explain the genetic basis of discontinuous variation and continuous variation

Variation

Objective: Explain the genetic basis of discontinuous variation and continuous variation, and compare their inheritance patterns and underlying mechanisms.

1. Introduction

Variation refers to the differences that exist between individuals of the same species. It provides the raw material for natural selection and is essential for the evolution of populations. In A‑Level Biology, variation is classified into two broad categories:

• Discontinuous (or qualitative) variation
• Continuous (or quantitative) variation

2. Discontinuous \cdot ariation

Discontinuous variation produces distinct, separate phenotypic classes. The differences are usually caused by single genes (or a small number of genes) that have a large effect on the trait.

2.1 Genetic basis

Most discontinuous traits follow Mendelian inheritance patterns:

• Complete dominance: one allele completely masks the other (e.g., flower colour in Antirrhinum).
• Incomplete dominance: heterozygotes show an intermediate phenotype (e.g., snapdragon flower colour).
• Codominance: both alleles are expressed simultaneously (e.g., ABO blood groups).
• Multiple alleles: more than two alleles segregate in the population (e.g., human blood type).
• Sex‑linked inheritance: genes located on sex chromosomes (e.g., colour blindness).

2.2 Examples

2.3 Key characteristics

• Phenotypic classes are clearly separated.
• Often controlled by a single locus with large effect.
• Environmental influence is minimal.
• Inheritance can be predicted using Punnett squares.

3. Continuous \cdot ariation

Continuous variation produces a range of phenotypes that blend into one another, forming a normal distribution in a large population.

3.1 Genetic basis – polygenic inheritance

Traits are controlled by many genes (polygenes), each contributing a small additive effect. The combined effect of these genes, together with environmental factors, generates the observed variation.

Mathematically, the phenotypic variance (\$V_P\$) can be expressed as:

\$VP = VG + VE + V{GE}\$

where:

• \$V_G\$ = genetic variance (sum of additive, dominance, and epistatic components)
• \$V_E\$ = environmental variance
• \$V_{GE}\$ = genotype‑environment interaction variance

3.2 Examples

3.3 Characteristics of continuous traits

• Phenotypic distribution approximates a bell‑shaped curve.
• No clear-cut categories; intermediate forms are common.
• Both genotype and environment contribute substantially.
• Quantitative genetics methods (e.g., regression, heritability estimates) are used to analyse inheritance.

4. Comparison of Discontinuous and Continuous \cdot ariation

5. Summary

• Variation is essential for evolution; it can be discontinuous or continuous.
• Discontinuous variation is typically governed by single genes and shows clear phenotypic classes.
• Continuous variation results from polygenic inheritance and environmental effects, producing a spectrum of phenotypes.
• Understanding the genetic basis of each type aids in predicting inheritance patterns and in interpreting population studies.