How many statements are correct regarding non-disjunction of chromosomes?
Only statement iii is correct.
Chrosomes get distributed in the Anaphase stage of cell division. Non-disjunction happens at the chromosomal level. Non–disjunction is the process in which paired chromosomes fail to move to opposite poles during the Anaphase I stage of Meiosis.
Non-disjunction of chromosomes leads to the non-separation of homologous chromosomes and thus one cell will be having an extra chromosome while another a chromosome less. Thus non-disjunction changes the ploidy of cells.
Usually chromosomes get distributed equally in organisms; non–disjunction happens rarely, in certain specific conditions like old age.