He doesn't use genes to deny personhood or to define it. The human genes are the only non-subjective means to verify human life. If it is alive and has a full set of genes making it a human life then it would easily be defined as both human and alive. Attempting to make a subjective declaration of "personhood" is what he is stating is objectionable. Pointing out that such subjective means only can serve to minimize the importance of a group of human life, and it is based on a subjective and not empirical measure should have made this clear to you.
I guess stating that using "personhood" is arbitrary and can be used to take rights from a group (Dredd Scott) isn't clear enough to you.
Ah, yes, the obligatory Dred Scott reference. What took so long? I thought it would follow shortly after SF's post.