A genetic locus is read by understanding its location on a chromosome, specified using a combination of chromosome number, arm, region, and band.
Understanding Chromosome Location
A genetic locus is essentially the address of a gene on a chromosome. This address is described using specific notations.
Chromosome Number
First, the chromosome number is indicated. Humans have 23 pairs of chromosomes, numbered 1 through 22 (autosomes) and the sex chromosomes (X and Y).
- Example: "11" indicates chromosome 11.
Chromosome Arm
Next, the arm of the chromosome is indicated:
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p - short arm of the chromosome
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q - long arm of the chromosome
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Example: "11q" indicates the long arm of chromosome 11.
Regions and Bands
The chromosome arms are then divided into regions and bands, each numbered sequentially from the centromere outwards.
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Regions are major divisions within the chromosome arm.
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Bands are subdivisions within a region.
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Example: "11q1" indicates region 1 of the long arm of chromosome 11. "11q1.4" indicates sub-band 4 of region 1 in the long arm of chromosome 11.
Reading a Range of Loci
Sometimes, the genetic locus specifies a range.
- Example: "11q1.4-q2.1" means the locus is located anywhere from sub-band 4 of region 1 to sub-band 1 of region 2 on the long arm of chromosome 11.
Chromosome Termini
The ends of a chromosome are labeled:
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pter - end of the short arm
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qter - end of the long arm
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Example: "2qter" refers to the terminus of the long arm of chromosome 2.
Example: Reading the OCA1 Locus
The example provided in the reference shows how to read the genetic locus of OCA1.
- The locus of gene OCA1 may be written "11q1.4-q2.1".
- This means that the gene is on the long arm (q) of chromosome 11.
- Specifically, the gene is located somewhere in the range from sub-band 4 of region 1 to sub-band 1 of region 2.
Summary Table
| Component | Description | Example |
|---|---|---|
| Chromosome Number | Identifies the chromosome. | 11 |
| Arm | Short (p) or long (q) arm of the chromosome | q |
| Region | Major division within the chromosome arm. | 1 |
| Band | Subdivisions within a region | 4, 2.1 |
| Range | A location spanning between bands | 1.4-2.1 |
| Termini | End of a chromosome arm | pter, qter |
By understanding this notation, scientists can accurately pinpoint the location of genes on chromosomes, facilitating genetic studies and disease research.
