Loss of heterozygosity means

“A child with inherited mutant RB allele in somatic cells is perfectly normal. Because such a child is a heterozygous at the Rb locus, it implies that heterozygosity for the Rb gene does not affect cell behavior. Cancer develops when the cell becomes homozygous for the mutant allele or in other words when the cell loses heterozygosity for the normal Rb gene (a condition known as LOH loss of heterozygosity)” Thus, from these lines we interpret that loss of heterozygosity means loss of normal allele in mutant gene.