Chromosome Theory of Heredity and Organization of the Genome
Section 2.6. The chromosome theory of heredity is one of the great
achievements in biology. The rules of inheriting the features in the next
generations discovered by Mendel have laid the ground for its buildup and
further development. In his time these discoveries have not found their due
explanation since cell division and meiosis responsible for the formation of
the gametes (i.e. the sexual cells) taking part in the fertilization have not
been known yet (see Chapter 3, Section 3. 2 and 3. 4).
In 1884 K. Nägeli has admitted hypothetically the existence of
idioplasm as a specific hereditary substance. Almost simultaneously E.
van Beneden, E. Strasburger, O. Hertwig, R. Köllicker, A. Weissmann,
etc. have come to the conclusion that Nägeli’s idioplasm could be found
in the chromosomes.
The chromosome-heredity link has greatly captured the imagination of
many researchers. The cytological studies on the behaviour of the
chromosomes, in the process of cell division and meiosis have shown that
the sexual cells — spermatozoa and eggs — possess a single (haploid) set
of chromosomes and the fertilized ones — the zygotes, contain a double
(diploid) set of chromosomes. T. Boveri and W. Sutton have related these
results to the rules observed by Mendel in the inheritance of the features in
the process of hybridization in peas.
In his classical paper “The Chromosomes in Heredity” the American
biologist W. Sutton (1903) has for the first time paid attention to the fact that
the diploid chromosome set consist of two morphologically similar haploid
sets of chromosomes and in the process of meiosis the gametes receive
one haploid chromosome from each homologous pair. He has made the
assumption that the genes are a part of the chromosomes, and being
based on the data available at that time from the cytological studies has
explained some of Mendel’s results in the hybridization. This work has
played a great role in the creation of the chromosome theory and in
connecting the cytology with genetics.
The view that heredity is to be found in the chromosomes and the
chromosome number is specific and exactly determined for each separate
species of organisms has been already shared by most of the researchers.
Around 1890, however, a chromosome has been discovered which was not
always observed in two copies in the cells. This chromosome was
interpreted as an additional or sexual chromosome (McClung, 1901) and
has later become known as X-chromosome. Its significance has been
clarified by E. Wilson and his student Stevens. They have assumed that
there are normally two X-chromosomes in the cells of the female individuals
and in the cells of the male individuals — only one. Besides, it has been
established that in some species (the human including) yet another