What is the Difference Between Continuous and Discontinuous Variation

The main difference between continuous and discontinuous variation is that continuous variation shows an unbroken range of phenotypes of a particular character in the population whereas discontinuous variation shows two or more separate forms of a character in the population. Furthermore, the presence of many genes for the determination of a particular trait causes continuous variation while one or few genes are responsible for the discontinuous variation. In addition to these, weight, height, and length of organisms are examples of continuous variation while the color of petals, blood groups of animals, etc. are examples of discontinuous variation. 

Continuous and discontinuous variation are two types of genetic variation which occurs in organisms. Generally, genetic variation is the difference in DNA among individuals, which are caused by mutations and genetic recombination. 

Key Areas Covered 

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

Key Terms 

Alleles, Continuous Variation, Discontinuous Variation, Genetic Variation, Phenotypes

What is Continuous Variation 

Continuous variation is the type of genetic variation showing an unbroken range of phenotypes in a population. Generally, measurable characters such as height, weight, and skin color are classic examples of continuous genetic variation. Moreover, the main feature of phenotype-wise continuous variation is the presence of intermediate phenotypes. Therefore, it gives a bell-shaped distribution curve of phenotypes. 

Figure 1: Variation of Intelligence

Furthermore, phenotypic variation in continuous variation occurs either under the influence of environmental factors or due to a genetic basis. Generally, many genes are involved in the determination of a particular phenotype in continuous variation. Therefore, this type of trait is also known as a quantitative trait. 

What is Discontinuous Variation 

Discontinuous variation is the second type of genetic variation, showing two or more distinct phenotypes for a particular character in a population. Generally, it is the most simple type of genetic variation to analyze. Basically, one gene determining a particular character usually has two alleles whose dominant forms exhibit one phenotype while the recessive form exhibits the second phenotype. Therefore, there is a predictable relationship between the genotype and phenotype of discontinuous variation. However, in some cases, a single gene with more than two types of alleles produce polymorphism.

Figure 2: Blood Group Variation

Moreover, another type of discontinuous variation is the presence of mutants with the normal companion phenotype called the wild type. Basically, both mutants and the wild type are still determined by different forms of a particular gene or in other words, alleles. Furthermore, some examples of discontinuous variation include characters with discrete phenotypes such as flower color, blood groups, gender, etc. 

Similarities Between Continuous and Discontinuous Variation 

• Continuous and discontinuous variation are two types of genetic variation seen among individuals of populations. 
• Both occur due to the differences in DNA in different genomes caused by mutations and genetic recombination. 
• Moreover, both cause phenotypic variation in different aspects including physical appearance, metabolism, fertility, mode of reproduction, behavior, learning and mental ability, and other obvious or measurable characters.  

Difference Between Continuous and Discontinuous Variation 

Definition 

Continuous variation refers to the type of genetic variation, which shows an unbroken range of phenotypes of a particular character in the population while discontinuous variation refers to the type of genetic variation, which shows to or more separate forms of a character in the population.  

Significance of Phenotypes 

Phenotypes of continuous variation have a continuous range, and they are difficult to classify into specific categories. In contrast, phenotypes of discontinuous variation have a discontinuous range, and they can be categorized easily. 

Intermediate Groupings 

Intermediate groupings occur in continuous variation while no intermediate groupings occur in discontinuous variation. 

Graphical Representation 

Continuous variation gives a smooth, bell-shaped curve, while discontinuous variation does not give a curve. 

Significance of Genetic Variation 

Moreover, continuous variation is the more common type of genetic variation, while discontinuous variation is the simplest form of genetic variation. 

Number of Genes 

The presence of many genes for the determination of a particular trait causes continuous variation while one or few genes are responsible for the discontinuous variation.  

Correspondence of Genotype and Phenotype 

Furthermore, there is no one-to-one correspondence of genotype and phenotype in continuous variation while there is a predictable one-to-one relation between genotype and phenotype in discontinuous variation. 

Examples 

Weight, height, and length of organisms are examples of continuous variation while the color of petals, blood groups of animals, the gender of animals, etc. are examples of discontinuous variation.  

Conclusion 

Continuous variation is the type of genetic variation in which a particular character shows an unbroken range of phenotypes. Generally, it is the most common type of genetic variation governed by the presence of many genes. The weight and height of animals are examples of continuous variation. In contrast, discontinuous variation is the second type of genetic variation in which a particular character shows only several phenotypes. Therefore, the continuous and discontinuous variation is the range of phenotypes for a particular character. 

References:

1. Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Genetic variation Available Here. 

Image Courtesy:

1. “Sketch-4race-transparent” (CC BY-SA 3.0) via Commons Wikimedia   
2. “ABO system codominance” By GYassineMrabet – Own work based on Codominant.jpg (Public Domain) via Commons Wikimedia