When parents raise their own children they are augmenting the effects of the genetics which the child has inherited. This is because the parent also shares a significant portion of these same genes, and in the course of parenting inevitably acts with respect to these genes to a large degree. Since these same genes tend to be shared, more or less, by the child himself, this means that the effects of these genes not only occurs within the child's own DNA, which is to say internally, but also externally through the environment, since the "mature" effects of these genes are tending to be reflected onto the child in the course of the influence which the child experiences by his parents.
This might lead to beneficial or detrimental consequences, depending upon a whole host of factors. Yet it remains true that if a child is raised by parents who do not share his own genetic tendencies then the child's genetic influence is not reflected and augmented in this manner, at least certainly not to the same degree as were the child to be reared by biological parents -- having non-biological parents would "dampen" this effect, causing a chance for less overt genetic influences to gain more possibility of traction in influencing the individual, more chance (less resistance) for subtler and less powerful genetic influence to potentiate. This means there would be more diversity of potential genetic influences. On the other hand, having biological parents tends to supplement genetic predispositions with learned behavioral traits as a consequence of absorbed environmental influences, leading to a decrease in this sort of diversity of potential genetic influence. As mentioned above, it is easy to come up with scenarios where either approach would be beneficial or harmful. But from a standpoint of concern purely for issues of diversity of potential conditioning influences which the child is exposed to, which translates to less strong and overwhelming particular influences at the expense of potentially competing influences, being reared by non-biological parents would be superior. And conversely, of course, if one is concerned more with exposure to several particularly strong influencing tendencies rather than a diversity of many more or less "equal" and competing influences, being reared by biological parents tends to be superior.
“Be clever, Ariadne! ...
You have little ears; you have my ears:
Put a clever word in them! —
Must one not first hate oneself, in order to love oneself? ...
I am your labyrinth ...”. -N
“A man is not great if he is not small, and he is not small if he is not great. Concepts flirt with the loss of their significance in the oscillation between ambiguous states, and this is in part the function and purpose of concepts.” -Primer on Meaning