nomenclature The shorter arm of a chromosome is termed the
p arm or
p-arm, while the longer arm is the
q arm or
q-arm. The chromosomal locus of a typical gene, for example, might be written
3p22.1, where: •
3 =
chromosome 3 •
p = p-arm •
22 = region 2,
band 2 (read as "two, two", not "twenty-two") •
1 = sub-band 1 Thus the entire locus of the example above would be read as "three P two two point one". The
cytogenetic bands are areas of the chromosome either rich in actively-transcribed DNA (
euchromatin) or packaged DNA (
heterochromatin). They appear differently upon staining (for example, euchromatin appears white and heterochromatin appears black on
Giemsa staining). They are counted from the
centromere out toward the
telomeres. A range of loci is specified in a similar way. For example, the locus of gene
OCA1 may be written "11q1.4-q2.1", meaning it is on the long arm of chromosome 11, somewhere in the range from sub-band 4 of region 1 to sub-band 1 of region 2. The ends of a chromosome are labeled
"pter" and
"qter", and so
"2qter" refers to the terminus of the long arm of chromosome 2. == See also ==