Sullivan nodded to indicate understanding, and Allison hid another mental smile. Interesting that he showed no discomfort at all with the way the conversation was headed. Of course, with the high percentage of stillborn boys on this planet, Graysons had been fanatical about prenatal care for centuries, and men were just as involved in the process (at one remove, of course! she amended) as women.

“They had no real option about that,” she went on. “Not if they wanted the change to be a permanent addition to the planetary genome. But in the process, they also got an unintended mutation. Their intervention introduced a stable trinucleotide repeat on the X chromosome, which wouldn’t have been a problem… except that it in turn affected one of the AGG codons.” Sullivan looked blank. “AGG codons are adenine-guanine-guanine sequences that act as locks on the expansion of other trinucleotide repeats,” she explained helpfully.

“Of course,” Sullivan agreed. He didn’t look too terribly enlightened, but he nodded for her to continue, and she punched a new command into her holo unit. The imagery changed to a color-coded schematic of nucleotides—an enormous chain composed of the color-coded letters “A,” “C,” “G,” and “T,” repeating again and again in jumbled patterns. As Sullivan watched, the image zoomed in on a single section—two three-letter groups of “CGG” in yellow, green, and green, separated by an “AGG” in red, green, and green.

“Essentially, it was a very tiny change,” Allison told the Reverend. “An adenine here—” she touched another key, and the “AGG” code flashed brilliantly “—mutated to cytosine—” another key, and the flashing red “A” turned into a yellow “C” and the three-letter group to its right grew suddenly into an enormous chain of the same codes, repeating again and again “—which deactivated the lock and allowed unstable expansion of—”

“Excuse me, My Lady,” Sullivan interrupted, “but I think we’re drifting into deep water here. What, precisely, does that m— No.” He stopped and raised one hand. “I’m certain that if you told me what it meant, I would be no closer to understanding than I am now. What I truly need to know, I suppose, is what the consequences of this… unstable whatever are.”

“Um.” Allison sipped some more tea, then shrugged.

“DNA is composed of four nucleotides, Your Grace: adenine, cytosine, guanine, and thymine. They link together in thousands of repeats—codes, if you will—which combine to carry the blueprint for our bodies… and transmit it to the next generation. They link in groups of three, hence the term ‘trinucleotide,’ which usually occur in ‘runs’ of thirty or less, but there are several diseases, such as the one we call ‘Fragile X,’ in which the number of repeats expands enormously, often into the thousands, effectively… well, scrambling a portion of the master code, as it were. Are you with me so far?”

“I believe so,” he said cautiously.

“All right. This schematic represents a portion of the nucleotides—in this case cytosine, adenine, and guanine—from the Grayson genome. This trinucleotide here—” she touched her controls and the holo reverted to its original form with the “AGG” flashing once more “—is what we call a ‘lock,’ sort of a blocker to prevent the CGG repeats on either side of it from expanding in a way that would scramble the code. What happened, though, was that when the adenine mutated into cytosine, the ‘lock’ disappeared… and that allowed an unstable expansion of the CGG chain ‘downstream’ of it.”