Chromosome polymorphism and twin zygosity

To demonstrate the heritability, stability, and frequency of chromosome polymorphism and to assess the value of chromosome studies in the determination of twin zygosity, Q- and G-band chromosome studies were performed on blood samples from MZ and like-sexed DZ twin pairs, and one set of male quadruplets. All of the karyotypic examinations were performed without knowledge of true zygosity. The quadruplets were shown to be asymmetrically dizygotic. The scoring results of the quadruplets and their parents were consistent with simple Mendelian segregation of the variants. There were 46 sib-sib comparisons available: 23 MZ twin pairs, 17 DZ twin pairs, and among the quadruplets, three MZ and three DZ comparisons. No differences were found between any of the 26 MZ pairs at any of 12 variable regions. Between the pairs shown to be dizygous on genotyping, 15 of 20 had three or more karyotypic discordances. Thus about 90% of the twin and quadruplet pairs were assigned their correct zygosity solely by comparisons of chromosome variant scores, with complete concordance of the MZ pairs' scores. The karyotype of a genetically distinct individual is virtually unique to himself. Two people taken at random have about one chance in 1,000 of sharing identical karyotypes using scoring criteria of this study. Furthermore, only one pair of sibs in 25 would be expected to have identical karyotypes. Chromosome variants scored here included 1qh, 9qh, and 16qh scored with G-banding; and variable regions of 3, 4, 13, and 22 and the acrocentric satellites scored with Q-banding. Variants seen at these regions are heritable and stable, since differences were found between DZ twins only, with complete concordance between MZ twins.