The key to this problem is:

• The correct puzzle piece must be PRESENT in all AFFECTED individuals. 
• The correct puzzle piece must be ABSENT from all UNAFFECTED individuals. 

Only one puzzle piece fits these criteria: #14. 

See for yourself:

All affected individuals (1, 3, 6, and 8) must have the deafness puzzle piece. Your first clue is color. Which color puzzle pieces do all affected individuals share? PURPLE. Here's another way to think about it: since Individual #1 on the pedigree is affected and contains only one color, PURPLE, then you know that the puzzle piece representing the deafness gene must be PURPLE as well.

1	2
3	4
5	6
7	8

There are lots of purple puzzle pieces, though. How do you know which one contains the deafness gene? To narrow your choices, find the puzzles of the affected individuals' (1, 3, 6, 8) that have the fewest purple pieces. Puzzle 6 has 5 purple pieces and 8 has 2 purple pieces.

Which purple pieces do puzzles 6 and 8 share? Only one piece fits the bill: #14.

Now, check your work:

Do all AFFECTED individuals (1, 3, 6, 8) have a purple #14 puzzle piece? YES.
Do any UNAFFECTED individuals (2, 4, 5, 7) have a purple #14 puzzle piece? NO. 

Congratulations! You discovered the deafness gene!

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