This is one of those examples of similarities and so-called patterns that are falsely assumed to prove common descent. The fact that different species have similar genes does not show that the species are related by ancestry or descent.
Turn this around into an "if/then" framework.
If different species are related by ancestry or descent, would you then expect them to have similar genes? Why or why not?
For what other reasons might you expect them to have similar genes?
Now I will grant you that in the first instance finding similar genes in different species does not tell us whether they are there for reasons of relatedness or for some other reason.
But it doesn't rule out relatedness as a reason either. Relatedness is at least one reason why similar genes are in similar species, so it has to be considered unless there is a reason to rule it out.
It isn't reasonable to assume that genetic similarities could only have resulted from common ancestry,because different species with many common traits could have come into being separately.
For what reason, other than relatedness could that happen?
Given that all genetic material is composed of the same materials that come from the earth - amino acids,proteins - it stands to reason that there should be genetic similarities between species in accordance with the similarities in structure.
Underlying all structures are the genes that build them. Structures can act as a proxy for genes, but at bottom it is all genes.
To focus on genetic similarities just pushes back the question of structural similarities to the microscopic level,but the genetic evidence does not demonstrate reproductive connections any more than the structural evidence.
True, but when we are looking for an explanation for the similarities, reproductive connections can't be ruled out without sufficient reason. What would be a sufficient reason to rule them out?
Also, consider, that it is not just similarities that lead to the conclusion of common ancestry, but also differences and above all the pattern of similarities and differences. Differences are just as important in establishing relationships as similarities are and are often key in deciding which way a lineage is branching.
Descent has to do with the reproductive connections between organisms,not just the inheritance of genetic material,
Can there be any inheritance of genetic material without reproduction?
and you can't prove reproductive connections if you can't show that there is or was reproductive compatibility.
Prove, no. Find evidence, yes.