Any organism which shows asexual reproduction will produce clones. The offspring are all genetically identical. Some crop plants are reproduced exclusively in this way, e.g. potato (Solanum). These are grown from stem tubers produced the previous year (these are confusingly called ‘seed’ potatoes which they are not). Another example is sugar cane (Saccharum officinarum), which is reproduced yearly by planting stem cuttings.

Cloning, therefore, has a long history and it is very useful if the desired combination of genes is to be maintained. Recently cloning has been carried out using the technique of tissue culture. Cells are taken from the parent plant and are grown on an agar jelly which is impregnated with a special cocktail of plant hormones and minerals to encourage cell division. The recipes are a closely guarded secret of the commercial plant breeders.

The first stage is to produce a callus, which is a lump of undifferentiated tissue.

The second stage is to cut the callus up into small pieces from which are grown fully differentiated clones. This requires a different mixture of plant hormones and minerals.

Once these clones have reached a sufficient stage of development, with roots and leaves, they can be planted out and grown into adult plants.

In this way millions of identical plants can be produced from a small amount of tissue.

Clones have a major disadvantage in that they are genetically identical. If one plant is susceptible to a disease all the rest will be too. The tragic story-of the potato famine in the 19th century illustrates this.

The potato originates from the Altoplano in the South American Andes. Samples of one cultivar (cultivated variety) were brought to Europe towards the end of the 17th century. 50 years later it had become the staple diet of peasants in many parts of Europe, especially Ireland. 7lb of potatoes (about 3.5 kg) and 1 pint of milk (about 0.5 litre) per day would support an adult.

The potato plant does produce a flower but it rarely produces any viable seeds. However, as the plant readily reproduces asexually this posed no problems. That is until 1845 when the potato blight fungus (Phytophthora infestans) attacked the crop. Because the potato plants were all genetically identical the fungus spread rapidly and ‘widespread famine occurred. In Ireland it caused an estimated 1 million deaths and 1 million emigrated to the USA and other parts of Europe — out of population of 8 million.

Today searches are being carried out in countries such as Bolivia for the wild ancestors or other cultivars of the potato. These will then be used to create new varieties which are resistant to disease and suitable for new habitats.