Spider Bones

Snorting in disgust, Lo gestured to Cotton.

“It turned out Atoa was actually only sixteen. The confession’s out. Since there’s really nothing else, he couldn’t be held.”

Down the hall, Schoon was still questioning Face Mask.

“Did I miss much?” I asked, gesturing at the screen.

“Spider’s reborn,” Ryan said. “Plans to join the Jesuits.”

“I know what happened.” I was so jazzed I showed no empathy for Lo’s frustration. “Spider. Xander. Lapasa. I just needed some medical info.”

“Lecture alert,” Ryan whispered to Lo and Cotton.

“I’ll keep it brief.” I was too pumped to take offense.

“And intelligible.”

“Yeah, yeah. No jargon.”

Deep breath.

“In two thousand two a pregnant woman named Lydia Fairchild applied for welfare in the UK. In addition to her unborn infant, she had two children by a man named Jamie Townsend. As part of the application process, Fairchild had to provide DNA evidence that Townsend was the father. Results showed that he was, but indicated that she wasn’t the mother.”

“Bummer,” Ryan said.

“No kidding. Fairchild was accused of fraud and her kids were taken into care. A judge ordered that a witness be present when she delivered, and that blood samples be taken from Fairchild and the baby. DNA indicated she was not the mother of that child either, even though it was a witnessed birth. A breakthrough came when lawyers discovered a similar case in Boston.”

“Thank the Lord for defense attorneys.” Lo, the king of sarcasm.

“In fact, it was the prosecutor.” I smiled at Cotton. “In nineteen ninety-eight a woman named Karen Keegan needed a kidney transplant. Her adult sons were tested for suitability as donors. Two of the three failed to match her DNA to the extent a biological child should. More sophisticated testing showed that Keegan was a chimera, a combination of two separate sets of cell lines with two separate sets of chromosomes.”

“How’d they figure that?” Ryan asked.

“Different DNA sequencing was found in tissues other than the ones originally taken from Keegan. Fairchild’s prosecutors suggested this possibility to her lawyers, and DNA samples were collected from members of the extended family. The DNA for Fairchild’s children matched that of her mother to the extent expected for a grandmother.”

“Showing she was the mother.” Cotton looked confused.

“Further tests showed that while DNA obtained from Fairchild’s skin and hair didn’t match her children’s, DNA obtained from a cervical smear was different and did match them.”

“Fairchild was carrying two different sets of genes.” Ryan simplified, but basically got it right.

“Yep.”

“That’s what this chimera thing is?” Lo.

“Yep.” I glanced at my notes.

“This is where she tells us all about it,” Ryan warned the other two.

“Two types of chimerism occur in humans. With microchimerism only a small portion of the body has a distinct cell line. Typically that arises because foreign cells have managed to stabilize inside a host.”

“Foreign?” Cotton asked.

“Could be cells originating from maternal-fetal exchange during pregnancy. For example, the fetus may pass on its stem and progenitor cells to the mother via the placenta. Because they’re undifferentiated, these cells may be able to survive and proliferate in the mother’s system. Maternal stem cells may be transferred to the fetus in the same way.”

No one said anything, so I continued.

“Microchimerism can also occur between twins. Actually, the most common form of human chimera is called a blood chimera. That results when fraternal twins share some portion of the same placenta. Blood is exchanged and takes up residence in the bone marrow. Each twin is genetically distinct except for their blood, which has two distinct sets of genes, maybe even two distinct blood types.”

“How common is it?” Ryan.

“It’s estimated that up to eight percent of fraternal twins are blood chimeras.” I thought a moment. “Things like blood transfusions or organ transplants can also produce microchimerism in a recipient.”

“That what happened to these ladies you’re talking about?” Lo asked.

“No. What Fairchild and Keegan had is a much rarer type, tetragametic chimerism. This occurs when two separate ova are fertilized by two separate sperm and produce two zygotes.”

Ryan raised a cautioning finger. “Embryos.”

“Yes, sorry. It occurs with fraternal or nonidentical twins. The embryos fuse very early in development, creating a single baby with two distinct cell lines. One set of DNA may appear in the kidney and another set may appear in the pancreas.”

Cotton summarized. “So these women, Fairchild and Keegan, each merged with her twin to form one baby with