jpaatero’s is the kind of response that is both challenging and useful (which, unfortunately, is too rare today). Alexander Hoischen of Radboud University Medical Center in Nijmegen, the Netherlands, would agree with many of the comments.
Hoischen, who was part of a research project into post-zygotic mutations stated that:
The textbook knowledge that our genome is identical in all the cells of our body is probably not true
(see:
https://www.the-scientist.com/daily-new ... rare-35342 )
Post-zygotic mutations are an under-recognised source of genomic variation.
The reason why scientists assumed—and many still do—that normal cells have the same basic DNA sequences, relates to the predominance of observed similarities across most cell types. However, as sequencing techniques are becoming more widely used and as techniques are improving, an increasing number of anomalies are being found. Despite these advances, limitations of current technologies still hamper our understanding of the extent of these changes, but conventional wisdom suggests they are likely to be much more common than previously thought. How can these be accommodated in the definition of “personal identity”?
Where they occur, the repercussions of post-zygote stage development on the definition of personal identity require careful consideration. For instance, if a mutation occurs at an early stage in one stem cell whose descendants become a specialised organ, then the tissue of that organ will appear different to all other parts of the body with respect to that mutation. The term for this anomaly is a “mosaic mutation”. The appearance of such differences has even given rise to the suggestion that some of our cells carry different versions of our genomes. Others suggest that some people may carry different genomes at the same time (e.g. in the case of human chimerism resulting from fused zygotes). However, for the purposes of the present discussion, the genome is understood to mean the complete set of genetic material present in an organism, whatever its complexities. Therefore, all material—whether original, mutation, or even blended genotypes, as in the case of human chimeras—is included in the term.
We know that mutations are relatively low in the human body. Every mutation affects a minor part of the entire genome. In other words, “sameness” is retained in the bulk of our underlying genetic sequences across the whole organism, including those in the chromosomes and cells that contain the mutation. It may seem reasonable, therefore, in cases where post-zygotic mutations are found, to base “sameness” of the person on the fact that the vast majority of their genome remains the same.
But no matter how small the modification is—it
is a change and “sameness” is infringed. This makes sense. After all, if a tiny mutation gives rise to a gene that induces susceptibility to cancer or another malady, it is highly significant for the whole organism. So, even though the rates of change are relatively small, the implication that the genome is not the same at all times or in all circumstances in terms of its structural makeup is irrefutable in these instances. As mutations are part of the life of these genomes, DNA sequences—alone—cannot be used to define personal identity in all cases. For this reason, “sameness” must be qualified.
The question we must now answer is: How do we know that we are the same person if post-zygote stage mutations take place? In what way are we the same under these circumstances? It turns out that “sameness”—though not contained in all DNA sequences—is present in the individual’s genome, which conforms to a specific set of unchanging conditions. These include:
• Viability: the genome is able to survive as an entity;
• Continuity: development from its beginning to end-of-life is an uninterrupted series of events (in terms of life processes);
• Individuality: it is independent of all other genomes in the post-fertilised state (or after separation in the case of a monozygotic twin); and
• Uniqueness: it is unlike any other genome.
In other words, not only is the vast majority of the underlying genome retained, it remains viable at all times. Viability is an important quality in that it overcomes the suggestion that the chimera or mosaic genome is actually two or more people in one. A person cannot be divided up and remain viable. Continuity intimately links subsequent post-zygotic mutations to the same genome. A separate person is not created because mutations take place; they are part of the nature of that person’s genome. And individuality is maintained from the point of fertilization (or separation of monozygotic twins) until death. Furthermore, individuality is underlined by the uniqueness of the makeup of the genome itself.
Personal identity is defined as the
sameness of the individual at all times or in all circumstances; the condition or fact that the individual is itself and not something else. As sameness of the individual is shown to reside
at all times or in all circumstances in the nature of a person’s genome, this means that it is also the source of a succinct definition of personal identity.
Again, thanks to for the comments.
For more updates see Our Own Identity’s website
http://ourownidentity.com/
Resorting now to childish and personal insults. No surprises there.