Customise drop down list validation

How would I go about entering a second data validation drop down cell where
the values to be selected depended on the selection made in the first drop
down list validation?

In other words the list or choices to select in the second cell are different
each time depending on the value selected in the first box

Eg

first cell drop down list is 1 2 3

First
entry cell
1 - second cell allow A B C
2- - second cell allow D F G
3 - second cell allow Q A Z

Am I missing some simple lookup solution?

many thanks

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