Section 16 — Human Cytogenetics 



have been found between the terminal replication 

 patterns of lymphocyte chromosomes versus 

 chromosomes of embryonic fibroblasts grown in 

 tissue culture. Results of the application of the 

 labeling technique to a number of abnormal 

 human karyotypes, including trisomic and 

 translocation mongolism and chronic myeloid 

 leukemia, will also be demonstrated. 



1. Proc. Nat. Acad. Sci. U.S. 43, 122. 



2. Cytogenetics, in press. 



in the fifth, even though breakpoint is <0.2 

 c.o. -units from + c . (2) Inactivating portions of 

 the X exert their influence along a gradient. 

 Thus, the amount of variegation for a given 

 locus differs for different translocations, one 

 factor being distance of locus from rearrange- 

 ment point. Furthermore, we observe a typical 

 spreading effect for three of the translocations. 

 The finding that part of the X has no ability 

 to inactivate translocated autosome suggests 

 that this part may itself never become inactive 

 and therefore not fit the single active X hypo- 

 thesis. 



16.10. Structural Variability of Human Chromo- 

 somes. Thea Luers and Eva Struck (Berlin- 

 Dahlem, Germany). 



Studying the chromosomes in human diseases 

 sometimes it seems difficult to decide between 

 structural abnormalities, natural variants, and 

 artfacts. A collection of karyotypes from 

 leucocyte cultures is presented and discussed 

 showing atypical chromosomes such as size 

 differences between the two homologues, achro- 

 matic regions, elongated centromere region, 

 translocation with partial trisomy, ring chromo- 

 somes, dicentrics, variability of the Y chromo- 

 some and of chromosomes nos. 21 or 22. 



16.11. Evidence from Six X-autosome Translocations 

 Bearing on the Single-active X Hypothesis. 



Liane Brauch Russell (Oak Ridge, U.S.A.). 



Recent results confirm our original thesis that 

 V-type position effects from X-autosome trans- 

 locations in the mouse (first reported by us in 

 1959) are found when the translocated X can 

 act heterochromatically. This occurs only when 

 there are at least two X's present, one being 

 required for gene action. Influences from X 

 heterochromatin suppress near-by autosomal 

 genes, allowing the recessives on the standard 

 autosome to express themselves. Lyon later 

 suggested that one or the other X is entirely 

 inactive and that the X-translocated autosome 

 behaves completely like the X to which it is at- 

 tached. This would imply that, for any given 

 autosomal locus, variegation characteristics 

 should be independent of the position of the X 

 and autosome break points. In work with six 

 translocations — one T(X;8) and five T(X;l)'s — 

 we have obtained evidence to the contrary. 



(1) Apparently, not the entire X has inactivat- 

 ing functions. Thus, the r-locus, which shows 

 typical variegation in four T(X;l)'s, fails to do so 



16.12. The Frequency of Sex-chromatin-positive Cells 

 and the Number of Nucleoli per Cell in Cultures 

 of Human Tissues. A. J. Therkelsen (Aarhus, 

 Denmark). 



The number of sex-chromatin-positive cells 

 was counted in tissue cultures of human cells 

 from bone marrow and from embryonic and 

 adult human skin. The frequency of sex-chro- 

 matin-positive cells is low — 40-60 per cent — 

 in the logarithmic growth phase, but rises to 

 about 95 per cent in the post-logarithmic phase. 

 The variation was found in normal females as 

 well as in sex-chromatin-positive males. The 

 cause of the variation will be discussed. In the 

 same type of experiments, cultures from males 

 and females were grown simultaneously, and the 

 number of nucleoli per cell was counted in 

 specimens stained with Feulgen-light-green. 

 The average number of nucleoli per cell was 

 shown to be significantly higher in normal 

 females than in normal males. 



This difference between the two sexes may be 

 explained on the assumption that the female 

 X-chromosome which forms the sex-chromatin 

 is nucleolus-organizing, whereas the Y-chromo- 

 some in males is not. 



If this explanation be correct, the average 

 number of nucleoli per cell in Klinefelter patients 

 should be significantly higher than in normal 

 males, whereas the number in Turner patients 

 should be the same as in normal males. Preli- 

 minary experiments with Klinefelter and Turner 

 patients have been performed. The results are 

 discussed. 



16.13. Iso-chromosome X in Man. M. Fraccaro and 

 J. Lindsten (Pavia, Italy). 



In a group of females selected because phe- 

 notypically similar to XO individuals, we found 



302 



