Show only those options in a link field, which are linked to that Group

I have created doctype by the name of Group Linking, which has one data field(Item Group) and child table Description, which has a field description.

Now i have created a child table in other doctype
Both Group and Description are link field inside this child table.
Now what i want to do, when i select the Group for eg New, i want only New 1 and New 2 to display in description.