Donna sank down on the jump seat next to the Doctor, her grey eyes awash with compassion. "Doctor? I just wanted to say…I'm so sorry…"
"I know, Donna," the Doctor replied in a strangely hollow voice.
"Can I get you anything? Nice cuppa?" she offered tentatively.
He smiled, just a bit, in spite of himself, and reached out for her hand. "No, thank you, Donna. I'm fine, really."
"No, you're not," she answered with that bit of heartfelt wisdom that was peculiarly her own.
"No, I'm not," he agreed reluctantly.
They sat in companionable silence for a while, then Donna quietly asked, "Doctor? Can I ask you something?"
"Oh, I've a feeling you're going to anyway," he teased affectionately.
"Oi! Watch it, spaceboy!" she replied, smacking him lightly on the arm.
"Ow! All, right, all right!" he said in mock surrender. "What was your question?"
"Jenny was a sort of a clone, right?"
"Weeeell, technically, she was a genetically modified -" He watched Donna's eyes fill with confusion. "Right. She was a sort of a clone."
"So, how come she didn't look like you?"
"DNA is remarkably flexible stuff. You don't look exactly like your mother, do you? Well, Time Lord DNA is infinitely more flexible than human. Has to be. Do you remember what Martha said about regeneration?"
"Yeah, but I didn't know what she meant."
"It's a thing that my people do when we're on the point of death. Our bodies heal themselves, and they change."
"What do you mean, change?"
"I mean I'm over nine hundred years old, and I haven't always looked like this. As it happens, Jenny looked an awful lot like I did, several centuries ago."
"What? You mean you were a little blonde slip of a girl?"
"No! No, I wasn't a…girl. Never been a girl, actually. That's a pretty rare trick, even for my people. I was, in fact, a fairly dashing blonde haired gentleman with a penchant for cricket."
Donna shot him a look that warned he'd better not be messing about at her expense.
"Really, Donna," he assured her. "Although…Time Lords are a telepathic race. I wonder if that didn't somehow figure into things, if the genetic sampling mechanism didn't somehow pick something - an image, or an idea - out of my mind."
"How do you mean?"
"Jenny…she looked like…" He paused and sucked in a harsh breath.
"Who did she look like Doctor?" Donna prompted kindly.
He was so very still that it almost seemed he'd stopped breathing altogether, then he let out an equally violent exhalation. "Rose. She looked like…it was as if…" He struggled silently, his face reddening as he tried so desperately to restrain his tears. "If Rose and I had a daughter…I think…Jenny…"
Donna slid her arms around the Doctor and gently pulled his head down to her shoulder. "If you and Rose had a daughter together, you think she'd look like Jenny?"
"Yeah…" he breathed out raggedly.