The search for the mechanisms of inheritance led in 1903/4 to the postulation of a new theory, the chromosome theory of inheritance. It claims that the chromosomes are the structures harboring the hereditary factors as MENDEL called them. It remained controversial for several years since no decisive proof could be given.
Around the turn of the century the details of mitosis and meiosis
were already known. Furthermore exceptions of the general scheme had
been observed that remained mysterious at first. In the year 1887 the
German zoologist A. WEISMANN postulated the germ line theory that
demanded a continuity of the karyoplasm for generations:
" The body or soma dies, the germ plasma continues to live...(it) produces the parts of the body by its development. Conversely new, acquired changes of the body have no influence on the germ plasm. An inheritance of acquired abilities does therefore not exist."
Around the middle of the 1880th several researchers like T.
BOVERI, O. HERTWIG, E. STRASBURGER and A. WEISMANN pointed out that
the hereditary factors might be localized on the chromosomes.
Furthermore an individuality of single chromosomes during the whole
life of the cell nucleus was postulated.
When MENDEL's laws were rediscovered and when they got known by a broad public in 1900, it did seem natural to bring the mechanism of chromosome dispersal used in the production of the germ cells and the fusion of the two parental nuclei at fertilization in connection with the dispersal of the hereditary factors among the offspring, since the ideas of MENDEL demanded also a kind of reduction of the factors. In 1903/4 W. S. SUTTON and T. BOVERI combined these ideas to the chromosome theory of inheritance. It remained controversial for years, because a final proof could not be found. E. STRASBURGER wrote in 1909:
" The studies on heredity show now that the phenomenons described by MENDEL's laws correspond completely to the observations made of the division of the nucleus and the fusion of the nuclei of the chromosomes."
In the "Textbook on Botany" mitosis and meiosis on one hand and heredity on the other hand were described by different authors. STRASBURGER, who was up until the 11th edition (1911) in charge of the morphological part that included also the division of the nucleus mentioned the connection to the laws of inheritance with not a single word. The physiologist L. JOST wrote in this and in the twelfth edition of 1913:
" In spite of many hypotheses and speculations we have until now no secure knowledge about the material consistence of these 'factors', and even less knowledge about the way, in which the exert their influence on the development..."
In the 13th edition was for the first time written:
" That these factors are bound to the chromosomes of the nucleus seems likely, but we know nothing about the way, in which they influence the development..."
This was the state of the affairs at the beginning of the 20th century. We will later on discuss the evidences for the chromosome theory of inheritance. At first it has to be settled, which further changes of the chromosomes were possible and which influence they have on geno- and phenotype.
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