenylalanine hydroxylase. The symptoms of phenylketonuria are mental retardation, reduced hair, and skin pigmentation. A gene carrying a mixture of both beneficial and harmful traits with the same gene is referred to as an antagonistic pleiotropy. The p53 gene suppresses the undifferentiated cell proliferation, preventing cancer. At the same time, it suppresses stem cell proliferation, preventing the tissue regeneration in the old. Aging is another example of antagonistic pleiotropy, which increases the fitness in youth but decreases the fitness as one grows old.

Figure 2: Albinism

Albinism occurs when the melanin production of melanin is altered by a mutation. It affects the organism’s skin, hair, and eye. Autism, schizophrenia, sickle cell anemia, and Marfan syndrome are examples of diseases caused by pleiotropy. Albinism in a peacock is shown in figure 2.

What is Polygenic Inheritance

In polygenic inheritance, a particular trait is determined by more than one gene. Thus, the effect of one gene on the trait is small. Here, the contributing genes exhibit incomplete dominance. Thus, the trait in the offspring is a mixture of parental traits. The external environmental factors also have an effect on polygenic inheritance. Most of the metric and meristic traits are under the influence of polygenic inheritance. The polygenic traits exhibit a continuous distribution in a  population. Thus, the distribution curve of the polygenic inheritance is bell-shaped. A great variability of genotypes can be observed within a population in polygenic traits. The organisms at the middle of the distribution curve consist of a combination of both dominant and recessive alleles. The individuals with many of the dominant alleles or recessive alleles may appear at the end of the curve. The distribution curve of the polygenic inheritance of height in humans is shown in figure 3.

Figure 3: Polygenic inheritance of height

The color of the human eye is controlled by 16 different genes. The eye color is determined by the amount of melanin produced in front of the iris. The color can be either black, brown, green, hazel or blue. The skin color of humans is another example of polygenic inheritance. The color of the skin is determined by the amount of melanin produced in the skin. When the number of dark alleles present in the skin is high, the color of the skin becomes darker. 

Similarities Between Pleiotropy and Polygenic Inheritance

• Pleiotropy and polygenic inheritance describe the relationship between genes and their traits.
• Both pleiotropy and polygenic inheritance can occur in all living organisms.

Difference Between Pleiotropy and Polygenic Inheritance

Definition

Pleiotropy: Pleiotropy is the controlling of multiple traits by a single gene.

Polygenic Inheritance: Polygenic inheritance is the controlling of a single trait by multiple genes.

Outcome

Pleiotropy: Pleiotropy has only three genotypic outcomes.

Polygenic Inheritance: Polygenic inheritance has many genotypic outcomes.

Trait

Pleiotropy: In pleiotropy, a particular trait is influenced by one gene.

Polygenic Inheritance: In polygenic inheritance, a particular trait is influenced by many genes.

Effect of One Gene on the Trait

Pleiotropy: The effect of one gene on its trait is 100%.

Polygenic Inheritance: The effect of one gene on the trait is small.

Mendelian Inheritance

Pleiotropy: The pleiotropy follows Mendelian inheritance patterns.

Polygenic Inheritance: Polygenic inheritance is a non-Mendelian inheritance pattern.

Effect of the Environmental Factors

Pleiotropy: Typically, pleiotropy is not affected by the environmental factors.

Polygenic Inheritance: The traits of the polygenic inheritance are highly affected by the environmental factors.

Examples

Pleiotropy: Albinism, phenylketonuria, autism, schizophrenia, sickle cell anemia, and Marfan syndrome are examples of pleiotropy.

Polygenic Inheritance: Height, weight, body shape, eye color, skin color, and hair color of humans are controlled by polygenic inheritance.

Conclusion

Pleiotropy and polygenic inheritance describe the influence of genes on their phenotypes. In pleiotropy, a single gene controls a particular character, obeying Mendelian inheritance patterns. In polygenic inheritance, a single trait is controlled by many genes. The main difference between pleiotropy and polygenic inheritance is the patterns of influences of genes on their traits.

Reference:

1. “Pleiotropy.” Genetics-notes. N.p., n.d. Web. Available here. 13 July 2017. 
2. Bailey, Regina. “What Is Polygenic Inheritance?” ThoughtCo. N.p., n.d. Web. Available here. 13 July 2017.

Image Courtesy:

“Pavo cristatus -Southwicks Zoo, Massachusetts, USA -albino-8a (1)” By Eric Kilby from USA – White Peacock Uploaded by Snowmanradio (CC BY-SA 2.0) via Commons Wikimedia
“Polygenic inheritance” By Source (WP:NFCC#4) (Fair use) via Commons Wikimedia