Individual variation of centric heterochromatin in man

Summary

C-banded chromosome preparations were obtained from cord blood of normal newborns, from venous blood of abnormal newborns, and from both parents of all the babies studied. Heterochromatin and euchromatin length was recorded in five cells per person for chromosomes 1, 9, and 16. 1655 karyotypes from 231 subjects have been studied. Standardised heterochromatin lengths were estimated from the regression of heterochromatin on euchromatin measurements. A maximum likelihood procedure was used to estimate heterochromatin lengths in chromosomes of either maternal or paternal origin and the distributions were compared with those of average absolute differences. An error variance of difference between homologues was derived from measurements of heterochromatin length in cells from carriers of structurally altered chromosomes in which it was possible to identify parental origin.

There was no evidence of systematic difference in heterochromatin or euchromatin length between normal and abnormal babies nor in the parents of such. Babies, however, tended to have shorter chromosomes than adults. The distribution of standardised heterochromatin length was symmetrical and there was a 2-${\text{2}}\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$}$-fold range between the ends of the distribution, apart from individual very large segments noted for each chromosome. The evidence is discussed in relation to criteria for detecting real differences between homologues as well the probable number of base pairs involved in the heterochromatin differences.