Read by format statement in Line 5. Population codes must be consecutive integers. Null alleles occur when there is no detectable enzyme activity at a locus. The program first defines all ETs including nulls and then pools the ETs that differ only by nulls. However, in this case, you are assuming, as with other electromorphs, that representative of an allele are identical by descent.
Distance is measured as the proportion of mismatched loci between pairs of ETs.
Null alleles that are scored as '0' are not used in the calculation of pairwise distances. I do not use this program anymore. It calculates genetic distance between pairs of ETs and writes a file in the MEGA input format. Note that MEGA does not except blanks spaces within the strain labels, so replace these blank spaces with some other symbol. For information and references about these measures, see Whittam et al.