How likely is this mutation? In my previous bioinformatics challenge, I asked you to mutate a DNA sequence. This time, I'd like you to evaluate how likely a mutation, or a series of mutations, is.
The two types of substitutions are transitions and transversions, and due to the chemical structure of DNA bases, transitions are more likely to occur than transversions. A transition is when a base is turned into one of the same size (purine -&gt; purine or pyrimidine -&gt; pyrimidine), and a transversion involves two bases of different sizes (purine &lt;-&gt; pyrimidine). Following Kimura's model, postulated in 1980, we can define a as the probability of a transition occurring for each unit of time, and b as the probability of a transversion occurring.
See diagram below. A and G are purines, C and T are pyrimidines.

Although the exact values of a and b change from organism to organism (and even between different areas in an organism's genome), we can set a=0.25 and b=0.1 for this challenge.
Given two DNA strings of same length as input, I would like you to calculate how likely it would be for string B to be a mutated version of string A. This is code golf, so fewest bytes wins!
Test cases:
Input String A | Input string B | Output probability
tgcctatc       | tgcctata       |  0.1
aggttcctt      | gggttcctt      |  0.25
aactgg         | aaccgg         |  0.25
atgccct        | atcgcct        |  0.01
tatcactaag     | tgtcaatgag     |  0.00625
ctcctgggca     | cttttgcgca     |  0.00625
ctgtgtgct      | cagagagca      |  0.0001
atcgctttca     | ttggctttca     |  0.01
attcac         | taagcg         |  0.000001
attttcattg     | attttttacg     |  0.000625

Specifications:

The input DNA strings can be as strings of characters, binary or numerical.
We will assume that all of the mutations are independent events.
The output can be in plain notation (0.00015) or scientific (\$1.5\times10^{-4}\$).

If you have any questions or require more specifications, send them my way in the comments!
Jelly, 9 bytes
^ị“½¥¢‘İP

Try it online!
Takes input as 2376 instead of acgt respectively. The Footer does this for you
-1 byte thanks to Jonathan Allan!
How it works
^ị“½¥¢‘İP - Main link. Takes the first input on the left and the second on the right
^         - XOR the corresponding elements
  “½¥¢‘   - Yield [10, 4, 1]
 ị        - Index into this array, 1-indexed and modularly
       İ  - Take the inverse of each
        P - Take the product

Why this works




Characters
As integers
XOR
1-index into [10,4,1]
Inverse




aa
2, 2
\$0\$
\$1\$
\$1\$


cc
3, 3
\$0\$
\$1\$
\$1\$


gg
7, 7
\$0\$
\$1\$
\$1\$


tt
6, 6
\$0\$
\$1\$
\$1\$


ac
2, 3
\$1\$
\$10\$
\$0.1\$


ag
2, 7
\$5\$
\$4\$
\$0.25\$


at
2, 6
\$4\$
\$10\$
\$0.1\$


cg
3, 7
\$4\$
\$10\$
\$0.1\$


ct
3, 6
\$5\$
\$4\$
\$0.25\$


gt
7, 6
\$1\$
\$10\$
\$0.1\$