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LYONISATION OF THE X CHROMOSOME
987 and whether the observations the microscope.
are
made
on
photographs
or at
We feel that, until techniques for demonstrating the secondary constrictions. and reproducing them on film have advanced further, it may be better not to make assumptions about the morphological differences in homologues or about autosomal inactivation related to them. Medical Genetics Program, Department of Medicine, Indiana University Medical Center,
Indianapolis, Indiana.
C. G. PALMER S. S. FUNDERBURK.
LYONISATION OF THE X CHROMOSOME SiR,-In the discussion of the Lyon hypothesis in your leading article of Oct. 12, a number of facts discordant with the hypothesis were mentioned. Some of them might be explained by observations made in this laboratory. Dr. Petersen and Ifound that in tissue-cultures of human cells the proportion of sex-chromatin-positive cells was small in the
logarithmic growth phase and rose to almost 100% in the postlogarithmic phase. We have confirmed these findings in further
experiments (unpublished) with cultures
of human
skin, both from
nor-
mal females and from
sex-chromatin-positive males. The results of an experiment with cultures of skin from a male with the chromosome constitution 48-XXXY and a normal female are shown in the figure. In the normal female control, the frequency of sex-
chromatin-positive cells ranges from a minimum of 51% at day 2 to a maximum of 99-5% at day 14. The same type of
variation, although pronounced, is
less
seen
in the 48-XXXY
male, when the total number of positive cells-i.e., cells with
one or two
Barr bodies-is counted. On the other
hand, the frequency of cells with two Barr bodies seems to vary very widely (from 20% at day 3 to 93% at day 14). The
of this is
far uncertain, but we have sug1 explanation that all female cells are sexchromatin-negative in a certain part of the interphase. Whether this is correct or not, the variation in the frequency of sex-chromatin-positive cells in relation to the growth phase may explain the following facts mentioned in your leading article as discordant with the Lyon cause
so
gested the
hypothesis: 1.
People with karyotypes 45-XO, 46-XX, 47-XXX, and are not phenotypically identical; neither are
48-XXXX 1.
Therkelsen,
A.
J., Petersen, G. B. Exp. Cell Res. 1962, 28,
588.
individuals with karyotypes 46-XY, 47-XXY, 48-XXXY, and 49-XXXXY-which would be expected if all but one of the Xs are inactive. Our experiments seem to show that inactivation of the extra Xs is present in only some of the cells in the logarithmic growth phase, whereas it is almost complete in the postlogarithmic phase. If inactivation is absent from some of the cells (or if it is absent at some intermitotic phase) in rapidly growing tissues, the karyotypes mentioned would not be expected to be identical. 2. The fact that all the red cells are Xgca +) in women heterozygous for this blood-group gene is also discordant with the Lyon hypothesis ; but if inactivation of the X in normal women is dependent on growth activity, inactivation is likely to be very incomplete in the bone-marrow, for the mitotic activity in this tissue is high. Of the two types of cellsXgca+) and Xg(a—)—only a few of the Xg(a-) type would be formed when inactivation is incomplete. If all cells are sexchromatin-negative at some intermitotic phase, and if bloodgroups antigens are formed during this phase, no Xg(a-) cells at all would be formed. Pronounced mosaicism in female heterozygotes would be expected only in tissues with low mitotic activity, which is in good agreement with the finding of mosaicism in the pigmentproducing cells of mouse skin and of human retina.22 The recent demonstration by Bach and Hirschhorn3 of two types of lymphocytes-one with and one without gammaglobulin-in women heterozygous for a sex-linked gene concerned in gamma-globulin production would also be expected as mitotic divisions in lymphocytes are very rare in vivo. 3. As mentioned above, the fact that 45-XO individuals are not phenotypically identical with 46-XX individuals is discordant with the Lyon hypothesis if complete inactivation of one X of all cells at all mitotic phases is assumed in normal women; and, as mentioned in your leading article, it is very difficult to explain the observation that patients with Turner’s syndrome show several anomalies apparently unrelated to sexual development. If inactivation of the extra X in normal females is incomplete during periods of rapid growth, 45-XO individuals are not genetically identical with normal women, and the genetic insufficiency is greatest during periods of rapid growth. It is therefore not unlikely that the genetic imbalance will be manifested phenotypically by impaired growth (dwarfing,
coarctation). Institute of General Pathology and Bacteriology, University of Aarhus, Denmark.
A.
J. THERKELSEN.
Gm LOCUS ON TRANSLOCATION CHROMOSOME?
SIR,-We wish
to call attention to a family with a translocation in whom there is a slight suggestion that the Gm locus is on the translocation chromosome. Though this is no more than a suggestion, we report it in the hope that other investigators will examine the Gm inheritance in families with a similar chromosomal translocation. Such information could quickly affirm or deny the suggested relationship. Gm typing is not mentioned in most published accounts of families with a D/D translocation or other heritable variations of the D
D/D
group. The present family was ascertained through a member with Down’s syndrome whose chromosomal analysis revealed a D/D translocation as well as a trisomy-21. Chromosomal analysis of available relatives demonstrated that other members of the family were carriers of the D/D translocation. No other individuals with mental retardation were discovered and no miscarriages had occurred among the progeny of the carriers. No linkage with the translocation was suggested for any blood-group system other than the Gm locus. Of the three children of one translocation carrier, one appeared to demonstrate a crossover between the translocation and the Gm locus. One of these three children had, in 2. Lyon, M. F. Amer. J. hum. Genet. 1962, 14, 135. 3. Bach ,F., Hirschhorn, K. Proc. 11th int. Congr. Genet.
1963, p. 312.
are
made
on
photographs
or at
We feel that, until techniques for demonstrating the secondary constrictions. and reproducing them on film have advanced further, it may be better not to make assumptions about the morphological differences in homologues or about autosomal inactivation related to them. Medical Genetics Program, Department of Medicine, Indiana University Medical Center,
Indianapolis, Indiana.
C. G. PALMER S. S. FUNDERBURK.
LYONISATION OF THE X CHROMOSOME SiR,-In the discussion of the Lyon hypothesis in your leading article of Oct. 12, a number of facts discordant with the hypothesis were mentioned. Some of them might be explained by observations made in this laboratory. Dr. Petersen and Ifound that in tissue-cultures of human cells the proportion of sex-chromatin-positive cells was small in the
logarithmic growth phase and rose to almost 100% in the postlogarithmic phase. We have confirmed these findings in further
experiments (unpublished) with cultures
of human
skin, both from
nor-
mal females and from
sex-chromatin-positive males. The results of an experiment with cultures of skin from a male with the chromosome constitution 48-XXXY and a normal female are shown in the figure. In the normal female control, the frequency of sex-
chromatin-positive cells ranges from a minimum of 51% at day 2 to a maximum of 99-5% at day 14. The same type of
variation, although pronounced, is
less
seen
in the 48-XXXY
male, when the total number of positive cells-i.e., cells with
one or two
Barr bodies-is counted. On the other
hand, the frequency of cells with two Barr bodies seems to vary very widely (from 20% at day 3 to 93% at day 14). The
of this is
far uncertain, but we have sug1 explanation that all female cells are sexchromatin-negative in a certain part of the interphase. Whether this is correct or not, the variation in the frequency of sex-chromatin-positive cells in relation to the growth phase may explain the following facts mentioned in your leading article as discordant with the Lyon cause
so
gested the
hypothesis: 1.
People with karyotypes 45-XO, 46-XX, 47-XXX, and are not phenotypically identical; neither are
48-XXXX 1.
Therkelsen,
A.
J., Petersen, G. B. Exp. Cell Res. 1962, 28,
588.
individuals with karyotypes 46-XY, 47-XXY, 48-XXXY, and 49-XXXXY-which would be expected if all but one of the Xs are inactive. Our experiments seem to show that inactivation of the extra Xs is present in only some of the cells in the logarithmic growth phase, whereas it is almost complete in the postlogarithmic phase. If inactivation is absent from some of the cells (or if it is absent at some intermitotic phase) in rapidly growing tissues, the karyotypes mentioned would not be expected to be identical. 2. The fact that all the red cells are Xgca +) in women heterozygous for this blood-group gene is also discordant with the Lyon hypothesis ; but if inactivation of the X in normal women is dependent on growth activity, inactivation is likely to be very incomplete in the bone-marrow, for the mitotic activity in this tissue is high. Of the two types of cellsXgca+) and Xg(a—)—only a few of the Xg(a-) type would be formed when inactivation is incomplete. If all cells are sexchromatin-negative at some intermitotic phase, and if bloodgroups antigens are formed during this phase, no Xg(a-) cells at all would be formed. Pronounced mosaicism in female heterozygotes would be expected only in tissues with low mitotic activity, which is in good agreement with the finding of mosaicism in the pigmentproducing cells of mouse skin and of human retina.22 The recent demonstration by Bach and Hirschhorn3 of two types of lymphocytes-one with and one without gammaglobulin-in women heterozygous for a sex-linked gene concerned in gamma-globulin production would also be expected as mitotic divisions in lymphocytes are very rare in vivo. 3. As mentioned above, the fact that 45-XO individuals are not phenotypically identical with 46-XX individuals is discordant with the Lyon hypothesis if complete inactivation of one X of all cells at all mitotic phases is assumed in normal women; and, as mentioned in your leading article, it is very difficult to explain the observation that patients with Turner’s syndrome show several anomalies apparently unrelated to sexual development. If inactivation of the extra X in normal females is incomplete during periods of rapid growth, 45-XO individuals are not genetically identical with normal women, and the genetic insufficiency is greatest during periods of rapid growth. It is therefore not unlikely that the genetic imbalance will be manifested phenotypically by impaired growth (dwarfing,
coarctation). Institute of General Pathology and Bacteriology, University of Aarhus, Denmark.
A.
J. THERKELSEN.
Gm LOCUS ON TRANSLOCATION CHROMOSOME?
SIR,-We wish
to call attention to a family with a translocation in whom there is a slight suggestion that the Gm locus is on the translocation chromosome. Though this is no more than a suggestion, we report it in the hope that other investigators will examine the Gm inheritance in families with a similar chromosomal translocation. Such information could quickly affirm or deny the suggested relationship. Gm typing is not mentioned in most published accounts of families with a D/D translocation or other heritable variations of the D
D/D
group. The present family was ascertained through a member with Down’s syndrome whose chromosomal analysis revealed a D/D translocation as well as a trisomy-21. Chromosomal analysis of available relatives demonstrated that other members of the family were carriers of the D/D translocation. No other individuals with mental retardation were discovered and no miscarriages had occurred among the progeny of the carriers. No linkage with the translocation was suggested for any blood-group system other than the Gm locus. Of the three children of one translocation carrier, one appeared to demonstrate a crossover between the translocation and the Gm locus. One of these three children had, in 2. Lyon, M. F. Amer. J. hum. Genet. 1962, 14, 135. 3. Bach ,F., Hirschhorn, K. Proc. 11th int. Congr. Genet.
1963, p. 312.