Mr. DeMoe turned back from the white board in classroom No. 7 at Palm Lane Elementary School in Phoenix. It was a Wednesday morning and the subject was fractions.

“Are we done?” Mr. DeMoe asked.

“Nooooo,” the fifth-graders said unison.

“What do we do?” he asked. A pause. “It starts with an ‘s.’ ” He waited, and then called on a boy in the front row. “We simplify it,” the boy said.

“It’s a lot of work for just one problem,” Mr. DeMoe said. The students nodded. One boy groaned.

He capped his marker and asked the kids to put their thumbs up if they were comfortable enough to do the next problems themselves, sideways if they were unsure, or down if they were completely lost.

“Be brave,” Mr. DeMoe said. “If I see your thumb down, I won’t be mad.” When he was their age, he had trouble with fractions, too.

Five students put their thumbs down. He gestured for them to follow him to a half-circle table on one side of the classroom, while the rest pulled out worksheets and pencils.

On the wall over the water fountain was a poster that read, “Life is full of choices. Choose carefully.” As students worked through the problems, a girl who had insisted she knew fractions slid into an open seat at the table with Mr. DeMoe. She needed help after all.

Just before 10 a.m., the students got up out of their seats and put their worksheets away. Mr. DeMoe looked up, surprised. “What’s going on?” he asked.

“We’ve got art,” Eddie Lierman told him. He pointed to the digital clock on the wall. “Did you forget?”

No, Mr. DeMoe didn’t forget. He just got caught up in fractions. But he is worried about the day that he might forget that his students go to art on Wednesday, or that they go to art at all.

Or what art is.

Or Wednesdays.

A family secret

Tyler DeMoe is 27, too young, you’d think, to worry about forgetting things. He’s just hitting his stride in adulthood, working at his dream job in a new city.

But he is not like other 27-year-old teachers. His family has a devastatingly high incidence of a rare form of early-onset Alzheimer’s disease called autosomal dominant Alzheimer’s disease, or ADAD.

His great-grandmother and his grandfather had it. So do his father and four of his father’s five siblings. Only one sibling of the six, Tyler’s aunt Karla DeMoe Hornstein, remains unaffected. It means her two children are in the clear. But the 11 children of the other siblings have a 50 percent chance of developing ADAD.

The disease is caused by a gene mutation. If you have the faulty gene, you will develop the disease. It’s a sure thing.

So far, one of Tyler’s 11 cousins has tested positive for the gene mutation. Two others have tested negative. The others have chosen not to know or are too young to be tested.

If Tyler has the gene mutation, the first symptoms of the debilitating disease could surface in his late 30s and grow worse in his 40s and 50s.

None of his family members with it has lived past their late 50s.

Tyler doesn’t know his status. It’s like a secret he can’t tell, even to himself. He was tested for the genetic mutation at age 21, as part of a study at the University of Pittsburgh. Only a couple of doctors know the result. Tyler never asked to know. But he thinks he may want to find out now.

Or maybe not.

Because it wouldn’t change how he’s living his life.

‘He looks out for me’

Mr. DeMoe’s class made two lines in the hallway. The last student to leave the classroom turned off the light. Mr. DeMoe waited until they were quiet. And waited.

This is a tough class. Mr. DeMoe took over for another teacher when he was hired in January, after he graduated from the University of North Dakota.

He had done his student teaching here last year. He had asked to teach in an inner-city school.

Mr. DeMoe had grown up in Thompson, a rural town of 1,000 or so people in North Dakota. He attended the same school with the same kids from kindergarten through 12th grade, the classes no bigger than 20 or so kids. Everyone was pretty much the same.

At Palm Lane, Mr. DeMoe has 30 students in his class — an extra four when another fifth-grade teacher called in sick once with pink eye. Palm Lane students are a mix of ethnicity and backgrounds, though mostly Hispanic and from low-income families.

“For my first teaching job, it’s been challenging, but I love it,” Mr. DeMoe said. He grinned. “I like a challenge.”

The students follow him across campus to the art room. Eddie was last in line, an orange ball with his name written on it in black marker under one arm. Another boy tried to take it from him but quit when he saw Mr. DeMoe looking.

Mr. DeMoe doesn’t put up with bullying, Eddie said. “He looks out for me.”

‘A lot of things that are worse’

Mr. DeMoe watched until the art room door closed behind Eddie, and then we walked back to classroom No. 7, empty now and quiet. Tyler pulled out a hardback copy of a new book called “The Inheritance” by Niki Kapsambelis, which includes the story of his family.

He had read it twice, quickly the first time, in just two nights, and more slowly the second. His family has been regularly featured in news articles, television shows and medical journals.

Sometimes it seems unreal, he said, flipping through the pages. Other times, it is all too real.

The book also features local researchers from Banner Alzheimer’s Institute in Phoenix, who are conducting research among the world’s largest autosomal dominant Alzheimer’s kindred in Medellin, Colombia.

“It is such a devastating story,” said Pierre Tariot, a geriatric psychiatrist and director of the Banner Alzheimer’s Institute. But it’s rare. Maybe 5,000 people are affected worldwide.

Tyler’s father, Dean DeMoe, lives in Tioga, still works for an oil company. His mother, Deb, works as a substitute teacher in Thompson.

In the spring of 2005, after his dad’s older siblings had been diagnosed with the disease, Dean DeMoe got tested and learned he, too, had the gene mutation. He was just 43. On the way out of the hospital that day, Tyler’s parents took a detour and walked through the pediatric cancer ward.

“I’ve lived 40-some years,” Dean said to his wife and later his children. “There are a lot of things that are worse.”

Tyler was 15, still a kid. “At that age, I was, like, ‘What in the world?’ ” His dad seemed fine. Alzheimer’s was something old people had. “I couldn’t understand it.”

They know what’s coming

Since then, he has learned more about the disease than he ever wished he knew. Tyler’s dad is 54 now. The symptoms are more noticeable. He misplaces his phone and his wallet. He asks the same questions again and again. He fumbles with the tiny buttons on his dress shirt.

“We know it’s happening, but we don’t really address it,” Tyler said. They talk about work and sports. Until a handful of years ago, Tyler said neither he nor his dad were much for the mushy stuff over the phone.“Now every time we talk, we are sure to say, ‘I love you,’ ” Tyler said.

He probably spends more time with his family than most people his age. He is particularly close to his two sisters (one older and one younger) and his cousins.

Last week, Tyler interviewed for a job as a second-grade teacher in Tioga. If he gets the job, he would like to live with his dad for the next year or two. He wants time with him, to watch sports, do yard work, drink beer and go fishing. It doesn’t matter that it would mean living in the middle of nowhere. “He has always been my best friend,” Tyler said. He is the kind of dad that all your friends like.

Because both men know what is coming.

Tyler and his dad each visited his Uncle Brian, his dad’s brother, in the nursing home. Brian DeMoe was a husband, a father of two, an oilman and NASCAR fan. He was 54 in 2010 when he became the first sibling to die from the disease.

Tyler sat with his mom and cousins at the bedside of his Aunt Lori in her final days in the nursing home last year. He was there when she died at age 56, a wife, mother and grandmother.

Tyler’s Uncle Doug is in that same nursing home. After visiting his brother, Tyler’s dad turned to his son. “That where I could be in a few years.”

His Uncle Jamie, the youngest at 45, has tested positive but is not showing symptoms yet. He has a 9-year-old daughter with his longtime girlfriend.

Soon Tyler’s father, like his siblings before him, won’t remember directions, or where he is, or the faces of his family. He’ll forget how to shave, and feed himself, and to walk. He’ll forget how to swallow and eventually even how to breathe.

“You watch that person slip away and the person who is left isn’t really them anymore,” Tyler said.

He wonders what that would be like. To be the person who slips away.

Would knowing make it worse?

For other people, the question of whether or not to open their test results, to find out if they have the gene mutation seems simple. Of course they would want to know.

Tyler struggles with the decision.

“Do I find out and know I have it and try to accomplish everything I can?” he asked. “Or, do you live your life just normally? Do you just not think about this?” He paused and then smiled. “Well, you really can’t,” he said.

“Some weeks I don’t even think about it. Some weeks it’s all I think about.”

In reality, fewer than half of people at risk don’t want to know, said Tariot of the Banner Alzheimer’s Institute. “It’s a very, very personal decision.” Some don’t want to know their time will be cut short. Others want to plan for that exact inevitability. In his study in Colombia, the standard response is not to know. Tariot thinks that will change when prevention therapies become available.

Tyler’s sister McKenna is 21 and still in college. His sister Lindsey is 30 and newly married. Like Tyler, his sisters don’t know if they have the gene mutation. Tyler said Lindsey may want to know before she decides whether to have children. Tyler would consider finding out if she does.

But if one sibling finds out, it has ramifications for the others. If Tyler and Lindsey were negative, would McKenna worry the odds were against her? If they were positive, would she have to worry their care might eventually fall to her?

Their parents don’t want to know if their children test positive. Tyler’s dad once told him if they had known what would happen, they may not have had children. The guilt is overwhelming.

So, for now, Tyler lives his life as if he does have it. Somehow, that way, he won’t be disappointed if he finds out it is true.

“Living like I do have it, I think it makes me live in the moment more. I worry less. I do what I need to do — I work, I pay my bills. In a weird way, it makes me appreciate life more,” he said.

It does make him hesitant to date. “I just don’t. When it comes to having to talk about this, I just find it hard to bring it up. When do you tell someone that? The first date, or the third? Do you wait until the second month?”

He shrugged. “I guess you just hope that someone comes along that totally gets it all.”

Because as hard as it has been, Tyler decided this: If he knew, one way or the other, it wouldn’t change what he is doing now. He would travel. Spend time with his family. He would teach.

“I live my life every day and try to be a positive influence,” he said. He looked around his classroom. He loves teaching. “I try to do what I can to make people remember me.”

‘What else are you going to do?’

Mr. DeMoe kept an eye on the digital clock on his classroom wall. He has to remember to pick up his students from art for lunch at 10:45 a.m. Some days, if he forgets how to get somewhere or can’t find his keys, he can’t help but wonder if it’s the beginning of something, even though it would be too early, even if he had the gene mutation.

A class of third graders was waiting quietly outside the art room when Mr. DeMoe’s fifth-graders come out, talking and laughing.

The other teacher patted Mr. DeMoe on the back. She asked, “How many more days?”

“Forty-eight,” he said. No hesitation. They both laughed.

In 48 days, (fewer now), school lets out for the summer. Mr. DeMoe will return to the University of Pittsburgh for his annual testing: cognitive tests, brain scans and a spinal tap. Many of his family members also participate in studies.

“It kind of beats the hell out of us, honestly, but they can do whatever they want to me as long as it is going to help,” he said. “We’re just hoping they can discover something from our family that can help lead to a cure or treatment.” Even if it doesn’t happen in time for the people he loves, or for him.

“What else are you going to do?” he said. “You can either contribute or do nothing. To me, contributing seems like a better idea.” Maybe he will be remembered for that.

Pittsburgh is also the place where his secret is kept.

On the walk to take his students to lunch, Mr. DeMoe turned his face up to the sun. If he gets the job in Tioga, he would miss Phoenix, particularly the weather. It was 42 degrees in North Dakota last week. He’d like to come back.

Mr. DeMoe hopes he made an impact on his students in the time he was here. He likes to imagine that when they’re older, 17 or 18, they will look back and say, “Oh, that Mr. DeMoe taught us about this.”

Maybe they’ll think back to the poster, the one about choosing carefully in life. More importantly, he hopes his students remember something that he told them again and again: “Anything, anything is possible.”

At the back of the line, Eddie tosses his orange ball into the air. On Mr. DeMoe’s first day of teaching, he had noticed that Eddie was sitting in the back of the room, his desk facing the wall. He moved the boy to the front of the class and told the other boys to be nice to him.

Eddie grinned at his teacher. “I’m never going to forget Mr. DeMoe.”

Reach Bland at [email protected] or 602-444-8614. Read more here.

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