Fróðskaparrit - 01.01.1965, Blaðsíða 88

96
True Hermaphroditism
somal abnormalities can be observed, in which more than two
X sex chromosomes and thus more than one Barr-body are
present.
Chromosomes.
Up to 1956 it was supposed that the number of human
chromosomes was 48. In 1956, Tjio and Levan8 were able to
correct this. They showed that man has 22 autosomal chromo-
some pairs and either two X sex chromosomes or one X sex
chromosome and one Y sex chromosome. By studying cells in
the metaphase after treatment with colchicine, the various
chromosomes were isolated and classified as pairs of homo-
logous chromosomes, the so-called Denver system being em-
ployed in most parts of the world for the purpose of classi-
fication and nomenclature. Under the Denver system the
chromosomes are classified according to decreasing size. Fig. 8
presents a survey of the chromosomes of a cell according to
the Denver system.
Chromosomal abnormalities may include changes in num-
ber and structure of the sex chromosomes and the autosomes.
The so-called mosaic formation designates a condition in which
cells with various chromosomal patterns, XX/XY, XX/XO,
XO/XY, XX/XXX, etc. appear in the same tissue. A chimaera
is an individual in whom various tissues present different
chromosomal patterns9 25.
During recent years it has been shown that many abnorma-
lities of the development of the gonad and the genitalia are
results of an abnormal number and structure of the sex
chromosomes. Only few autosomal chromosome abnormalities
are known as yet. In mongolism (Down’s syndrome) dupli-
cation of one of the smallest autosomes is found and conse-
quently, the total number of chromosomes will be 47 13. In
connexion with ehronic myelogenous leucaemia a reduction of
the structure of one of the smallest chromosomes has been
described, and in cases of multiple congenital malformations
abnormal structure or number of the autosomes has also been