These changes do not modify the amount of chromosomes in a cell, but determine the appearance of abnormal chromosomes.
The aberrations we describe below almost always lead to serious problems, including the formation of gametes. This is because during meiosis, the chromosome with the deficiency anomalously matches its unchanged counterpart, affecting the progress of the meiotic process. The severity of manifestations of a deficiency depends on the missing genes.
Disability or Deletion
A chromosome piece is lost in this type of anomaly, which involves the loss of many genes. Deficiencies are noticed during chromosome pairing in meiosis. A human example is the syndrome of cri du chat (cat meow syndrome), lacking a fragment of the short arm of chromosome 5. Characterized by mental retardation, microcephaly, rounded aspect of the face, presence of epicanthic folds in the eyes and a cat-meow cry.
Another example is short chromosome 22 ("Philadelphia chromosome"), associated with a form of leukemia.
A piece of chromosome breaks, rotates 180 degrees and welds back upside down. Because of the change in gene order, the pairing of homologs in meiosis.
It is the exchange of pieces between nonhomologous chromosomes, different from what occurs in crossing-over, normal and common phenomenon. We speak of reciprocal and heterozygous translocation, in which only one element of each pair undergoes exchange. At the time of the meiotic pareamet, there is a cross-shaped figure.
It is possible that translocation was a mechanism of formation of new species. There are hypotheses about some species of drosophila, all with a different number of chromosomes, that could have originated from an ancestral species, from translocations of various types.
In duplication, there is the formation of an additional segment in one of the chromosomes. In general, the consequences of duplication are well tolerated as there is no shortage of genetic material.