2. ๏จ In genetics, a locus (plural loci) is a specific,
fixed position on a chromosome where a
particular gene or genetic marker is located.
๏จ Each chromosome carries many genes, with
each gene occupying a different position or
locus; in humans, the total number of protein-
coding genes in a complete haploid set of 23
chromosomes is estimated at 19,000โ20,000.
3.
4.
5. ๏จ Genes may possess multiple variants known as
alleles, and an allele may also be said to reside
at a particular locus. Diploid and polyploid
cells whose chromosomes have the same allele
at a given locus are called homozygous with
respect to that locus, while those that have
different alleles at a given locus are called
heterozygous.
6. ๏จ The ordered list of loci known for a particular
genome is called a gene map. Gene mapping is the
process of determining the specific locus or loci
responsible for producing a particular phenotype
or biological trait. Association mapping, also
known as "linkage disequilibrium mapping", is a
method of mapping quantitative trait loci (QTLs)
that takes advantage of historic linkage
disequilibrium to link phenotypes (observable
characteristics) to genotypes (the genetic
constitution of organisms), uncovering genetic
associations.
7. ๏จ 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:
๏จ 3 = chromosome 3
๏จ p = p-arm
๏จ 22 = region 2, band 2 (read as "two, two", not
"twenty-two")
๏จ 1 = sub-band 1
8. ๏จ 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.
9. ๏จ Component Explanation
๏จ 3 The chromosome number
๏จ p The position is on the chromosome's short arm (a
common apocryphal explanation is that the p stands for
petit in French); q indicates the long arm (chosen as next
letter in alphabet after p; it is also said that q stands for
queue, meaning "tail" in French[4]).
๏จ 22.1 The numbers that follow the letter represent the
position on the arm: region 2, band 2, sub-band 1. The bands
are visible under a microscope when the chromosome is
suitably stained. Each of the bands is numbered, beginning
with 1 for the band nearest the centromere. Sub-bands and
sub-sub-bands are visible at higher resolution.
10. ๏จ The characteristic pattern of light and dark
transverse bands on a stained chromosome (as
viewed under a microscope), and describes the
location of genes
11. ๏จ The different types of banding are G-banding,
reverse-banding, C-banding, Q-banding, NOR-
banding, and T-banding. Giemsa stain is used
in G-banding whereas quinacrine is used in Q-
banding.
12. ๏จ Each chromosome arm is divided into regions,
or cytogenetic bands, that can be seen using a
microscope and special stains. The cytogenetic
bands are labeled p1, p2, p3, q1, q2, q3, etc.,
counting from the centromere out toward the
telomeres. At higher resolutions, sub-bands can
be seen within the bands. The sub-bands are
also numbered from the centromere out toward
the telomere.
13. ๏จ Chromosome ideograms, i.e. diagrams used in
identifying chromosomes, are used by
cytogeneticists to show the relative sizes and
the characteristic banding patterns of
chromosomes. Banding patterns are patterns of
light and dark transverse bands on
chromosomes.
14.
15.
16. ๏จ 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.
๏จ The ends of a chromosome are labeled "pter"
and "qter", and so "2qter" refers to the terminus
of the long arm of chromosome 2.
17. ๏จ For example, the cytogenetic map location of
the CFTR gene is 7q31.2, which indicates it is
on chromosome 7, q arm, band 3, sub-band 1,
and sub-sub-band 2.
๏จ The ends of the chromosomes are labeled ptel
and qtel. For example, the notation 7qtel refers
to the end of the long arm of chromosome 7
18. ๏จ The light and dark bands become apparent by
staining the chromosome with a chemical
solution and then viewed under a microscope.
These bands describe the location of genes on a
chromosome.1 The basis of the differential
staining, which is the same in most tissues, is
not understood: each band represents 5-10% of
the length, about 10exp7 base pairs, although
this is not true for polytenechromosomes in
Drosophila that show more than 4000 bands.
