WHAT ARE HOMOLOGOUS CHROMOSOMES? - BIOLOGY - 2025

The homologous chromosomes of an individual are those chromosomes that are part of the same pair in a diploid organism. In biology, homology refers to kinship, similarity, and / or function by common origin.

Each member of the homologous pair has a common origin, and they are found in the same organism by fusion of gametes. All chromosomes in an organism are somatic chromosomes, except those of the sexual pair.

Sex chromosomes, from the point of view of homology, are an exception. Both may have a different origin, but have regions of homology that make them behave like somatic chromosomes during cycles of cell division.

These homologous portions allow both to mate during mitosis and meiosis, and to recombine during the second of them.

Obviously, particular chromosome pairs from different closely related species are also phylogenetically homologous. However, they have recombined and changed so much that it is very difficult for the same chromosomes from different species to be completely homologous.

Most likely, when comparing the chromosomes of two species, the homology is a mosaic. That is, a chromosome of one species will share large or small homologous regions with different chromosomes of the other.

Sources of chromosomal changes

Chromosome-level mutations can be experienced at two major levels: changes in number, and changes in structure.

Changes at the sequence level are analyzed at the gene (and genome) level and give us an idea of ​​the similarity in informational content between genes, genomes and species.

Changes in number and structure allow us to show similarities and differences at the organizational level, either by analyzing individual chromosomes or all of them as a whole.

Ploidy changes

Changes in the number of an individual's chromosomes that affect one or a few chromosomes are called aneuploidies. For example, an individual with 3 chromosomes 21 instead of two is said to have a trisomy.

A trisomy on chromosome 21 is the most common cause of Down syndrome. On the other hand, a female of the human species with a single X chromosome is also aneuploid for that chromosome. XO women have what is known as Turner Syndrome.

Changes that affect the basic number of chromosomes in a species are called euploidies. That is, there is a repeat of the haploid chromosome set of the species.

If there are two, the organism is diploid - as is the case with most species that exhibit sexual reproduction. If they present three, the organism is triploid; if four, tetraploid, and so on.

This is very common in plants and has been an important source of evolutionary changes in this group of organisms.

Chromosomal rearrangements

Individual chromosomes can also present certain types of rearrangements that can have great consequences for both the individual and the species. These changes include deletions, insertions, translocations, mergers, and inversions.

In deletions, portions of the chromosome are completely lost, thus giving rise to alterations in the cycles of meiotic division with the consequent production of possibly unviable gametes.

The lack of regions of homology is the cause of abnormal recombination events. The same happens in the case of insertions, since the appearance of regions in one and not another chromosome has the same effect in the generation of regions that are not completely homologous.

A particular case of addition is that of duplication. In this case, a portion of the DNA that is generated in the chromosome is added to a region of the chromosome. That is, it is copied and pasted next to the source of the copy.

In the evolutionary history of chromosomes, batch duplications have played a fundamental role in the definition of centromeric regions.

Another way to partially change the homology between two chromosomes is by the appearance of inverted regions. The information of the inverted region is the same, but its orientation is opposite to that of the other member of the pair.

This forces homologous chromosomes to pair abnormally, giving rise to other types of additional rearrangements in gametes. The meiotic products of these meioses may not be viable .

An entire chromosomal region can migrate from one chromosome to another in an event called a translocation. Interestingly, translocations can be promoted by highly conserved regions between chromosomes not necessarily homologous. Finally, there is also the possibility of observing fusions between chromosomes.

Sythenia

Syntenia refers to the degree of conservation of the order of genes when two or more chromosomes or different genomic or genetic regions are compared.

Synthenia is not concerned with studying or measuring the degree of sequence similarity between homologous regions. Rather, to catalog the information content of these regions and analyze whether they are organized in the same way in the space they occupy.

All the rearrangements that we have mentioned above, obviously, decrease the synteny between the altered chromosome and its counterpart. They are still homologous because they share the same origin, but the degree of synthesis is much lower.

Synthenia is useful for analyzing phylogenetic relationships between species. It is also used to trace evolutionary trajectories, and to estimate the weight that chromosomal rearrangements have played in the appearance of species. As it makes use of large regions, these are macrosyntenia studies.

Microsyntenia, on the other hand, deals with doing the same type of analysis, but in smaller regions, usually at the gene or genes level. Genes, as well as chromosomes, can also undergo inversions, deletions, fusions, and additions.

Homology and sequence similarity

If they are homologous, two regions of DNA must have high similarity at the sequence level. In any case, here we are interested in pointing out that homology is an absolute term: one is homologous or not. The similarity, on the other hand, is measurable.

That is why, at the sequence level, two genes that code for the same thing in two different species can present a similarity of, for example, 92%.

But to say that both genes are 92% homologous is one of the worst conceptual errors that can exist at the biological level.

References

1. Alberts, B., Johnson, AD, Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P. (2014) Molecular Biology of the Cell (6 ^th Edition). WW Norton & Company, New York, NY, USA.
2. Brooker, RJ (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
3. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Philadelphia, PA, USA.
4. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11 ^th ed.). New York: WH Freeman, New York, NY, USA.
5. Philipsen, S., Hardison, RC (2018) Evolution of hemoglobin loci and their regulatory elements. Blood Cells, Molecules & Diseases, 70: 2-12.
6. Wright, WD, Shah, SS, Heyer, WD (2018) Homologous recombination and the repair of DNA double-strand breaks. Journal of Biological Chemistry, 293: 10524-10535