Four possible mechanisms for the formation of hyperdiploidy. (A) Doubling of a near-haploid set of chromosomes, resulting in widespread LOH caused by UPD. (B) Initial polyploidization with subsequent losses of chromosomes, resulting in LOH for approximately one third of the disomic chromosomes and equal allele dosages for tetrasomic chromosomes. (C) Sequential gains of individual chromosomes in consecutive cell divisions, resulting in unequal allele dosages for two thirds of the tetrasomic chromosomes. (D) Simultaneous gain of chromosomes in a single abnormal mitosis, resulting in equal allele dosages for tetrasomic chromosomes.