Possession is a specialization some telepaths use to transfer their mind to the victim's brain, allowing the telepath to use the victim's sensory perception, or even dominate their mind. The DNA Alternate accomplishes this by impressing their own synaptic activity on the victim's anterior cingulate cortex, effectively replicating their mind within the victim's brain. Dependent entirely on the potential and control of the Alternate, this possession could range from feeling everything going through the victim's mind, to actively taking control of the body as though it were their own.
The level of control the Alternate is able to exert on the victim is dependent on the will of the victim. Any victim can fight the possession, often futilely, but strong-willed victims may be able to limit the degree to which the Alternate can control them. Telepathic victims can project static to block the possession if they are aware of the attempt. If caught unawares, they are completely possessed. If they have time to prepare but not time to project static, they may put up walls around their memories, or their abilities, to prevent the Alternate from accessing them.