VMBO Year 2 · HAVO/VWO Years 2–3 · Inheritance and evolution · Section 5.3

Genes and alleles

DNA and genes

DNA contains paired bases: A–T and C–G. The sequence of bases carries genetic information used to make cellular products and regulate processes.

A gene is a section of DNA at a particular position on a chromosome. A single gene is not normally spread across several chromosomes, but several genes on different chromosomes may contribute to one characteristic.

• Genes: DNA sequences containing information for functional products, often proteins.
• Proteins: Molecules involved in body structure, colour, chemical reactions and many other functions.

Gene variants and alleles

Different versions of a gene are called alleles. For a typical gene on a matching pair of body-cell chromosomes, one allele occurs on each chromosome, forming an allele pair.

Different alleles may affect the phenotype in different ways. In a simple dominant–recessive relationship, a dominant allele affects the phenotype when one copy is present. This does not mean that it is physically stronger or necessarily more common.

Mutations

A mutation is a change in a DNA sequence. Its effects vary: it may have no noticeable effect, be harmful or sometimes be beneficial.

A mutation in an ordinary body cell affects that cell and its descendants, but is not usually inherited by children. A mutation in a sex cell can be inherited if that cell contributes to fertilisation.

An organism carrying a mutation may be called a mutant, whether or not the effect is visible. Albinism is an example of inherited changes affecting pigment production. People or animals with albinism do not all have the same eye or skin colour.

Mutagenic influences

Mutagens increase the chance of DNA changes. Examples include ultraviolet radiation, ionising radiation and certain chemicals in tobacco smoke. Some harmful substances, such as asbestos fibres, can contribute to DNA damage and cancer through several mechanisms.

Only changes reaching the cells that form future generations can be inherited through reproduction.

Cancer

Cancer involves cells escaping normal controls on growth and division, usually after several important genetic changes. Cells may form a tumour.

A malignant tumour can invade surrounding tissues. Some cancer cells spread through blood or lymph and establish new growths elsewhere. This is metastasis. Not every tumour is malignant, and not every cancer forms a solid tumour.