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Genetic Science Learning Center
Deafness
Educator's Pages - Map a Gene Activity
 

The key to this problem is:

  • The correct cell must have the SAME LETTER in all AFFECTED individuals. 
  • The correct cell must have a DIFFERENT LETTER (than affected) in all UNAFFECTED individuals. 

Only one cell fits these criteria: #14. 

See for yourself:

All affected individuals (1, 3, 6, and 8) must have the deafness cell. Your first clue is the letters of the cells. Which letters do all affected individuals share? A. Here's another way to think about it: since Individual #1 on the pedigree is affected and contains only one letter, A, then you know that the cell representing the deafness gene must contain A as well.

TABLE 1
1 - A 2 - A 3 - A 4 - A 5 - A
6 - A 7 - A 8 - A 9 - A 10 - A
11 - A 12 - A 13 - A 14 - A 15 - A
16 - A 17 - A 18 - A 19 - A 20 - A
21 - A 22 - A 23 - A 24 - A 25 - A

TABLE 2

1 - B 2 - B 3 - B 4 - B 5 - B
6 - B 7 - B 8 - B 9 - B 10 - B
11 - B 12 - B 13 - B 14 - B 15 - B
16 - B 17 - B 18 - B 19 - B 20 - B
21 - B 22 - B 23 - B 24 - B 25 - B

TABLE 3

1 - C 2 - A 3 - C 4 - A 5 - A
6 - A 7 - C 8 - A 9 - C 10 - C
11 - A 12 - C 13 - C 14 - A 15 - C
16 - C 17 - A 18 - A 19 - A 20 - A
21 - C 22 - A 23 - C 24 - C 25 - A

TABLE 4

1 - B 2 - D 3 - A 4 - D 5 - B
6 - D 7 - A 8 - D 9 - B 10 - D
11 - D 12 - C 13 - D 14 - D 15 - B
16 - C 17 - D 18 - D 19 - C 20 - A
21 - D 22 - B 23 - B 24 - D 25 - D

TABLE 5

1 - E 2 - E 3 - E 4 - E 5 - E
6 - E 7 - E 8 - E 9 - E 10 - E
11 - E 12 - E 13 - E 14 - E 15 - E
16 - E 17 - E 18 - E 19 - E 20 - E
21 - E 22 - E 23 - E 24 - E 25 - E

TABLE 6

1 - E 2 - C 3 - E 4 - E 5 - E
6 - A 7 - E 8 - E 9 - C 10 - E
11 - E 12 - C 13 - A 14 - A 15 - A
16 - C 17 - E 18 - E 19 - A 20 - E
21 - E 22 - E 23 - C 24 - E 25 - C

TABLE 7

1 - F 2 - F 3 - F 4 - F 5 - F
6 - F 7 - F 8 - F 9 - F 10 - F
11 - F 12 - F 13 - F 14 - F 15 - F
16 - F 17 - F 18 - F 19 - F 20 - F
21 - F 22 - F 23 - F 24 - F 25 - F

TABLE 8

1 - G 2 - C 3 - E 4 - G 5 - G
6 - G 7 - E 8 - G 9 - C 10 - E
11 - E 12 - G 13 - G 14 - A 15 - G
16 - G 17 - E 18 - G 19 - G 20 - E
21 - A 22 - C 23 - G 24 - E 25 - C

There are lots of cells with A, though. How do you know which one contains the deafness gene? To narrow your choices, find the tables of the affected individuals' (1, 3, 6, 8) that have the fewest A cells. Table 6 has 5 A cells and 8 has 2 A cells.

Which A cells do tables 6 and 8 share? Only one cell fits the bill: #14.

Now, check your work:

Do all AFFECTED individuals (1, 3, 6, 8) have an A in cell #14? YES.
Do any UNAFFECTED individuals (2, 4, 5, 7) have an A in cell #14? NO. 

Congratulations! You discovered the deafness gene!

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