The nuclei of all living cells contain chromosomes. The chromosomes are found in pairs and each type of animal has its own special number of chromosomes. For example in man the number of chromosomes found in the cells of the body is 46 (i.e. 23 pairs). One chromosome of each pair will have come from the mother and one from the father.

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Chromosomes:

© Department of Cytogenetics, Northern Region Genetics Service

The chromosomes are like long threads and they contain genes.
The genes are arranged along the chromosome like beads on a necklace and they control hereditary. They are the initial templates which determine which proteins are made.

A fertilised egg has 23 chromosomes from the mother paired with 23 from the father. When this fertilised egg cell divides all the new cells produced will have this same mix of 46 chromosomes from the parents and so the new offspring will have some characteristics from their mother and some from their father. The two chromosomes in each pair are therefore very similar but not quite identical. Each chromosome in the pair carries the same genes but their detailed structure will be slightly different. These variants of genes are called alleles. Any gene within an individual has two alleles, one from the mother and one from the father.

Let's say that chromosome pair 1 carries the gene for making enzyme A. Half of the chromosome pair 1 was inherited from the father and the other half from the mother. Therefore each of the pair has got a template for making enzyme A but the two templates - the alleles - may be slightly different. In the extreme case one of them may not work.

It is these variations within genes, alleles, which give rise to specific characteristics that an individual will show, for example, hair or eye colour, blood group, shape of nose, size of feet.

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