The genomic mutations are mutations (changes) that affect genomes in terms of alterations in the amount (mass) of DNA. They are represented by changes in the number of chromosomes and, therefore, in the decrease or increase in the amount of DNA in an organism.
These alterations in number also affect gene expression patterns, sometimes with undesirable consequences. The main genomic mutations are caused by errors or unexpected changes during cell division (mitosis and / or meiosis).
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Since a genomic mutation has to do with changes in the number of chromosomes of an individual, it is pertinent that we familiarize ourselves with some basic concepts:
- The genes are fragments of deoxyribonucleic acid (DNA) that contain the information that determines the characteristics of living beings.
- In eukaryotic organisms, all genes (genome) are organized into defined structures known as chromosomes and that they are contained within an organelle called core.
- All organisms are composed of cells, which are the product of one or more cell division events (mitosis or meiosis) and differentiation.
- During the division of a cell, either by mitosis or meiosis (reductional division), changes can occur in the ploidy of organisms that can result in genomic mutations.
- The ploidy It is the set of all the chromosomes that define the genome of a species. Human beings, for example, are diploid, since we have two complete sets of chromosomes of the species (24 from our mother and 24 from our father).
Genomic mutations have to do, fundamentally, with changes in the number of chromosomes of an individual. These changes can be of two types:
Aneuploidies are mutations or changes in the number of a particular chromosome. Thus, for example, if an organism completely lacks a given chromosome, it is said to suffer from a nullisomy.
Other changes are named as follows:
The most common cause of aneuploidies is the fusion of gametes (fertilization) in which one of them was the product of a gametic non-disjunction (separation failure) during meiosis.
In meiosis II, for example, separation of sister chromatids may fail. If a daughter cell receives both chromosomes of the pair (disomic gamete), its sister cell will receive neither (nullisomic gamete). The fusion of a disomic gamete with a monosomic one will give rise to a trisomic organism.
The fusion of a nullisomic gamete with a monosomic one will give rise to a monosomic individual for that chromosome.
In humans, the chromosome-based sex determination system requires that individuals who are XX be female, and XY male.
Sometimes meiotic nondisjunction leads to the production of gametes with an extra sex chromosome (disomic gamete) or without any sex chromosome (nullisomic gamete).
The fusion of some of these two gametes with a gamete carrying a sex chromosome will lead to the formation of a trisomic or monosomal zygote, respectively..
There are several sets of phenotypic alterations in humans, or syndromes, that are explained by changes in the number of the sex chromosomes. We will see three of them:
XO individuals are monosomic females who carry a single X chromosome instead of two (XX). Phenotypically they are female; are short, have a tendency to be more likely to develop certain diseases, and are infertile.
XXY individuals are males with two X chromosomes, rather than one (XY). Although phenotypically male, they preserve youthful features throughout adulthood, and sometimes develop gynecomastia. Hypogonadism is also frequent. They are infertile.
Erroneously associated in the past with a greater tendency to violent behavior, these individuals have a double Y chromosomal load.
These individuals are basically indistinguishable from XY men. Sometimes, however, they produce more testosterone and are higher than their blood relatives (parents and siblings). Sometimes they suffer from severe acne; are generally fertile.
Euploidies are "true" ploidies, that is, they represent changes (in integer multiples) in the numbers of complete sets of chromosomes of a species.
There are species in which the ploidy between males and females is different. In hymenopterans (wasps and bees, among others), for example, females are diploid and males are haploid. It is, for these species, however, a normal condition.
It is also normal, particularly in plants, that there are populations of the same species with different ploidies. This is very common in potatoes, a species in which we can find individuals with 2, 3, 4, 5 or 6 complete chromosome sets..
Species and individuals with different numbers of complete chromosome sets are called:
Changes in euploidy are frequent in plants and, in some cases, explain the formation of new species.
In animals, euploidies are rare, but not impossible. Polyploidies can arise from the fusion of gametes with a non-reduced chromosome number. In one of the parents, for example, meiosis II can lead to the production of gametes with all the chromosomes, while other gametic products did not receive any..
The fusion of non-reduced ploidy gametes with normal gametes of the same species can lead to the formation of autopolyploids. The fusion of gametes of different phylogenetically close species, but with different chromosomal sets, can lead to the formation of allopolyploids.
Finally, in normally diploid individuals, some somatic cells in certain tissues may be polyploid. In humans, it is common for this to occur in the liver, heart, bone marrow, etc..
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