When it was recognized that changes (mutations)
in genes occur spontaneously (T. H. Morgan,
1910) and can be induced by X-rays (H. J.
Muller, 1927), the mutation theory of heredity
became a cornerstone of early genetics. Genes
were defined asmutable units, but the question
what genes and mutations are remained. Today
we know that mutations are changes in the
structure of DNA and their functional consequences.
The study of mutations is important
for several reasons. Mutations cause diseases,
including all forms of cancer. They can be induced
by chemicals and by irradiation. Thus,
they represent a link between heredity and environment.
And without mutations, well-organized
forms of life would not have evolved.
The following two plates summarize the chemical
nature of mutations.
