Non-random segregation of chromosomes
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Non-random segregation of chromosomes is a deviation from the usual distribution of chromosomes during meiosis, that is, during segregation of the genome among gametes. While usually according to the 2nd Mendelian rule (“Law of Segregation of genes“) homologous chromosomes are randomly distributed among daughter nuclei, there are various modes deviating from this in numerous organisms that are "normal" in the relevant taxa. They may involve single chromosome pairs (bivalents) or single chromosomes without mating partners (univalents), or even whole sets of chromosomes, in that these are separated according to their parental origin and, as a rule, only those of maternal origin are passed on to the offspring. It also happens that non-homologous chromosomes segregate in a coordinated manner. As a result, this is a form of Non-Mendelian inheritance.
This article describes cases where non-random segregation is the normal case for the particular organisms or occurs very frequently. A related phenomenon is called meiotic drive or segregation distortion. This is a higher than average transmission of a single chromosome relative to the homologous chromosome in inheritance. This can be due to non-random segregation during meiosis, but also to processes after meiosis that reduce the transmission of the homologous chromosome.
In addition, there are pathological cases that result in aneuploidy and are almost always lethal.