There are six columns in a bim file
fid: family ID;
iid: individual ID, default row name used by [readBED]
;
mom: maternal ID;
dad: paternal ID;
sex: individual sex.
phe: phenotype, not often used;
The PLINK1 bim file has no header line, this is changed in PLINK2.
The columns "sex" and "phe" are mostly the legency of early GWAS, nowerdays
it is common to provide sex, among other covariates, and multiple phenotypes
in a separate file.