map <- data.frame(chr=c(1,1,1, 2,2,2, "X","X"),
pos=c(0,5,10, 0,8,16, 5,20),
marker=c("D1M1","D1M2","D1M3", "D2M1","D2M2","D2M3", "DXM1","DXM2"))
map_list <- map_df_to_list(map, pos_column="pos")
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