Discontinuous variation occurs when individuals within a population show distinct, non-overlapping categories of traits. These traits are often controlled by single genes with a limited number of alleles.
Characteristics of Discontinuous Variation
Traits fall into distinct categories (e.g., blood groups).
The number of categories is limited.
Often controlled by single genes with a few alleles.
Examples include blood groups, sex determination, and some inherited diseases.
Examples of Discontinuous Variation
Blood Groups: The ABO blood group system is a classic example. Individuals can have blood groups A, B, AB, or O. This is determined by the presence or absence of specific alleles for the ABO gene.
Sex Determination: In humans, sex is determined by the presence or absence of the Y chromosome. This results in two distinct phenotypes: male and female.
Cystic Fibrosis: This is a genetic disorder caused by a mutation in a single gene. Individuals either have the disease or they do not.
Trait
Alleles
Phenotypes
Blood Group (ABO system)
IA, IB, i
A, B, AB, O
Sex
XY
Male, Female
Cystic Fibrosis
FF, Ff
Affected, Non-affected
Continuous Variation
Continuous variation occurs when individuals within a population show a range of phenotypes for a particular trait. These traits are typically influenced by multiple genes and environmental factors.
Characteristics of Continuous Variation
Traits show a range of values (e.g., height).
The number of categories is unlimited.
Often controlled by multiple genes (polygenic inheritance) and environmental factors.
Examples include height, weight, and skin colour.
Examples of Continuous Variation
Height: Individuals can have a wide range of heights within a population. This is influenced by many genes and environmental factors like nutrition.
Weight: Similar to height, weight exhibits continuous variation due to the influence of multiple genes and diet.
Skin Colour: Skin colour is a complex trait influenced by multiple genes and exposure to sunlight.
Graphical Representation of Continuous Variation
Continuous variation is often displayed using a normal distribution curve. This curve shows the frequency of different phenotypes, with the most common phenotype in the centre and the less common phenotypes at the extremes.
Suggested diagram: A bell-shaped curve representing a normal distribution, showing the range of phenotypes for a continuous trait like height.