Secondary constrictions in the human karyotype are less intensely stained by plain Giemsa than the rest of the chromosome. By G-banding, this region (designated the 1qh region, which typically corresponds to band q12) is more variably stained. It may appear as a Giemsa positive band or as a combination of darker and lighter bands (Fig. 6.1a). Long 1qh regions may appear as two or more dark bands separated by Giemsa-negative bands (Fig. 6.1a5). By C-banding, the entire region is usually Giemsa-positive (Figs. 6.1b and 2.2a). Variations in the size of the secondary constriction of chromosome 1 by plain Giemsa were early noted to occur in 1/100 to 1/1000 newborns (Lubs and Ruddle, Nature 233:134–136, 1971). Variations in size by C-banding range from less than 1/2 the size of 16p (level 1,) to more than twice the size of 16p (level 5). From the New Haven study (Lubs et al., Population cytogenetic studies in humans, 1977), 7.5% of children showed size variations by C-banding, 80% of which were level 5 variants, and 20% of which were level 1 variants (Fig. 6.2-1).
KeywordsSecondary Constriction Light Band Alpha Satellite Normal Physical Examination Complete Inversion
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