(1) WJEC A2 Biology 4.4 Variation

Organisms differ in their phenotypes for several reasons: (3 reasons)

• Different genotype.

• Same genotype but different epigenetic modifications.

• They have different environments.

Heritable variation

Term used to describe differences resulting from different DNA sequences or epigenetic modifications that can be inherited.

The mechanisms whereby sexual reproduction can generate heritable variation:

• Crossing over between homologous chromosomes during prophase I of meiosis.

• Random distribution of chromosomes during metaphase I.

• Random distribution of chromatids at metaphase II.

• Mixing of two different parental genotypes at fertilisation.

Discontinuous variation

Type of variation, also usually drawn as a bar chart:

• Characteristics are discrete and have no intermediate types.

• Monogenic (controlled by a single gene).

• Gene expression is not influenced by the environment.

Continuous variation

Type of variation, forms a normal distribution curve:

• Characteristics normally have many possible values.

• Generally polygenic (being controlled by many genes).

• Gene expression is influenced by environment.