12.
G E N E T I C
C Y T O L O G Y
441
Once in the cell cycle at least, i.e. when heterochromatin replicates
itself, it must synthesize its D N A and protein. Thus the heterochromatin
must also uncoil, even though briefly, during interphase, to facilitate
synthetic activities. Perhaps this stage is reached when the euchromatic
parts have completed their D N A synthesis, at which time the cell is
nearly ready to enter the G 2 phase.
When a gene is brought to the juxtaposition, by translocation or
inversion, of heterochromatin, the phenotype determined by the said
gene may be variegated. In Drosophila, if the wild-type (dominant)
allele of the white-eye locus is brought to the proximity of heterochromatin, the heterozygote w
+
/w does not show a wild-type eye color
as anticipated. The eyes will be mottled, i.e. with patches of wild-type
eye color and patches of white color. It appears that the phenomenon
is a type of position effect; the expression of a gene in this particular
category depends largely upon its position, a position near or away
from heterochromatin. Lewis (1950) called it the V-type (variegated)
position effect. V-type position effect has been found in Drosophila in
many cases and in the plant Oenothera (Catcheside, 1947). In Drosophila
variegation usually involves such characteristics as eye or body color,
facet arrangement, bristle structure, and wing patterns. Phenotypes
showing variegated or mottled effect usually involve those affecting
localized characters; other genes do not necessarily respond in such
a way. The expression of the mutant phenotype, in the majority of
cases, occurs only when the dominant gene has been moved and is
heterozygous with a recessive allele.
The V-type position effect was interpreted as the result of a change
in stability of the gene because of its proximity to the heterochromatin.
Schultz (1939) explained the effect as the result of alteration of nucleic
acid metabolism leading to heterochromatization of the euchromatic
genes. The process appears to be a sort of inactivation, with the point
of breakage acting as a center from which the disturbance spreads.
Demerec (1939) proposed a similar concept, namely, the instability
may result from a reversible chemical change, or a reversible suppression of the gene activity.
According to our present working hypothesis, namely, that physical
condensation (heteropyknosis) of a chromosome or a part of a chromosome in interphase inactivates the metabolic activities, specifically R N A
synthesis, of the genes, we may interpret the variegation phenomenon
somewhat differently. Since a euchromatic gene is brought to the
proximity of a heterochromatin, this gene may or may not be involved
in the periphery of the pyknotic section. The involvement is passive,
but inactivation of synthesis results. If the wild-type allele for the white
eye locus is involved in the condensation procedure, the white gene
G E N E T I C
C Y T O L O G Y
441
Once in the cell cycle at least, i.e. when heterochromatin replicates
itself, it must synthesize its D N A and protein. Thus the heterochromatin
must also uncoil, even though briefly, during interphase, to facilitate
synthetic activities. Perhaps this stage is reached when the euchromatic
parts have completed their D N A synthesis, at which time the cell is
nearly ready to enter the G 2 phase.
When a gene is brought to the juxtaposition, by translocation or
inversion, of heterochromatin, the phenotype determined by the said
gene may be variegated. In Drosophila, if the wild-type (dominant)
allele of the white-eye locus is brought to the proximity of heterochromatin, the heterozygote w
+
/w does not show a wild-type eye color
as anticipated. The eyes will be mottled, i.e. with patches of wild-type
eye color and patches of white color. It appears that the phenomenon
is a type of position effect; the expression of a gene in this particular
category depends largely upon its position, a position near or away
from heterochromatin. Lewis (1950) called it the V-type (variegated)
position effect. V-type position effect has been found in Drosophila in
many cases and in the plant Oenothera (Catcheside, 1947). In Drosophila
variegation usually involves such characteristics as eye or body color,
facet arrangement, bristle structure, and wing patterns. Phenotypes
showing variegated or mottled effect usually involve those affecting
localized characters; other genes do not necessarily respond in such
a way. The expression of the mutant phenotype, in the majority of
cases, occurs only when the dominant gene has been moved and is
heterozygous with a recessive allele.
The V-type position effect was interpreted as the result of a change
in stability of the gene because of its proximity to the heterochromatin.
Schultz (1939) explained the effect as the result of alteration of nucleic
acid metabolism leading to heterochromatization of the euchromatic
genes. The process appears to be a sort of inactivation, with the point
of breakage acting as a center from which the disturbance spreads.
Demerec (1939) proposed a similar concept, namely, the instability
may result from a reversible chemical change, or a reversible suppression of the gene activity.
According to our present working hypothesis, namely, that physical
condensation (heteropyknosis) of a chromosome or a part of a chromosome in interphase inactivates the metabolic activities, specifically R N A
synthesis, of the genes, we may interpret the variegation phenomenon
somewhat differently. Since a euchromatic gene is brought to the
proximity of a heterochromatin, this gene may or may not be involved
in the periphery of the pyknotic section. The involvement is passive,
but inactivation of synthesis results. If the wild-type allele for the white
eye locus is involved in the condensation procedure, the white gene