7 min readConcept sheetPart II (2 marks) on definitions or Down syndrome; Part III (4 marks) or Part IV (7 marks) on the types

Mutations

A mutation is a sudden change in genes or chromosomes that can be inherited. It may be a gene mutation or a chromosomal change in structure or number.

Term coined byHugo de Vries, from his work on the evening primrose (Oenothera lamarckiana)

MutagensAgents that cause mutations: X-rays, gamma rays, UV rays, some chemicals

Down syndromeTrisomy 21: an extra chromosome 21, giving 47 chromosomes

Gene mutation exampleSickle cell anaemia

Kinds of mutation

Kind	What changes	Example
Gene (point) mutation	The base sequence of one gene changes: a base is replaced (substitution), lost (deletion) or added (insertion)	Sickle cell anaemia
Chromosomal: structure	Part of a chromosome is lost (deletion), repeated (duplication), turned round (inversion) or moved to another chromosome (translocation)	Can cause serious disorders
Chromosomal: number (euploidy)	Whole sets of chromosomes are added: polyploidy (3n, 4n)	Seedless watermelon and banana (triploid, 3n)
Chromosomal: number (aneuploidy)	One or a few chromosomes are added or lost: 2n + 1 (trisomy), 2n − 1 (monosomy)	Down syndrome (2n + 1)

Down syndrome: a chromosomal disorder

Disorder	Chromosomes	Main features
Down syndrome	Trisomy 21: an extra chromosome 21 (47 in all)	Slower learning, short height, flat round face, upward-slanting eyes, a single crease across the palm

Gene mutation vs chromosomal mutation

Gene mutationA change inside one gene (in its DNA bases). It cannot be seen under a microscope. Example: sickle cell anaemia, where red blood cells become sickle-shaped.

Chromosomal mutationA change in the structure or number of chromosomes. It can often be seen under a microscope. Example: Down syndrome.

Causes and importance of mutations

Mutagens cause mutations: radiation such as X-rays, gamma rays and UV rays, and chemicals such as mustard gas.
Most mutations are harmful or have no effect. A few are useful.
Mutations in the gametes are passed on to offspring.
Mutations are the main source of new variation, the raw material for evolution.
Plant breeders use mutations to produce new crop varieties.

In real life

Cover up in strong sun and avoid unnecessary X-rays, because radiation can damage DNA. The seedless watermelons and bananas sold in our markets are polyploid plants with extra sets of chromosomes.

Exam angle

2 marks: “What is a mutation? Who coined the term?” A sudden change in genes or chromosomes that can be inherited. The term was coined by Hugo de Vries.

2 marks: “What is Down syndrome?” A disorder due to an extra chromosome 21 (trisomy 21, 47 chromosomes), with slower learning, short height and slanting eyes.

4 marks: “Explain the types of chromosomal mutation.” Structural: deletion, duplication, inversion, translocation. Numerical: euploidy (polyploidy, e.g. seedless watermelon) and aneuploidy (2n + 1, e.g. Down syndrome, or 2n − 1).

Remember it

Four structural changes, 'DDIT': Delete, Duplicate, Invert, Translocate.