Monoploid is defined as the half of the normal number of chromosomes. This is also known as gene reduction due to the reduced number of chromosomes in the genome.
Monoploidy is very infrequent because detection might not be possible as recessive lethal mutations will then become unmasked, causing death. However, this is not the same with diploids due to that the dominant alleles will mask the effect in the genome, thus avoiding death.
Cause of monoploidy:
Due to non disjunction - This is where separation of chromosome pairs is unsuccessful during the process of cell division. Deletion on one chromosome and chromosome duplication on the other arise due to unequal crossing over. Thus, this leads to gene reduction in one and subsequently gene duplication on the other chromosome.