432
T . G . HSU
without the Y ( X O males) are normal and healthy except that they are
sterile. Thus the idea was advanced that the Y chromosome is genetically
inert. Putting two and two together, the conclusion was, therefore,
a straightforward one: heterochromatin is genetically inert.
The lack of genetic activity of heterochromatin finds equivalent
cases in plant materials. The so-called B chromosomes in maize
(McClintock, 1933; Rhoades and McClintock, 1935) are primarily
composed of heterochromatin and are genetically inert. The presence
of one or many B-type chromosomes has no visible effect on the
morphological characters of the plant. During meiosis, the B chromosomes may show nonhomologous pairing, a property rarely seen among
euchromatic segments.
Schultz (1947) summarized the data on the function of heterochromatin. Apparently the Y chromosome, inert in development of
morphological and metabolic characters, is essential for male fertility.
He also inferred, from analyses of translocations involving heterochromatin, that the heterochromatin in different chromosomes has
similar functions. When a euchromatic gene is brought to the proximity
of a heterochromatin, by inversion or by translocation, its phenotypic
expression may be variegated. That is to say, some of the cells show
mutant characteristics, while others around them appear normal,
suggesting that heterochromatin changes the stability of the genes.
Schultz also thinks that the heterochromatin is involved in the synthesis
of nucleolar substance. Schultz, Caspersson and Aquilonius (1940) postulated that heterochromatic regions have something to do with cytoplasmic synthesis in general, by way of precursors derived from the
nucleoli.
When polytene chromosomes were rediscovered, cytologists naturally
attempted to correlate the characteristics found in the giant chromosomes with those described from regular mitosis, which, in Drosophila,
were principally observed from neuroblasts. T o students of genetics,
it is a familiar picture that in the salivary-gland cell there is a mass of
diffused, mesh-like chromocenter from which several long, banded
elements radiate. This configuration supposedly corresponds to meiotic
prophase, save that in the salivary-gland the chromosomes are multistranded. As mentioned previously, heterochromatin lies adjacent to
the centromeres. It appears that in the salivary-gland cells, as in the
meiosis of maize, heterochromomeres tend to attract one another
in a nonspecific manner. The chromocenter (diffused mesh-work)
thus represents all the heterochromatin of all chromosomes, with the
heterochromomeres irregularly paired in a disorganized pattern. In
the cells of male larvae, the Y chromosome also joins the chromocenter.
Close examination of the chromocenter or the heterochromatic mass
T . G . HSU
without the Y ( X O males) are normal and healthy except that they are
sterile. Thus the idea was advanced that the Y chromosome is genetically
inert. Putting two and two together, the conclusion was, therefore,
a straightforward one: heterochromatin is genetically inert.
The lack of genetic activity of heterochromatin finds equivalent
cases in plant materials. The so-called B chromosomes in maize
(McClintock, 1933; Rhoades and McClintock, 1935) are primarily
composed of heterochromatin and are genetically inert. The presence
of one or many B-type chromosomes has no visible effect on the
morphological characters of the plant. During meiosis, the B chromosomes may show nonhomologous pairing, a property rarely seen among
euchromatic segments.
Schultz (1947) summarized the data on the function of heterochromatin. Apparently the Y chromosome, inert in development of
morphological and metabolic characters, is essential for male fertility.
He also inferred, from analyses of translocations involving heterochromatin, that the heterochromatin in different chromosomes has
similar functions. When a euchromatic gene is brought to the proximity
of a heterochromatin, by inversion or by translocation, its phenotypic
expression may be variegated. That is to say, some of the cells show
mutant characteristics, while others around them appear normal,
suggesting that heterochromatin changes the stability of the genes.
Schultz also thinks that the heterochromatin is involved in the synthesis
of nucleolar substance. Schultz, Caspersson and Aquilonius (1940) postulated that heterochromatic regions have something to do with cytoplasmic synthesis in general, by way of precursors derived from the
nucleoli.
When polytene chromosomes were rediscovered, cytologists naturally
attempted to correlate the characteristics found in the giant chromosomes with those described from regular mitosis, which, in Drosophila,
were principally observed from neuroblasts. T o students of genetics,
it is a familiar picture that in the salivary-gland cell there is a mass of
diffused, mesh-like chromocenter from which several long, banded
elements radiate. This configuration supposedly corresponds to meiotic
prophase, save that in the salivary-gland the chromosomes are multistranded. As mentioned previously, heterochromatin lies adjacent to
the centromeres. It appears that in the salivary-gland cells, as in the
meiosis of maize, heterochromomeres tend to attract one another
in a nonspecific manner. The chromocenter (diffused mesh-work)
thus represents all the heterochromatin of all chromosomes, with the
heterochromomeres irregularly paired in a disorganized pattern. In
the cells of male larvae, the Y chromosome also joins the chromocenter.
Close examination of the chromocenter or the heterochromatic mass