Understanding:
• A gene is a heritable factor that consists of a length of DNA and influences a specific trait
• A gene occupies a specific position on a chromosome
DNA is the genetic blueprint which codes for, and determines, the
characteristics of an organism
- This includes the physical, behavioural and physiological features of the organism
DNA is packaged and organised into discrete structures called chromosomes
- A gene is a sequence of DNA that encodes for a specific trait [traits may also be influenced by multiple genes]
- The position of a gene on a particular chromosome is called the locus [plural = loci]
Genes and Gene Loci
In genetics, a locus [plural loci] is a specific, fixed position on a chromosome where a particular gene or genetic marker is located.[1] 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 19,000–20,000.[2]
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.[3] 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.
Nomenclature[edit]
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.
Example of cytogenetic bands
| Component | Explanation |
| 3 | The chromosome number |
| 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]]. |
| 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. |
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[edit]
- Chromosomal translocation
- Cytogenetic notation
- Karyotype
- Null allele
- International System for Human Cytogenetic Nomenclature
References[edit]
- ^ Wood, E.J. [1995]. "The encyclopedia of molecular biology". Biochemical Education. 23 [2]: 1165. doi:10.1016/0307-4412[95]90659-2.
- ^ Ezkurdia, Iakes; Juan, David; Rodriguez, Jose Manuel; Frankish, Adam; Diekhans, Mark; Harrow, Jennifer; Vazquez, Jesus; Valencia, Alfonso; Tress, Michael L. [2014-11-15]. "Multiple evidence strands suggest that there may be as few as 19,000 human protein-coding genes". Human Molecular Genetics. 23 [22]: 5866–5878. doi:10.1093/hmg/ddu309. ISSN 1460-2083. PMC 4204768. PMID 24939910.
- ^ "NCI Dictionary of Genetics". National Cancer Institute. Retrieved 13 December 2014.
- ^ "NCBI Genetics Review". National Center for Biotechnology Information. Retrieved 10 March 2021.
Michael, R. Cummings. [2011]. Human Heredity. Belmont, California: Brooks/Cole.
External links[edit]
- Overview at ornl.gov
- Chromosome Banding and Nomenclature from NCBI.