Variation
Variation refers to the difference in specific characteristics that exist between members of a species e.g. in humans, characteristics such as height, blood group.
There are two types of variation:
(a) Continuous variations
(b) Discontinuous variations
Continuous variation
Refers to the existence of a characteristic in a continuous gradation between two extremes i.e.
A characteristic that changes gradually over a range of values e.g.
Examples
- Height
- Weight
- Foot length
- Fruit size in trees.
Discontinuous variation
Refers to the existence of two or more distinct forms between extremes
A characteristic with only a limited number of possible values.
Individuals can be grouped into distinct and discrete phenotypes.
The character cannot be measured or graded.
Examples;
- Sex
- Blood groups
- Fingerprints
- Pink or white flowers in pea plants
Human blood is an example of discontinuous variation. There are only 4 types of blood group. There are other possibilities and there are no values in between.
Causes of variations
- Independent assortment (during Gamete formation)
During metaphase I, the homologous chromosomes come together in pairs and subsequently segregate into daughter cells independently of each other. This independent assortment produces a wide variety of different gametes.
- Crossing over (during Gamete formation)
During prophase I homologous chromosomes associate, their chromatids break and rejoin at chiasmata where genetic exchanges occur, a process called crossing over that lead to variations.
- Fertilisation
This permits parental genes to be brought together in different combinations; hence, different qualities of parents can be combined in the offspring.
- Mutations
Mutation is a spontaneous change in the genetic make-up of an organism.