Q represents a common relationship based on similarities. Therefore it has logical significance as follows:
Prediction: If X and Y share certain calculated similarities in their genetic codes, then they are of common descent.
Test: Compare the genomes of X and Y for predicted similarities.
Conclusion: X and Y do share the calculated similarities, therefore they are of common descent.
Corollary: If X and Y share common descent, it follows that they have a common ancestor, Q.
Here is another fallacy I didnt' mention: The attributes of a part cannot be applied to the whole.
For this to be the case, the statement would have to be 'X has Z attributes. X and Y are both part of Q. Therefore Y has Z attributes'. That is not what the statement says.
Notice the first part of the logical premise by evolutionistis: X is SIMILAR to Y in Z. Similar is not the same as IDENTICAL, so the fallacy of equovication also applies. Humans and giraffes have the same number of neck vertabrae - should we assume we share a common ancestor with them?
If this is a fallacy of equivocation, then what word was misconstrued? If you are claiming it is the use of the word 'identical' where 'similar' is meant, then I challenge you to show where I used the word 'identical'. And if you are saying 'similar' was used when 'identical' was meant, then allow me to clarify: Wherever I said 'similar' I MEANT 'similar'.
As for the giraffe, no we should not assume common ancestry based solely on the evidence you cite. But making such a claim after taking all of the evidence available into account is another story.
Here's another important point: The evolutionist is assuming that the existence of similarity in designs or processes is evidence AGAINST design, when in fact this type of evidence is perfectly consistent with a designer.
Scientifically, evolutionists don't care in the least if there was a designer or not. That of course isn't to say that there are no evolutionists out there that use evolution as a club to bash religion. Just that they are overstepping the realm of science when they do so.
The relationship does not appear in the equation so this is false. You can't just SAY there is a relationship - it has to be logically demonstrated.