Heterogeneous gene distribution reflects human genome complexity as detected at the cytogenetic level.

Human chromosomes are heterogeneous in structure and function and this is the base for the specific banding patterns produced by various chromosome staining techniques. The Human Genome Data Base as of January 2001 was searched for genes mapped to individual chromosomal bands to study the different aspects of human genome organization as they appear at the cytogenetic level of resolution. Genes are unequally distributed both on human chromosomes and chromosome bands. Among more than 5000 genes mapped at individual bands, 81% were located in G-negative bands, which correspond to half of the human genome. The main practical value of having a dense genetic physical map of genes is to accelerate the discovery by positional candidate cloning of human disease genes. Gene content agrees with H3 family isochores and with GC-rich flavors. Interestingly, two G-positive bands, namely 2p chromosome bandings is 12 and 7q35, contain a high number of genes. The finding of heterogeneity in gene content suggests that chromosome banding is not only due to differences in gene content.