The simplest structural change, but perhaps the most damaging, is a deletion (the complete loss of a part of a chromosome).
In a haploid cell, this is lethal, because part of the essential genome is lost. However, even in diploid cells, the deletions are usually lethal or have other serious consequences.
In a diploid, a heterozygous deletion results in a cell that has a normal chromosome set and another set that contains a truncated chromosome. Such cells show genomic imbalance , which increases in severity with the size of the shed.
Another potential source of damage is that any recessive, deleterious, or lethal alleles that are in the normal homolog of the deleted region will be expressed in the phenotype.
The spontaneous removal process must include two chromosomal breaks to cut the middle segment. If the two ends meet and one of them carries the centromere , a shortened chromosome is produced, which is said to carry a deletion. The removed fragment is eccentric; consequently, it is immobile and is lost.
An effective mutagen to induce chromosomal rearrangements of all kinds is ionizing radiation. This type of radiation, of which X-rays and gamma rays are examples, is very energetic and causes chromosome breaks. How the breaks are rejoined determines the type of rearrangement that occurs.
There are two types of removal possible. Two jumps can produce an interstitial deletion.
In principle, a single cut can cause terminal removal; But, because of the need for the special chromosomal tips (telomeres), the apparently terminal deletions are likely to include two breaks, one near the telomere.
The effects of eliminations depend on their size. A small deletion within a gene, called an intragenic deletion, inactivates the gene and has the same effect as other null mutations in that gene.
If the homozygous null phenotype is viable (as, for example, in human albinism), then the homozygous deletion will also be viable. Intragenic deletions can be distinguished from single nucleotide changes because they are not reversible.
Multigene deletions are those that delete several genes. Multigene deletions have serious consequences. If by inbreeding this deletion becomes homozygous (that is, if both homologs have the same deletion), then the combination is almost always lethal.
This result suggests that most regions of the chromosomes are essential for normal viability and that the complete removal of any segment of the genome is detrimental. Even individuals heterozygous for a multigenic deletion: those with a homologue may not survive.
There are several possible reasons for this failure. First, a genome has been “tuned” during evolution to require a specific balance of genes, and deletion alters this balance.
Second, in many organisms there are lethal recessive and other deleterious mutations throughout the genome. If they are “covered” by wild-type alleles in the other homolog, these recessives are not expressed. However, a deletion can “uncover” recessions, allowing their expression at the phenotypic level.
The lethality of heterozygous deletions can be explained by genome imbalance and unmasking of recessive lethal alleles.
However, some small deletions are viable in combination with a normal homologue. In these cases, the deletion can sometimes be identified by cytogenetic analysis.
If the meiotic chromosomes are examined in an individual carrying a heterozygous deletion, the region of the deletion can be determined by the failure of the corresponding segment in the normal homolog of the pair, resulting in a deletion cycle.
In insects, deletion loops are detected in polyene chromosomes, in which the homologues fuse. A deletion can be assigned to a specific chromosome by determining which chromosome shows the deletion loop and the position of the loop along the chromosome.
Deletions of some chromosomal regions produce their own unique phenotypes. A good example is the deletion of a small Drosophila-specific chromosomal region.
When a homologue carries the deletion, the fly shows a unique notched wing phenotype, so the deletion acts as a dominant mutation in this regard.
But elimination is lethal when homozygous and therefore acts recessive in relation to its lethal effect. The specific dominant phenotypic effect of certain deletions could be caused by one of the chromosomal breaks within a gene, which when altered will act as a dominant mutation.
What are the genetic properties of the deletions?
In addition to the cytogenetic criteria, there are several purely genetic criteria to infer the presence of a deletion. These criteria are particularly useful in species whose chromosomes are not easily analyzed cytogenetically.
The first is the failure of the chromosome to survive as a homozygous; however, this effect could also be produced by any lethal mutation. Second, the chromosomes with deletions can never return to a normal condition. This criterion is useful only if there is a specific phenotype associated with the deletion.
A third criterion is that, in heterozygous deletions, the recombinant frequencies between the genes flanking the deletion are lower than in control crosses. This makes intuitive sense because part of the region contains an unpaired chromosomal region, which cannot participate in the crossover.
Inversions have a similar effect on recombinant frequencies, but can be distinguished in other ways.
A fourth criterion for inferring the presence of a deletion is that the deletion of a segment in one homologue sometimes unmasks the recessive alleles present in the other homolog, leading to their unexpected expression.