def recode_missing(geno_df):
    """Recode missing data from '0' to 'NA'. Plink codes missing genotypes
    using '0', but Rqtl2 does not allow '0' character to represent missing
    genotypes because in R, fread() considers '0' as type boolean."""
    for snp_col in geno_df.columns:
        geno_df[snp_col] = geno_df[snp_col].replace('00', 'NA')
    return(geno_df)