Is it possible for an individual with dissociative personality disorder, with two distinct personalities, for the second personality (the 'alter') to take over and become the primary personality? More specifically, can the individual's second personality delete or suppress the individual's primary personality, therefore becoming the individual's sole personality?
I think this question is asking about the movie version of multiple personality disorder, rather than the real dissociative identity disorder (DID) as it presents clinically. As-is, this question is not meaningful, but it is worth answering, if only to address the confusion over what DID is really like.
Alters and Behaviours
All humans have "personas". For example, any person may behave like a different person when they are intoxicated, stressed, or in love; they may present different personalities at parties, at work, and around family; they may demonstrate different skills at sports, games, and hobbies; all people mature over time. It is not really meaningful to say that a person's "adult" persona took over their "child" persona when they grew up; or that a "recovered" alcoholic suppressed their "intoxicated" persona when they permanently sobered up; or that someone's "daughter" persona was deleted when her parents died.
Dissociative alters differ from such personas in a few ways. For one thing, they are not socially appropriate in the way that putting on a "game face" might be in a competitive context, and are often problematic to their hosts as a result. Alters can also report quite distinct identities, including having different names, personal histories, perceived memories, personalities, skills, and even differing in age, gender, race, and other characteristics. Nonetheless, much like common personas, they share far more with their host than they are distinct. As such, it is not meaningful to suggest that some alter took over, suppressed, or deleted their host.
Alters and Cognitions
Personas are not merely behavioural - they are cognitive as well. For example, people may hear the voice of their parent when slacking off at school or work; they may ruminate on exceptionally negative thoughts when depressed; they may rationalize impulsive behaviours in one moment, and deliberate long-term consequences the next; they routinely compartmentalize beliefs, preferences, and memories - remembering certain events in one context, and different ones in another.
Again, dissociative alters differ in that voices, thoughts, feelings, and compartmentalizations are not context appropriate, are attributed their own identity, and vary in the ways that information is shared from one context to the next. However, research into cognitions of DID patients demonstrates that contrary to subjective experience, alters do in fact share memories (Huntjens, Verschuere, & McNally, 2012; Marsh et al, 2021; Kong, Allen, & Glisk, 2008; Marsh et al, 2018), and while alters do differ from each other cognitively, they are more similar to each other than to healthy controls, in attentional bias (Hermans et al, 2006), memory speficity (Huntjens et al, 2014), and neural activation patterns (Reinders et al, 2012). Alters share motor and perceptual skills, semantic memory, and language and vocabulary (Maiese, 2016). Overall, alters have more in common with each other cognitively than they are different, as would be expected, given that they share the same physical body and brain.
Integration and Fusion
A common misconception about alters is that how they present - as different persons - must be how they are. Indeed, during therapy, psychiatrists may treat different alters as separate individuals, as it is natural for anyone to do given how they behave. However, researchers do not view alters as actually distinct entities or separate individuals. As explained above, they are more like dispositions, states, or personas of a single individual, that have certain maladaptive traits. They most certainly share more than they segregate, including the stream of consciousness.
Accordingly, alters are not "deleted" during treatment. Rather, the process of integration or fusion is a common progression in treatment, involving the merger of alters, such that their separate memories, skills, personas, and cognitions become accessible to the host in the same way that they are readily accessible to healthy individuals.