Chromosomal aberrations are abnormalities in the structure or number of chromosomes which are often responsible for genetic disorders. The structural changes in the chromosomes include:
- Deletion or deficiency -loss of a part of chromosome
- Duplication -addition of a part of chromosome
- Translocation- exchange of segment between non homologous chromosomes
- Inversion- reversal of order of genes in a part of a chromosome
Deletion or Deficiency
Deficiency is due to loss of a part of chromosome. If the lost part of chromosome does not have a centromere, it is left behind during anaphasic movement. This part of chromosome, thus gets excluded from the newly formed nucleus. Such a condition is generally lethal for an organism.
Smaller deficiencies, present in heterozygous condition (only on one of the two homologous chromosomes) can be tolerated by an organism. Such individuals at meiosis will form a loop in a bivalent that can be observed at pachytene stage. Loops also occur in salivary gland chromosomes of Drosophila which are found in a permanent state of pairing, so that even small deficiencies could be detected in these chromosomes.
Deficiencies have an effect on inheritance also. In presence of a deficiency, a recessive allele will behave like a dominant allele (pseudodominance). This principle of pseudodominance exhibited by deficiency heterozygotes have been utilised for location of genes on specific chromosomes in Drosophila, maize and other organisms.
Duplications are obtained due to addition of a part of a chromosome. Duplications shows the presence of the same part of chromosome, two or more times. These are not harmful like deletions. Duplications often change the phenotype of an organism and are, therefore, important in evolution.
One of the classical examples of duplication in Drosophila is Bar eye. Bar eye is a character, where eyes are narrower as compared to normal eye shape. This phenotypic character is due to duplication for a part of a chromosome. By the study of giant salivary gland chromosomes, it could be demonstrated that Bar character was due a duplication in region 16A of X-chromosome. Barred eyes will have slightly different phenotype in heterozygous and homozygous individuals. Barred individuals (16A 16A) gave rise to ultra bar(16A 16A 16A) and normal wild-type (16A) due to unequal crossing over. Some other duplications known in Drosophila lead to following phenotyping effects:
- A reverse repeat in chromosome 4 causes eyeless dominant.
- A tandem duplication in chromosome 3 causes confluens resulting in thickened veins.
- Another duplication causes hairy wing.
These are the transfer of chromosome segments from one chromosome to another. The important amongst these are reciprocal translocations or segmental interchanges which involve mutual exchange of chromosome segments between two pairs of non homologous chromosomes.
Cytology of a translocation heterozygote
If a translocation is present in one of the two sets of chromosomes, this will be a translocation heterozygote. In such a plant, normal pairing into bivalents will not be possible among chromosomes involved in translocation. Due to pairing between homologous segments of chromosomes, a cross shaped figure involving 4 chromosomes will be observed at pachytene. This ring of four chromosomes at metaphase 1 can have one of the following three orientations:
Alternate: In alternate orientation, alternate chromosomes will be oriented towards the same pole . In other words, adjacent chromosomes will orient towards opposite poles. This will be possible by formation of a figure of eight.
Adjacent I : In adjacent I orientation, adjacent chromosome having non-homologous centromeres will orient towards the same pole. In other words, chromosomes having homologous centromeres will orient towards opposite poles.
Adjacent II: In adjacent II orientation, adjacent chromosome having homologous centromeres will orient towards the same pole.
Alternate disjunctions will give functional gametes. Adjacent I and adjacent II disjunctions will form gametes, which would carry duplications or deficiencies and as a result would be non-functional or sterile. Therefore, in a plant having a translocation heterozygous condition, there will be considerable pollen sterility.
Inversion occurs when a part of a chromosome breaks away and reunites in reverse order. There are two types of inversions paracentric inversion and pericentric inversion.
Inversions result in considerable gametic or zygotic sterility and pollen sterility. The recombination frequencies get highly reduced, therefore, inversions are often known as crossover suppressors.
Inversions plays a significant role in evolution of different species and races of Drosophila.
Cytology of inversions
Due to an inverted segment in one of the two homologous chromosomes, the normal kind of pairing is not possible in an inversion heterozygote. In order to enable pairing of segments, each of the two chromosomes form the shape of a loop. This kind of configuration occurs both in paracentric as well as in pericentric inversions.
Paracentric inversions are those inversions, where inverted segment does not include centromere. A single crossing over or an odd number of crossovers in inverted region will result into formation of a dicentric chromosome (i.e., having two centromeres) and acentric chromosome ( with no centromere). Of the remaining two chromatids, one will be normal and the other will carry the inversion. The dicentric chromatid and the acentric chromatid occur at anaphase 1 in the form of a bridge and a fragment. Double crossovers and crossovers within and outside inversion will give various kinds of deficiencies and duplications.
In a pericentric (means surrounding the centromere) inversion, inverted segment includes centromere.
In a pericentric inversion the pachytene configuration observed is similar to the one described above for paracentric inversion. However, the products of crossing over and configurations at subsequent stages of meiosis differ. In this case, two of the four chromatids resulting after meiosis will have deficiencies and duplications.
However, unlike paracentric inversion, no dicentric bridge or acentric fragment will be observed. Consequently, at anaphase 1 no bridge or fragment will be seen. However, in pericentric inversion, if two breaks does not occur equidistant from the centromere, this will result in a change in shape of this chromosome. For instance, a metacentric chromosome (with chromosome in the centre) may become submetacentric and vice versa.
Sometimes a second inversion occurs in a chromosome which already has one inversion. This results in an overlapping inversion, if the segments involved in first and second inversions contain a common region.