Section 16 — Human Cytogenetics 



46 chromosomes instead of the expected 45 and 

 a structurally abnormal X chromosome. Mea- 

 surement of this metacentric chromosome 

 showed that its arms were approximately 

 equal to the long arm of the X. These individuals 

 were consistently sex chromatin positive and 

 measurements of the DNA content of Barr 

 bodies in their cells (in collaboration with 

 H. Klinger) revealed that these Barr bodies had 

 higher DNA content than those from suitable 

 controls. When labelled with 3 H-thymidine (in 

 collaboration with S. Muldal) late in the syn- 

 thesizing period the abnormal chromosome was 

 heavily labelled (i.e. it behaved as one of the 

 two X chromosomes usually does in this sys- 

 tem), and symmetrically so along its length. 

 We tentatively interpret this abnormal chro- 

 mosome as an iso-chromosome for the long 

 arm of X. 



If the cytological interpretation is correct, it 

 follows that the carriers of these iso-chromo- 

 somes are monosomic for the short and triso- 

 mic for the long arm of X. In one family in our 

 material there was informative segregation of the 

 genes for deutan colourblindness and the sex- 

 linked blood-group Xg. Analysis of this family 

 (in collaboration with Ruth Sanger and R. R. 

 Race) gave evidence that both genes are located 

 on the short arm of the X chromosome. The 

 origin of the iso-chromosome is probably meiotic 

 and likely paternal. There was an apparent 

 deficiency of females among the sibships of 

 the index cases. 



16.14. DNA Replication of X-chromosomes in Cul- 

 tured Leucocytes from Presumptive XXX, 

 X XX X Y and XO/ X Iso-chromosome- X Mosaic 

 Human Subjects. Barid B. Mukherjee, 

 J. Orlando Miller, W. Roy Breg and Saul 

 Bader (New York, U.S.A.). 



An autoradiographic study of DNA replica- 

 tion in X-chromosomes was carried out by ex- 

 posing the cultured leucocytes to tritiated 

 thymidine for 3-9 hours in the presence of col- 

 chicine and then harvesting the cells directly 

 from the isotope-containing medium. Two 

 chromosomes in group 6-1 2 +X in metaphase 

 figures from three chromatin two positive 

 presumptive XXX females and three chromo- 

 somes of the same group in metaphase figures 

 from a chromatin three positive presumptive 

 XXXXY male were consistantly the last chro- 

 mosomes in the whole complement to complete 

 DNA synthesis. Two sex chromatin masses were 

 observed in only two-ten per cent of the inter- 

 phase nuclei of the three XXX females and three 



sex chromatin masses were present in only seven 

 per cent of the interphase nuclei of the XXXXY 

 individual. These results support the concept 

 of a one-to-one correspondence between the 

 maximum number of sex-chromatin masses and 

 the number of late-replicating X-chromosomes. 

 The isochromosome-X was always the last 

 chromosome to terminate duplication in cultured 

 leucocytes from the XO/X iso-chromosome-X 

 female, and the normal X chromosome never 

 terminated duplication late. According to the 

 Lyon hypothesis, random differentiation of the 

 paternally or maternally derived X chromosome 

 should occur in individual cells during mamma- 

 lian embryogenesis. In this case, however, the 

 structurally abnormal X chromosome always 

 becomes the heteropycnotic, metabolically inert 

 member of the pair. 



16.15. The Incidence of Drumsticks in Normal 

 Women and in Patients with Chromosomal 

 Abnormalities. Ursula Mittwoch (London, 

 Great Britain). 



The demonstration of sex chromatin has be- 

 come an important aspect of human genetics, 

 both for theoretical and for practical reasons. 

 Although it is now generally accepted that the 

 drumstick of the polymorphonuclear leucocyte 

 bears a direct relationship to the X-chromosome 

 and is therefore comparable to the Barr body, 

 the interpretation of findings on drumsticks has 

 not always been straightforward. For this there 

 are three main reasons: (1) The low incidence 

 of drumsticks, which is on the average less than 

 3 per cent of polymorphonuclear leucocytes; 

 (2) the wide variation in frequency encountered 

 in different women; (3) the fact that incidence 

 of drumsticks is to some extent determined by 

 the degree of segmentation of the polymorpho- 

 nuclear nuclei. 



An investigation has been made relating the 

 incidence of drumsticks to the number of nu- 

 clear lobes in apparently normal women and in 

 patients with various chromosomal abnormali- 

 ties. The following individuals were tested: 12 

 apparently normal women; 27 girls with mon- 

 golism (Down's syndrome) in association with 

 27 non-mongol mentally defective girls for con- 

 trol purposes; 12 patients with Klinefelter's 

 syndrome (XXY), 3 patients with XXXY chro- 

 mosome constitution; and 3 patients with XXX 

 chromosome constitution. 



It was found that in the apparently normal 

 women the correlation coefficient between the 

 incidence of drumsticks and the average number 

 of nuclear lobes was +0.44. In all women there 



303 



