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There are several causes for variation being present within a population:

  1. Each gene has different alleles. Therefore, different individuals may have different alleles.
  2. During prophase I of meiosis, chiasmata (crossing over) occurs whereby sections of DNA are swapped between sister chromatids.
  3. During metaphase I of meiosis, there is independent assortment of the homologous pairs of chromatids.
  4. Mutation - gene and chromosome.
  5. Random fertilisation.

This shows that there is variation of genotype and phenotype between individuals. The environment also exerts an effect and can cause variation.

For each characteristic, the population may show either continuous or discontinuous variation.

The genetic basis for discontinuous variation

This is where different alleles for one gene have a large effect on the phenotype.


ABO blood groups, there are no intermediates; you are either A, B, AB or O.

The genetic basis for continuous variation

Different alleles for one gene have small effects.

Different loci have the same or additive effects. (When a large number of loci produce a combined effect it is called polygeny.)


Imagine height is controlled just by two genes (though in reality, many genes will contribute to height). Each has two alleles; E and e, F and f.

E and F contribute 2cm to height whereas e and f contribute just 1cm to height.


EEFF = 8cm

eeff = 4cm

If EeFf is crossed with EeFf, the outcome will be...

Parental genotypes: EEFF x eeff
Gametes: All EF x All ef
F1: all EeFf

Parental genotypes: EeFf x EeFf
Gametes: EF, Ef, eF, ef x EF, Ef, eF, ef

F2 punnett square: EF Ef eF ef
Ef EEFf EEff EeFf Eeff
eF EeFF EeFf eeFF eeFf
ef EeFf Eeff eeFf eeff

Phenotypes: 8 cm tall 7 cm tall 6 cm tall 5 cm tall 4 cm tall
Ratio: 1 4 6 4 1

Variation in a population with regard to height

This crudely shows the continuous variation in a population with regard to height.

Don't forget that environment also causes variation of the phenotype. This will not be passed on to offspring.

In any population, the total variety of genes and alleles present is called the gene pool.

This gene pool can change in content (new alleles arriving, existing alleles being lost) or the ratio of alleles altering due to the following:

  1. Mutation
  2. Natural selection
  3. Emigration
  4. Immigration
  5. Mate selection

The factors favouring stability of the gene pool are:

  1. No mutation
  2. No natural selection
  3. The population being large
  4. No gene flow (due to individuals emigrating or immigrating)
  5. Random mating

If these factors favouring stability are fulfilled, the ratio of the alleles for a gene can be established using the Hardy-Weinberg Equilibrium.