Genetic mutations that result in half the normal level of the gene product leading to phenotypic effects (i.e., they are sensitive to gene dosage). It occurs in individuals who are heterozygous for a certain gene mutation or homozygous at a particular locus, often due to a deletion of the corresponding allele. Turner’s syndrome is thought to result from the haploid insufficiency of genes on the sex chromosomes, but these genes have not been identified yet.