WHAT IS A LOCUS? (GENETICS) - BIOLOGY - 2024

A locus, in genetics, refers to the physical position of a gene or a specific sequence within a chromosome. The term originates from Latin roots, and the plural is loci. Knowing the loci is very useful in the biological sciences, since they allow us to locate genes.

Genes are DNA sequences that code for a phenotype. Some genes are transcribed into messenger RNA, which is subsequently translated into an amino acid sequence. Other genes generate various RNAs and may also be related to regulatory roles.

Source: Doc. RNDr. Josef Reischig, CSc., via Wikimedia Commons

Another relevant concept in the nomenclature used in genetics is allele, which some students often confuse with locus. An allele is each of the variants or forms that a gene can take.

For example, in a hypothetical butterfly population, gene A is located at a certain locus and may have two alleles, A and a. Each one associated with a particular characteristic - A can be related by the dark coloration of the wings, while a is related to a lighter variant.

Today, it is possible to locate a gene on a chromosome by adding a fluorescent dye that makes the particular sequence stand out.

Definition

A locus is the point location of a gene on a chromosome. Chromosomes are structures characterized by exhibiting complex packaging, made up of DNA and proteins.

If we go from the most basic levels of organization in chromosomes, we will find a very long DNA strand wrapped in a special type of protein called histones. The union between both molecules forms the nucleosomes, which resemble the beads of a pearl necklace.

Next, the described structure is grouped in the 30 nanometer fiber. Thus various levels of organization are achieved. When the cell is in the process of cell division, the chromosomes compact to such an extent that they are visible.

In this way, within these complex and structured biological entities are the genes located at their respective locus.

Nomenclature

Biologists need to be able to refer to a locus precisely and their colleagues to understand the address.

For example, when we want to give the address of our houses, we use the reference system we are used to, be it house number, avenues, streets - depending on the city.

Similarly, to deliver the information about a specific locus, we must do it using the correct format. The components of a gene location include:

The number of chromosomes: In humans, for example, we have 23 pairs of chromosomes.

Chromosome arm: Immediately after referring to the chromosome number we will indicate in which arm the gene is found. The p indicates that it is in the short arm and the q in the long arm.

Arm position: The last term indicates where the gene is on the short or long arm. The numbers are read as region, band, and sub-band.

Genetic mapping

What are genetic maps?

Techniques exist to determine the location of each gene on chromosomes and this type of analysis is crucial for understanding genomes.

The location of each gene (or its relative position) is expressed on a genetic map. Note that genetic maps do not require knowledge of the gene's operation, only its position needs to be known.

In the same way, genetic maps can be constructed starting from variable segments of DNA that are not part of a specific gene.

Linkage disequilibrium

What does it mean that one gene is "linked" to another? In recombination events, we say that a gene is linked if they do not recombine and stay together in the process. This occurs due to the physical closeness between the two loci.

In contrast, if two loci inherit independently, we can conclude that they are far apart.

The linkage disequilibrium is the central point for the construction of gene maps through linkage analysis, as we will see below.

Markers for the construction of genetic maps

Suppose we want to determine the position of a certain gene on the chromosome. This gene is the cause of a fatal disease, so we want to know its location. Through pedigree analysis, we have determined that the gene has traditional Mendelian inheritance.

In order to find the position of the gene, we will need a series of marker loci that are distributed throughout the genome. Then, we must ask ourselves if the gene of interest is linked to any (or more than one) of the markers of which we are aware.

Obviously, for a marker to be useful, it must be highly polymorphic, thus there is a high probability that the person with the disease is heterozygous for the marker. "Polymorphism" means that a given locus has more than two alleles.

That there are two alleles is essential, since the analysis seeks to answer whether a particular allele of the marker is inherited together with the study locus and this generates a phenotype that we can identify.

Furthermore, the marker must exist in a significant frequency, close to 20% in heterozygotes.

How do we build a genetic map?

Continuing with our analysis, we choose a series of markers that are separated from each other by about 10 cM - this is the unit in which we measure the separation and it is read centimorgans. Therefore, we assume that our gene is at a distance no greater than 5 cM from the markers.

Then, we rely on a pedigree that allows us to obtain information about the inheritance of the gene. The studied family must have enough individuals to yield data with statistical significance. For example, a family group with six children would be sufficient in some cases.

With this information, we locate a gene to which the condition is linked. Suppose we find that the B locus is linked to our deleterious allele.

The above values ​​are expressed as a ratio between the probability of linkage and the absence of this phenomenon. Today, the subsequent statistical calculation is done by a computer.

References

1. Campbell, NA (2001). Biology: Concepts and relationships. Pearson Education.
2. Elston, RC, Olson, JM, & Palmer, L. (Eds.). (2002). Biostatistical genetics and genetic epidemiology. John Wiley & Sons.
3. Lewin, B., & Dover, G. (1994). Genes V. Oxford: Oxford University Press.
4. McConkey, EH (2004). How the human genome works. Jones & Bartlett Learning.
5. Passarge, E. (2009). Genetics text and atlas. Panamerican Medical Ed.
6. Ruiz-Narváez EA (2011). What is a functional locus? Understanding the genetic basis of complex phenotypic traits. Medical hypotheses, 76 (5), 638-42.
7. Wolffe, A. (1998). Chromatin: structure and function. Academic press.