library(paramlink) 

y = read.table("markers.txt", header=TRUE) 
x = linkdat(y, model=1)

lod(x)

Result:

LOD:    NaN

Data:

FAMID   ID  FID MID SEX AFF M_A1 M_A2
1   1   0   0   1   1   2   3
1   2   0   0   2   1   1   2
1   3   1   2   2   2   2   2
1   4   1   2   1   2   2   3
1   5   1   2   2   1   1   2
1   6   1   2   2   1   1   2
1   7   1   2   2   1   1   2
1   8   1   2   1   1   1   2