awk approach:
awk 'BEGIN{OFS="\t"; print "pos" OFS "COL1"}NR>1{for(i=2;i<=NF;i++) print $1,$i}' real2.txt
The output:
pos COL1
18691441 C
18691441 A
18691441 G
18691572 G
18691572 C
18691572 G
18691620 A
18691620 T
18691620 G
18691716 C
18691716 G
18691716 C
OFS="\t" - output field separator
print "pos" OFS "COL1" - prints header line
NR>1 - start processing from 2nd line
for(i=2;i<=NF;i++) print $1,$i - printing each column (COL...) value "rowwise" regarding to respective pos column value