CHROMOSOMES carry most of the genetic material and therefore they:
The normal human karyotype is made of 46 chromosomes:
-->22 pairs of autosomes, numbered from 1 to 22 by order of decreasing length
-->1 pair of gonosomes, or sex chromosomes: XX in the female, XY in the male.
In mammalian cells, the p-arm of many acrocentric chromosomes carry nucleolar organising regions (NORs) which contain genes coding for ribosomal RNA. This is true for all five pairs of acrocentrics in human cells.
A chromosome anomaly can be:
Visually, chromosomes can appear to break, and broken ends can rejoin in various ways:
A mutual exchange between terminal segments from the arms of 2 chromosomes. Provided that there is no loss or alteration at the points of exchange, the new arrangement is genetically balanced, and called a:
At meiosis, where there is pairing of homologous chromosome segments (normal chromosomes form a bivalent), followed by crossing-over, translocations may form a quadrivalent (tetravalent, in Greek) and this leads to segregation problems. At meiosis anaphase I, chromosomes separate without centromere separation; this separation occurs at anaphase 2. Segregation of chromatids in the case of a quadrivalent (Figure) can be according the following:
Huret JL, Leonard C, Savage JRK
Atlas of Genetics and Cytogenetics in Oncology and Haematology 2000-05-01
Chromosomes, Chromosome Anomalies
Online version: http://atlasgeneticsoncology.org/teaching/30084/chromosomes-chromosome-anomalies