SNP with possible null alleles. The possible extra peaks observed between the simplex and duplex, and between the duplex and triplex peak may indicate the presence of null alleles for the SNP; this is not detected by the software. For a general explanation see Figure 1.
Voorrips et al. BMC Bioinformatics 2011 12:172 doi:10.1186/1471-2105-12-172