Assertion: The ADA gene gives instruction for producing the enzyme adenosine deaminases.
Reason: Throughout the body this enzyme is found but is most active in lymphocytes.

Both Assertion (A) and Reason (R) are correct  but R is not the correct explanation of A.

The ADA gene gives instructions for producing the enzyme adenosine deaminase. In all cells this enzyme is produced, but the highest levels of adenosine deaminase takes place in immune system cells called lymphocytes, that develop in lymphoid tissues. Lymphocytes form the immune system, that defends the body against potentially harmful invaders, such as viruses or bacteria.