Each plant and animal is characterized by a set of chromosomes, represented once in haploid cells (eg gametes and spores) and twice in diploid cells.
Each species has a specific number of chromosomes. But irregularities sometimes occur in nuclear division, or "accidents" (such as radiation) can occur during interphase so that whole cells or organisms with aberrant genomes can form. Such chromosomal aberrations may include whole genomes, whole isolated chromosomes, or just parts of chromosomes.
At chromosomal aberrations can be numeric or structural and involve one or more autosomes, sex chromosomes, or both. Numeric chromosomal aberrations include cases where there is an increase or decrease in the normal karyotype number of the human species, while structural chromosomal aberrations include cases in which one or more chromosomes exhibit changes in their structure. Thus, cytologists recognize:
(1) Changes in Chromosome Number (Heteroploidy)
(2) Changes in Chromosome Structure.
THE heteroploidy can reach entire sets of chromosomes (euploidy) or loss or addition of isolated whole chromosomes (aneuploidy). All of these changes have an important effect on development, as altering the normal nuclear structure can produce phenotypic changes.
Changes in chromosome number
Numerical variations are of two types: euploidies, which give rise to cells with multiple chromosomes number to haploid number, and the aneuploidies, which give rise to cells lacking or excessing any chromosome (s). Like this, euploidies they are alterations of the whole genome; In this respect, individuals may be haploid (n), diploid (2n), triploid (3n), tetraploid (4n), and finally polyploid (when there are many excess genomes). Euploidies are rare in animals, but quite common and important evolutionary mechanisms in plants. In humans, the occurrence of euploidies is incompatible with the development of the embryo, determining the occurrence of abortion. Polyploid cells whose chromosome number reaches 16n are found in bone marrow, liver and normal kidneys, and occur in solid tumor cells and leukemia.
Origin of Aneuploidies
Aneuploidies may originate from anomalies that occur in meiosis (ie, prezygotic) or zygote mitoses (postzygotic).
When non-segregation is prezygotic, it may have occurred in spermatogenesis or ovulogenesis. In the origin of individuals with two X and one Y chromosomes, the female contribution is greater than the male; On the other hand, 77% of cases where there is only one X originate from errors in spermatogenesis. In autosomal aneuploidies, the influence of maternal age leads us to assume that female participation is higher than male participation. Aneuploidies produced by errors in zygote mitosis or blastomer segmentation are less frequent.
At aneuploidies due to non-segregation (or non-segregation) of one (or more) chromosome (s) for daughter cells during meiosis or during zygote mitosis. Non-segregation in mitosis results from non-disruption of the centromere in the the beginning of anaphase or the loss of some chromosome because it did not bind to the spindle.
The non-segregation in meiosis is due to failures in the separation of chromosomes or chromatids, which randomly separate to one pole or the other. In meiosis non-segregation can occur in both the first and second divisions. In the first case, the gamete with the excess chromosome, instead of having only one of the chromosomes of a given pair, that is, will have a paternal and a maternal chromosome. In the second, the gamete with the excess chromosome will have two paternal or two maternal chromosomes, for example.
When as a result of these non-segregation processes a chromosome of a given pair is missing, that is, when the number of chromosomes in the cell is 2n - 1, it is said that the cell has monosomy for this chromosome. If both elements of the same pair 2n - 2 are missing, one has nullisomy. If, on the contrary, there is an increase in the number of chromosomes of a given pair, the cell will be polysomic for the chromosome in question; She will be trisomictetrasomic etc., as it has 1, 2 or 3 more chromosomes, in which case its chromosome number designated by (2n + 1), (2n + 2), (2n + 3), etc.