col 1: pedigree ID
col 2: person ID
col 3: father ID
col 4: mother ID
col 5: gender (1 for male, 2 for female)
col 6: affection status (0 for unknown, 1 for unaffected, 2 for affected)

the next column is optional (be careful! since you don't
know whether you have this column or not until you check
the LINKAGE parameter file): the liability class this person
belongs to.

the rest are the marker information. note that one marker
contributes two columns, for both parents, although it
is not clear which one is from father and which is from 
mother.  the coding of the marker allele values is
arbitrary as long as it is consistent.