Examination of genome databases suggests that CpG
island clustering can be extrapolated to gene density itself
Hence the highest densities of genes in our genomes are
located in the T-and R-bands. This explains why human
chromosomes with a high G-band content, e.g. 13, 18 and
21, are seen as viable trisomies in the population, whereas
trisomies for small but T-/R-band-rich chromosomes (e.g.
chromosome 22) are lost early in embryonic development.
Whole genome sequencing has confirmed the low gene
density of human chromosome 21 compared to that of 22
(Dunham et al., 1999; The Chromosome 21 Mapping and
Sequencing Consortium, 2000).