Part Eight: The Question

For several seconds, neither of us could speak.

Are you absolutely certain the two of you aren’t biologically related?

I looked at Emily.

She looked at me.

Same nose.

Same stubborn chin.

Are you sure the hospital didn’t mix them up?

“That’s—” Emily started. “That’s impossible. We’re not related. We met when I was six. Over a dead goldfish.”

“Our parents are completely different people,” I said. “My dad was Swedish. From Minnesota. Her dad’s family is Irish. From Boston.”

Dr. Harrison nodded slowly.

“I understand,” he said. “And I’m not saying you are. I’m saying the numbers suggest it.”

He pointed to a colored bar on one of the screens.

“If two mothers are unrelated,” he said, “and they use the same donor, their babies share about twenty-five percent of their DNA. Through the father.”

He pointed to another number.

“If the two mothers are themselves half-sisters,” he said, “the babies would share more. About thirty-seven percent. Through the father and through the mothers.”

He looked at us.

“That’s what these numbers show,” he said.

“Half-sisters,” I repeated.

“There’s something else,” Dr. Harrison said.

Part Nine: Runs of Homozygosity

He pulled up a different screen.

A long graph. Colored bars. Some of them highlighted in red.

“These are called runs of homozygosity,” he said. “Long stretches of DNA where a baby inherited the exact same sequence from both parents.”

He pointed.

“Every person has a few,” he said. “Short ones. It’s normal.”

He pointed at the red bars.

“These are long,” he said. “Longer than we’d expect from two unrelated parents.”

He looked at both of us.

“Both of your babies show this pattern,” he said. “To a similar degree.”

“What does that mean?” I asked. My voice was barely a whisper.

Dr. Harrison took off his glasses.

“It means,” he said carefully, “that in addition to the two of you possibly being related to each other, there may also be a biological relationship between each of you and the donor.”

The room went completely silent.

“Between us,” Emily said slowly, “and Daniel.”

“Yes,” Dr. Harrison said.

“What kind of relationship?” I asked.

He was quiet for a long moment.

“The pattern is consistent,” he said, “with the mother and the donor being half-siblings.”

I couldn’t breathe.

“That’s not possible,” Emily said. “We don’t know him. We don’t even know his real name. He’s an anonymous donor from a sperm bank.”

“I know,” Dr. Harrison said gently.

He put his glasses back on.

“That’s why I asked about your families,” he said.