fill in blank values based on other column values

This should be simple, however I cant find a solution

Input dataframe

id      Y/N
1       
2       
50      
6       
70      Y
100     Y

I would like to fill in blanks using condition: if id>49 and Y/N is null then Y/N = Y, else leave Y/N

so the output is

id      Y/N
1       
2       
50      Y
6       
70      Y
100     Y