When comparing the results of Y-chromosome STR (short tandem repeat) tests, the difference between the two values at a marker is the Y-DNA genetic distance or the step-mutation count. That is, a one-step mutation means that the count for a single STR has changed by one. A two-step mutation means that the count for a single STR has changed by two.

For example, when comparing kit B193 and kit B173, the STR marker DYS393 has changed from 13 to 12. The difference between 13 and 12 is 1 {13-12= 1}. This is considered a single or one-step mutation.

Example: One-Step Mutation
Kit Surname Hg D
Y
S
3
9
3
D
Y
S
3
9
0
D
Y
S
1
9
D
Y
S
3
9
1
D
Y
S
3
8
5
a
D
Y
S
3
8
5
b
D
Y
S
4
2
6
D
Y
S
3
8
8
D
Y
S
4
3
9
D
Y
S
3
8
9
|
1
D
Y
S
3
9
2
D
Y
S
3
8
9
|
2
B193 Sypline I1 12 24 14 10 14 14 11 14 11 12 11 28
B173 Sypline I1 13 24 14 10 14 14 11 14 11 12 11 28
B132 Sypline I1 13 24 14 8 14 14 11 14 11 12 11 28

In this example, kit B193 has a DYS391 value of 10 and kit B132’s value is 8. The difference between 10 and 8 is 2 {10-8= 2}. This is then considered a two-step mutation.

Example: Two-Step Mutation
Kit Surname Hg D
Y
S
3
9
3
D
Y
S
3
9
0
D
Y
S
1
9
D
Y
S
3
9
1
D
Y
S
3
8
5
a
D
Y
S
3
8
5
b
D
Y
S
4
2
6
D
Y
S
3
8
8
D
Y
S
4
3
9
D
Y
S
3
8
9
|
1
D
Y
S
3
9
2
D
Y
S
3
8
9
|
2
B193 Sypline I1 12 24 14 10 14 14 11 14 11 12 11 28
B173 Sypline I1 13 24 14 10 14 14 11 14 11 12 11 28
B132 Sypline I1 13 24 14 8 14 14 11 14 11 12 11 28