Final answer:
The primary difference in the case studies of Isabelle and that of Genie is the nature of their upbringing and the impact it had on their language development abilities.
Step-by-step explanation:
The primary difference in the case studies of Isabelle and that of Genie is the nature of their upbringing and the impact it had on their language development abilities. Genie, who was raised in extreme neglect and abuse, was unable to speak until the age of 13.
Her language acquisition abilities were significantly delayed compared to typical development. On the other hand, Isabelle, who was discovered at the age of 6 after being isolated in a dark and silent room, quickly caught up in language development once she was exposed to a supportive environment.