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Sickle cell anaemia

An inherited blood disease in which the red blood cells contain an abnormal type of haemoglobin.

Natural selection

The process in nature where the fittest individuals survive, reproduce and pass their characteristics on to their offspring.

Huntington's disease

An inherited disease caused by a single dominant gene. It typically develops between the ages of 35-50 and at present there is no cure.

Gene therapy

A new, experimental method of fighting disease by replacing a defective gene with a healthy gene

Down's syndrome

A disorder in which there is an extra number 21 chromosome in the body cells so that there are 47 chromosomes altogether. This results in a variable degree of learning difficulties and a characteristic appearance.

Cystic fibrosis

A genetic disease caused by a defective, recessive gene. It is characterised by the production of thick, sticky mucous in the lungs and pancreas which cause respiratory and digestive problems.


The emission of heat, light or other electromagnetic waves.


A theory, supported by much evidence, which suggests that the animal and plant species inhabiting the earth today are descended from simpler forms by a gradual process of change.


Any substance that causes cancer.


A mass of abnormal cells which keep multiplying in an uncontrolled way.

What can go wrong?


drawing of budgies
The basic colour of wild budgies is green. The many colour variations are a result of mutations in specific genes.

Mutation is a sudden change in a gene or a whole chromosome and will be passed on to new cells by mitosis, or to new individuals by meiosis.

In chromosome mutation an individual may be missing a chromosome or have an extra one. Down's syndrome is caused in humans by the presence of an extra copy of chromosome 21. A gene mutation is a change in the order of bases in the DNA molecule. The change may be very small but could have a severe effect on the individual as in certain diseases, for example, sickle cell anaemia .

Mutations happen randomly and by chance but the risk of mutation is increased by exposure to radiation and to certain chemicals, known as mutagens. If a mutation results in cancer, the mutagens responsible are known as carcinogens.

Most mutations are harmful or even fatal to an organism. However, on rare occasions, a mutation results in a new characteristic which can help the organism survive in unfavourable environmental conditions. The new characteristic will be passed to its offspring who can survive where others may die. This is an example of how natural selection and, over a long period, evolution, occurs.

Inherited diseases

Cystic Fibrosis, one of the most common genetic diseases, develops as a result of a faulty recessive gene (c) which was identified as recently as 1989. To develop the disease a child must inherit the gene from both parents. If, as in the diagram, both parents are carriers, there is a one-in-four chance that a child will have the disease, a two-in-four chance of a child being a carrier and a one-in-four chance of the child being unaffected.

Huntington's family tree and cross diagram (affected people in blue)
Huntington's family tree (affected people in blue)

Unlike cystic fibrosis, Huntington's Disease is dominant, that is, it can be inherited if just one parent has this gene.

The diagram shows what can happen if one parent has the HD gene (and therefore has Huntington's disease) and the other parent does not. In this diagram the presence of the disease is shown by a capital H. In Huntington's the symptoms do not show until the sufferer is 35-40 years of age, by which time the faulty gene may have been passed on to children and even grandchildren.

Continuing genetic research has meant development of tests for some genetic diseases both in utero and after birth and treatments such as gene therapy.

Question 4

Quiz Print
Study the sentences below, and drag and drop in the missing words.
1. In Down's Syndrome there is an extra
2. A small change in the bases of
  is called a
3. Mutagens can cause
 are chemicals that cause mutations in cells.
5. Natural selection could be said to be
  of the fittest.
6. Cystic Fibrosis occurs in children that have inherited both
7. In genetics, a carrier is someone who has the
  for a disease but does not actually
  from it.