Difference Between Transient and Balanced Polymorphism

The key difference between transient and balanced polymorphism is based on how the alleles behave. Transient polymorphism is the progressive replacement of one allele of a gene by another allele while balanced polymorphism is the maintenance of both two different alleles of a gene over time

Genetic polymorphism is one or two or more variants of a particular DNA. Transient and balanced polymorphism are two types of polymorphism that arise due to two alternate alleles of a gene. In transient polymorphism, from the two alternate alleles of a given locus, one allele is progressively replaced by another. In balanced polymorphism, the two alleles are in equilibrium with each other. Both versions of the gene are maintained in the population.

CONTENTS

1. Overview and Key Difference
2. What is Transient Polymorphism
3. What is Balanced Polymorphism
4. Similarities Between Transient and Balanced Polymorphism
5. Side by Side Comparison – Transient vs Balanced Polymorphism in Tabular Form
6. Summary

What is Transient Polymorphism?

Transient polymorphism can be seen in a population when there are two alleles present in a gene pool. These polymorphisms are located in a particular gene locus. One of the two alternate forms of the allele is progressively replaced by another allele during inheritance. This happens due to strong environmental pressure to eliminate one allele from the gene pool. This as an unbalanced, unstable polymorphism and is not predictive.

For example, we can notice moths inhabiting industrial regions get blackened by smoke. Moths become more black or blackish in polluted areas due to the progressive replacement of one allele from the allele responsible for excess melanin production. Therefore, light and dark melanin variant of the moth is a transient polymorphism directed by industrial melanism or directional selection.

What is Balanced Polymorphism?

Balanced polymorphism is a stable polymorphism that can be maintained throughout inheritance. In this polymorphism, the organism will have both alleles of the gene, rather than having two copies of either version alone. Therefore, in a population, both versions of the gene will be maintained. Usually, a balanced polymorphism is maintained in the heterozygous condition. In some cases, this brings about a heterozygote advantage. For example, we can consider two alleles of a gene that are responsible for producing enzymes that detoxify poisons and other chemicals. One allele has a higher activity of detoxifying chemicals, but it causes accumulation of harmful intermediates. The other allele has low activity, but the harm is less. Hence, the best situation for the organism is to have one copy of each allele. This is a type of a balanced polymorphism.

Moreover, sickle cell anemia in Africans can be nicely explained by balanced polymorphism. Heterozygotes do not get anemia. They are also resistant to malaria. Hence, heterozygous advantage in sickle cell anemia is a result of balanced polymorphism.

What are the Similarities Between Transient and Balanced Polymorphism?

• Transient and balanced polymorphism can lead to phenotypic changes.
• They may alter protein expression.
• They involve alleles of a gene.
• Both take place at a locus of a gene.

What is the Difference Between Transient and Balanced Polymorphism?

Transient and balanced polymorphisms are two types of polymorphisms visible in populations. Transient polymorphism refers to the progressive replacement of one allele of a gene by another allele. In contrast, balanced polymorphism refers to the maintaining of both two different alleles of a gene over time. So, this is the key difference between transient and balanced polymorphism. In other words, one allele is involved in transient polymorphism while both alleles of a gene are involved in the balanced polymorphism.

Below infographic summarizes the difference between transient and balanced polymorphism.

Summary –  Transient vs Balanced Polymorphism

Transient and Balanced polymorphisms are two types of polymorphisms seen in populations. Both give rise to phenotypic changes and affect gene expression. Transient polymorphism takes place when one allele is progressive replaced by another casing polymorphism. Hence, only one allele is affected in transient polymorphism. In contrast, two different versions of the gene (two different alleles) are maintained over time in balanced polymorphism. Therefore, both types of alleles are maintained. Thus, this is the key difference between transient and balanced polymorphism.

Reference:

1. Ford, E. B. “POLYMORPHISM.” Wiley Online Library, John Wiley & Sons, Ltd, 21 Jan. 2008, onlinelibrary.wiley.com/doi/10.1111/j.1469-185X.1945.tb00315.x.
2. “Balanced Polymorphism | Encyclopedia.Com”. Encyclopedia.Com, 2020, Available here.

Image Courtesy:

1. “Biston.betularia.f.carbonaria.7209” By Chiswick Chap – Self-published work by Chiswick Chap (CC BY-SA 3.0) via Commons Wikimedia