Discontinuous And Continuous Version , Continuous variation



Discontinuous and continuous variation

The individuals within a species of plants or animals are alike in all major respects; indeed, it is these likenesses which determine that they belong to the same species. Nevertheless, even though an organism recognizably belongs to a particular Species, it may differ in many minor respects from another individual of the same species. A mouse may be black, brown, white or other colours, the size of its cars and tail may vary but despite these variations, it is still recognizably a mouse.

Discontinuous variation. The variations in coat colour are examples of discontinuous variation because there are no intermediates. If black and brown mice are bred together they will produce black or brown offspring. There are no intermediate colours and no problems arise in deciding in which colour category to place the individuals. It is not possible to arrange the mice in a continuous series of colours ranging irom brown to black with almost imperceptible differences of colour between adjacent members of the series. The way sex is inherited is another example of discontinuous variation. With the exception of a small number of abnormalities, one is either male or female and there are no intermediates.

Discontinuous variation in humans is rather more difficult to illustrate. There are, for example, four major blood groups designated A, B, AB and O. Blood from different groups cannot be mixed without causing a clumping of the red cells. A person must be one or other of these four groups: he cannot, for example, be intermediate between group O and group A, he must be one or the other. In general terms, eye colour in white races is inherited in a discontinuous manner; one has blue eyes or pigmented eyes, but there are some individuals who would be difficult to classify. Clear-cut examples of discontinuous variation occur among the more serious variants, e.g. one is either an achondroplastic dwarf or one is not; intermediates do not occur.

The features of discontinuous variation are clearly genetically determined; they cannot be altered during the lifetime of the individual. You cannot alter your eye colour by changing your diet. An achondroplastic dwart cannot grow to full height by cating more food. An albino cannot acquire a dark skin by Sunbathing. Moreover, the variations are likely to be under the control of a small number of genes. One dominant gene makes you an achondroplastic dwart: the absence of one gene for making pigment causes albinism.

Continuous variation. When one tries to classify individuals according to height or weight rather than eye colour, the decisions become more difficult and the classes more arbitrary

There are not merely two classes of people, tall and short, but a whole range of intermediate sizes differing from each other by barely measurable distances. Categories can be invented for convenience e.g. people from 1-4 to 1-6 m, 1-6 to 18 m, 1-8 to 2-0 m, but they do not represent discontinuous variations of 0-2 m between individuals.

There is no reason why continuous variations should not be genetically controlled but they are likely to be influenced by several genes. Height might be influenced by 20 genes, each gene contributing a few centimetres to the stature. A person who inherited all 20 would be tall whereas a person with only 5 would be short. Although this example is purely hypothetical, it is known that height is at least partially genetically determined because tall parents have, on average, tall children and vice versa, but how many genes are involved is not known.

Continuous variations are also those most likely to be influenced by the environment. A person may inherit genes for tallness but if he is undernourished in his years of growth he will not grow as tall as he might if he had received adequate food. In fact, most continuous variations result from the interaction of the genotype with the environment. A person will grow fat if he eats too much food ; he will lose weightif he goes on a diet. This seems to be an entirely environmental effect until one realizes that another person may eat just as much food and yet remain slim because of his different, inherited constitution. Whether one catches a disease or not would appear to be dependent ot exposure to the disease germs, an exclusively environment effect, and yet it is apparent that one may inherit susceptibillly or resistance to a disease. If a person with inherited susceptabiu to an intectious disease is never exposed to the infection, he W not develop the disease.


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