From ISOGG Wiki
Heterochromatin is "a chromosomal region that remains highly condensed throughout the cell cycle and shows little or no evidence of active gene expression".
Heterochromatin contains very long repetitive sequences, and is very difficult to sequence with current technology. About half of the Y chromosome is made up of heterochromatin and has not yet been sequenced. The p arms of chromosomes 13, 14, 15, 21, and 22 contain heterochromatin and have also not been sequenced. There are also large regions of heterochromatin near the centromeres of chromosomes 1 and 9. See the NCBI Genetics Review website for a diagram showing the regions of the human genome where there are large portions that are heterochromatin.
- Strachan, T. and Read, A.P. 1999. Glossary in Human Molecular Genetics, 2nd ed. New York: John Wiley & Sons. Cited by the NBCI Genetics Review.