I'm not really into the Doctor Who fandom, but isn't Susan from Season 1 referred to as the Doctor's granddaughter? I understand that that has been named canon/uncanon by various people, but it seems like a viable choice even if it isn't realistic.

Maybe someone more familiar with DW canon could expand on Susan?

-MB

Unless there are canonical hybrids, then no, they're not going to be able to reproduce.

For a start, the one thing everyone knows about Gallifreyans is they have two hearts, right? Humans have one. Anyone want to work out how the maternal-to-foetal bloodflow issue is going to work?

In the Real World? Nope, it's extremely unlikely. See Darwin's Origin of Species for a much more in-depth discussion of why hybrid animals tend to be sterile.

In a sci-fi canon, however, it depends. If we're talking, say, Star Trek, then hybrids are possible even when it makes my brain hurt (eg Vulcan copper-based blood not causing any problems with Human blood). If it's a sci-fi continuum with no hybrids, then it'd be a no. I'm trying to think if there are any hybrids in Doctor Who canon, and drawing a blank.

Mind you, I suppose there was that woman and that cat in Gridlock. But I'd rather not touch the biology of that one with a ten foot pole because it makes my inner biologist cry.

of species (from wikipedia): "a group of organisms capable of interbreeding and producing fertile offspring of both genders" Or as I remember from my biology lesson: different species don't interbreed and produce fertile offspring (mules, and such).

So, unless Gallifreyans are a human race (and not a different species) the Doctor could get it on with Sarah Jane's cousin, they could possibly have biological offspring, but those kids would be sterile.

On the other hand, scifi likes to ignore this definition of species and create multibrids all over the place (remember half-Klingon B'Elanna who had a baby?)

In one of the Doctor novels the Doctor was said to have a human mother. If you take that as canon, then canon further implies that the Doctor can have off-spring and isn't sterile.