12. G E N E T I C C Y T O L O G Y
443
carrying out the functions common to such regions. This state of
'heterochromatization' may be conceived either as reversible or irreversible. It may be noted that aside from its applicability to normal
differentiation, this point of view has consequences for theories of
tumor origin."
Blocking genie activities in toto, whether a microscopically imperceptible section or an entire chromosome, by physical condensation,
appears to be one way to achieve such a purpose. Biologists are familiar
with the fact that the nuclear morphology and the chromocenter
pattern differ from tissue to tissue and from stage to stage. This could
very well be an expression of the state of genie activity. In cell cultures,
Moorhead (personal communication) noted that nuclei of human
cells may develop a spotted appearance with large numbers of heterochromatic blocks. Exactly what happens to the cells illustrated in Fig.
13a and b is not known, but it is tempting to think in agreement with
Schultz that it is a process of differentiation. However, it is likely that
Schultz was also suggesting a sweeping generalization that all gene loci
not active in the particular tissue are heterochromatized, unless
heterochromatization is an all-inclusive term for gene inactivation
which once again becomes not very meaningful. W e must remember
that in neurons the only heteropyknotic piece seen is the sex chromatin;
yet it is quite clear that numerous genes should not be active in the
nerve cells. A number of enzymatic activities are known to be genetically
controlled (cf. Jacob and Monod, 1961), and it is highly probable
that the suppression of synthetic activity by physical condensation is
also a result of gene activity.
Tentatively, it may be concluded that chromatin showing heteropyknosis or allocycly may contain regular genes, but so long as the
condensed state prevails, the metabolic activities of the genes are
minimized. This conclusion can be applied to the X chromosome of the
grasshoppers, one X of female mammals, and, very likely, to the B-type
chromosomes of maize. The Y chromosome of Drosophila also probably
contains a number of genes because it affects, at least, the fertility of
the male flies. That no demonstrable Mendelian genes affecting
morphology were found in the Y does not prove that it is inert. Nevertheless, it is highly possible that the Y is truly inert in the larval neuroblasts, the main material for cytological observations.
Whether any gene exists in the classic heterochromatin, the Heitz
type heterochromatin located at both sides of the centromere discovered
in Drosophila, remains a question. Nor do we have any information
about the genetic function of the heterochromatin reported by Darlington and La Cour which, when showing heteropyknosis, is always
under-condensed or achromatic. Perhaps such questions can be
443
carrying out the functions common to such regions. This state of
'heterochromatization' may be conceived either as reversible or irreversible. It may be noted that aside from its applicability to normal
differentiation, this point of view has consequences for theories of
tumor origin."
Blocking genie activities in toto, whether a microscopically imperceptible section or an entire chromosome, by physical condensation,
appears to be one way to achieve such a purpose. Biologists are familiar
with the fact that the nuclear morphology and the chromocenter
pattern differ from tissue to tissue and from stage to stage. This could
very well be an expression of the state of genie activity. In cell cultures,
Moorhead (personal communication) noted that nuclei of human
cells may develop a spotted appearance with large numbers of heterochromatic blocks. Exactly what happens to the cells illustrated in Fig.
13a and b is not known, but it is tempting to think in agreement with
Schultz that it is a process of differentiation. However, it is likely that
Schultz was also suggesting a sweeping generalization that all gene loci
not active in the particular tissue are heterochromatized, unless
heterochromatization is an all-inclusive term for gene inactivation
which once again becomes not very meaningful. W e must remember
that in neurons the only heteropyknotic piece seen is the sex chromatin;
yet it is quite clear that numerous genes should not be active in the
nerve cells. A number of enzymatic activities are known to be genetically
controlled (cf. Jacob and Monod, 1961), and it is highly probable
that the suppression of synthetic activity by physical condensation is
also a result of gene activity.
Tentatively, it may be concluded that chromatin showing heteropyknosis or allocycly may contain regular genes, but so long as the
condensed state prevails, the metabolic activities of the genes are
minimized. This conclusion can be applied to the X chromosome of the
grasshoppers, one X of female mammals, and, very likely, to the B-type
chromosomes of maize. The Y chromosome of Drosophila also probably
contains a number of genes because it affects, at least, the fertility of
the male flies. That no demonstrable Mendelian genes affecting
morphology were found in the Y does not prove that it is inert. Nevertheless, it is highly possible that the Y is truly inert in the larval neuroblasts, the main material for cytological observations.
Whether any gene exists in the classic heterochromatin, the Heitz
type heterochromatin located at both sides of the centromere discovered
in Drosophila, remains a question. Nor do we have any information
about the genetic function of the heterochromatin reported by Darlington and La Cour which, when showing heteropyknosis, is always
under-condensed or achromatic. Perhaps such questions can be