nested If condition to match two halves of cells

hi to all,

I am working on some data there I would like to use nested if condition to solve my problem. This is how my table looks like
SX0001A SX0002A SX362 SX0001A SX0002A SX362
P1 C/C C/C A/A
P2 A/A G/G T/T
1 C/C -/- A/A 0 0 0
2 -/- C/C A/A 0 0 0
3 C/A G/G T/A 2 1 2
4 A/A C/G T/T 1 2 1

in E4 cell I would like to give score 1 if B4 cell matches exactly with B3 cell
in E4 cell I would like to give score 0 if B4 cell does not match with B3 cell
in E4 cell I would like to give score 2 if B4 cell have half of B2 and half of B3 i.e. C/A or A/C
I tried nested if condition to solve it but I am getting problem in matching two halves of to cells to score. I am requesting you to help me. I am attaching sample file for better understanding of my problem.
Thanks in advance
Regards