In the literature there are met over 5000 types of monogenic disorders
due to disturbances in the functions of only one gene are known by now.
More widely spread among them are thalassemia, phenylketonuria,
mucoviscidosis, hemophilia, etc.
Down’s syndrome is the most widely spread chromosome ailment in
the human population. It is caused by the trisomy of chromosome 21
(Fig. 2–23). The formula of Down’s syndrome is 47, XY, +21 for boys
and 47, XX+21 for girls.
image
Figure 2–23. Trisomy of chromosome 21. Arrous indicate the three copies of the chromosome (Courtesy of A. Andreev, Laboratory of Cytogenetics at the State University Pediatric Hospital, Sofia).
As was mentioned in Section 2. 5 giant and lampbrush type chromosomes
have been discovered in the cell nuclei from tissues specific of certain organisms
(Balbiani, 1881; Flemming, 1882). The interest towards them has considerably
grown in half a century when they were rediscovered by a number of authors
(Painter, 1933—35; Heitz, Bauer, 1933; King, Beams, 1934; Bridges, 1935; Bauer,
1935, 1936, etc.). Such chromosomes have been observed in the salivary glands
of Drosophila, Chironomus and some other two-winged insects of the Diptera
order.
The giant chromosomes (Fig. 2–24) are formed as a result from the
manifold replication cycles in them, which are not accompanied by release
and distribution of the chromatids in the daughter cells as it is normally
taking place in mitosis and cytokinesis. That is why there is a multiple