Things become slightly more complicated than this because organisms contain two (or more) copies of each gene,  known as the two alleles.  So, if a sample is taken from a patient and amplified using PCR both copies of the region of DNA (alleles)  of interest are amplified.  So if we are looking for mutation there are now three possibilities:
# Neither allele contains a mutation
# One or other allele contains a mutation
# Both alleles contain a mutation.
These three scenarios are known as "Wild–type", "Heterozygote" or "Homozygote" respectively.  Each gives a melt curve that is slightly different.  With a high quality HRM assay it is possible to distinguish between all three of these scenarios.
Homozygous allelic variants may be characterised by a temperature shift on the resulting melt curve produced by HRM analysis. In comparison, heterozygotes are characterised by changes in melt curve shape. This is due to base-pair mismatching generated as a result of destabilised heteroduplex annealing between wild-type and variant strands. These differences can be easily seen on the resulting melt curve and the melt profile differences between the different genotypes can be amplified visually via generating a difference curve