Definition of Haploinsufficiency
Haploinsufficiency is the phenomenon in which there is only one copy of a gene is present in a person that is potentially functional. This single functional copy of the gene is unable to give rise to a normal phenotype and hence haploinsufficiency shows the dominant pattern of inheritance.
The variant allele may have suffered from the loss of function mutation during inheritance or has arisen from a de novo mutation. This allele produces a very little or no product at all. On the contrary, the other allele produces the normal amount of product but the collective product is not enough to give the standard phenotype.