Continuous Variation: Variation showing a continuous spectrum of intermediate phenotypes between two extremes.
Characteristics: Quantitative, measurable, controlled by multiple genes (polygenic inheritance), and significantly influenced by environmental factors (e.g., height, weight, skin color, body mass index).
Graphical Representation: Normal distribution curve (bell-shaped curve).
Discontinuous Variation: Variation showing distinct, discrete phenotypic categories with no intermediate forms.
Characteristics: Qualitative, non-measurable, controlled by a single gene (monogenic) with two or three distinct alleles, and unaffected by environmental factors (e.g., ABO blood groups, presence of dimples, ability to roll tongue, earlobe type).
Graphical Representation: Discrete bar chart.
12.2 Causes of Variation
Genetic and Environmental Factors
Genetic Factors (Inherent Variation):
Meiosis - Crossing Over: Exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I creates new allele combinations.
Meiosis - Independent Assortment: Random arrangement and segregation of homologous chromosome pairs at Metaphase I and Anaphase I produces diverse gametes.
Random Fertilization: Fusion between any random sperm cell and any random ovum yields unique zygotic combinations.
Gene and Chromosomal Mutations: Spontaneous permanent changes in DNA base sequences or chromosome structures/numbers introduce entirely new genetic traits.
Environmental Factors:
Sunlight exposure, nutrient intake, climate, water availability, and physical exercise influence phenotypic expression in continuous variation.
Environmental factors cannot alter an organism's underlying genotype or be inherited by offspring.
12.3 Mutation
Types of Mutations and Mutagens
Gene Mutation: Chemical changes in the nucleotide base sequence of a single gene.