On the other hand, diseases like diabetes (not all types of diabetes), even though hereditary may not affect the individual until quite late in life. This means that hereditary diseases also interact with the environment and express themselves only in a suitable environment. In two other human diseases – albinism and xeroderma, the former is recognized by complete loss of pigments in skin, hair, iris of the eyes etc; the latter is also a skin ailment in which exposure to sun rays results in freckles on the skin.

But both these can be withstood. Albinos can protect themselves by not exposing themselves too much to light; the same is the case with xeroderma people. Till now there has been no total cure for a hereditary disease the remedies are only palliative. But recent techniques of genetic engineering have brought a ray of hope that the faulty genes can be identified and corrected very early. However this is still in the experimental stage.

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In order to cure a hereditary disease or rectify a genetic abnormality; it will be necessary for us to know the norm or range of reaction of the genotype in all the environmental conditions. Even though it is difficult to fully analyze the entire reaction range of a genotype it has been possible discovered that, if larvae of these flies are fed on silver salts, they develop into yellow flies irrespective of the fact whether they were genetically brown or yellow. The yellow bodied fly which is genotypically brown is a variant of the original yellow bodied fly.

This is now called a phenocopy to do so to some extent at least in some organisms. In the fruit fly Drosophila melanogaster, the normal body colour is brown with black margins. A hereditary mutant for this fly was discovered by Morghan (1910) where the body colour was yellow. This was a genotypic character in that the body colour was constant in both the flies in all environments. In 1939, Rapport It is easy to distinguish between the original yellow body and its phenocopy.

The offspring of a phenocopy would have brown body, it they are not fed again on silver salts. The temporary development of body colour (yellow) in brown flies is an example of norm of its genotype.