Today, C. is protective of her father. “He tried to get her help,” she said. “He had reached out to my grandfather, my mom’s dad, and said: ‘Something’s wrong with Christy. Something’s changing.’ And he just brushed it off.” She is equally protective of her own privacy. (She mentioned — and several others in the family told me this — that two of her aunts lost their jobs after speaking openly about their family’s illness.) She is also charitable toward Christy. “I do remember her being a wonderful person, just fun and active,” she said. But those happier memories seem less accessible to C. now, overshadowed by everything that occurred after the disease took over.

During her teenage years, she watched from a distance as her aunt Susan handled a host of challenges. Christy owed the I.R.S. $10,000 in back taxes. Christy ballooned to 250 pounds, until Susan finally padlocked the refrigerator. Once, Christy bolted from the mall on a shopping trip and wandered five miles in the cold and rain to a Wendy’s, where the police were called and bought her dinner. Susan was in tears when she caught up with her, but Christy was fine — unfazed, even cheerful. During C.’s visits, she could see for herself her mother’s mysterious, almost random new personality. Once, in front of C.’s boyfriend, Christy asked C. whether she was sleeping with David Hasselhoff, the star of “Baywatch,” Christy’s favorite show at the time. Watching her mother become so unrecognizable was excruciating. But with Susan looking after Christy, C. was at least free to be a teenager, to go to school, to one day start a life of her own.

Once she was in her mid-20s, building a career, that might have been that — her mother’s tragic disease, a difficult childhood, a safe landing with her father. Then her family learned about FTD. While others, particularly her older relatives, lined up for genetic tests, she, like Barb, froze in place, deciding that she didn’t want to know. She wanted to give herself time. “I was just like, ‘If I find out I have this right now, I’m not going to have any motivation,’” she said. “ ‘I’m not going to have any desire to move forward.’”

She made a bargain with herself: She would be tested in five years, when she turned 30. For her, the decision to delay knowing felt less like denial than a play for personal agency, for control over something she had no control over. For those five years, C. worked hard not to think about the family’s condition — to move forward as if it wasn’t there. Pretending was even less possible for her than for Barb, when the example of her own mother was always present, directly in front of her, living with full-time care, losing her ability to speak, losing herself.

When C. turned 30, she had a boyfriend, a serious one, whom she told about the risk of FTD almost as soon as they started dating several years earlier. Now they were engaged. She went through with her plan to find out the truth. “I wanted him to have the choice to opt out if he didn’t want to deal with me,” she said.