this may sound stupid but i would just like to know a basic few things....why is it that say a dog can not become impregnated with a human or cat baby?
Two species have to be very closely related in order for their chromosomes to be so compatible that they can join and create a live animal. (Or plant, or any other type of lifeform that reproduces sexually.) Generally they have to be members of the same genus. This is why a wolf,
Canis lupus, and a coyote,
Canis latrans, can mate and have viable offspring. Or a lion,
Panthera leo, and a tiger,
Panthera tigris, although it's almost impossible for them to arouse each other and trigger the urge to copulate so it has to be done by artificial insemination. Horses and donkeys, bison and cattle, scarlet macaws and greenwing macaws, black-headed grosbeaks and rose-breasted grosbeaks--it's always the same: two species within the same genus can crossbreed and produce live offspring. (I have vastly oversimplified but let's just call this Lesson One and say that it's about 98% correct.)
The problem with trying to hybridize humans with another species is
there is no other species within our genus. Homo is a genus with only one species. (There's a name for that, mono-something-or-other, but I can't remember it.) The last time modern humans were able to hybridize with another species was about 25,000 years ago, when Neanderthals,
Homo neanderthalensis, still existed. And yes, we did hybridize with them, the DNA evidence was discovered just this year.
In the interest of full disclosure, there have been occasional instances of animals being able to crossbreed despite being members of different genera. The blue-and-gold macaw with the hyacinthine macaw, for example. They are members of two different genera within the same family and their DNA just happens to be compatible. But this is very rare. (This is in Lesson Two, as I noted earlier.) It's far more common that
two species in the same genus cannot hybridize. (It has to do with number of chromosomes and a bunch of other stuff, and since it goes beyond my knowledge of biology, it's not in Lesson Two.
)
In any case, there are only five other species in the other genera within the family our species belongs to: the two species of chimpanzee, the two species of gorilla and the one species of orangutan. Their DNA has been examined in minute detail, and there's no way we could hybridize with one of them.
So the answer is that no other animal can crossbreed with a human because their chromosomes are not compatible with ours.
also say is it possible to say somehow cross breed say a dog and a cat or a turtle and a human and stuff like this, like through dna or something?? government would consider it inhumane so thats obviously why it hasn't really been done unless the government themselves are doing it, but what would you have to do to cross breed to different species?
There's no way to
literally crossbreed humans with any other species. We can't take a sperm cell from one and an egg cell from another, put them together in the right environment, and watch them merge into a zygote. As far as the human sperm cell is concerned, that foreign egg cell might as well be a sesame seed or a golf ball. It doesn't recognize it and doesn't have the capability to merge with it. End of story.
All we can do is splice individual genes into a sperm or an egg. Other members have discussed that at length in this thread. That isn't
hybridization in the original meaning of the word, because it is not the result of a mating, even an artificial mating. It's really
genetic engineering. The ethical questions you're wondering about have been raised in the discipline of genetic engineering, and they're still being sorted out. Many countries don't allow genetically modified crops to cross their borders--and those aren't even animals, much less people.
What about combining animal DNA with human? Will athletes have cheetah genes implanted into their leg muscles so they can run faster?
There's so much more to it than that. In order for the muscle to pump the leg faster, it has to have a thicker cross section so it can deliver more energy. But you're going to run into a limit on that type of expansion, because if you pump the legs much faster than they go now, it's going to increase the wear and tear on the joints where the rotation takes place and they won't hold up under the abuse. So in addition to thicker muscles, you're going to have to make the legs longer so one stride covers more ground. Now you're raising the pelvis further off the ground.
The human pelvis is an amazing piece of engineering, and if you start messing with it you're going to have to go back to the drawing board. The birth canal has to be wide enough for our enormous heads to slip through. This puts our hips much farther apart than other animals of comparable size, so when we walk or run we're performing a weight transfer from one leg to the other that spans considerable distance. Because we also have the ability to lock our knees for bipedal walking, this weight transfer requires enormous balancing muscles. Notice that no other animal has a gluteus maximus (the twin hemisperical muscles that shape our butt) like ours.
All of that will have to be redesigned. Cheetah DNA won't do it because cheetahs are quadrupeds. In fact, we're the only full-time bipedal mammal. You're going to have to invent your own genes!