Re: Why can't humans cross breed(in special labs) with other animals?

Dear Peter,

    What an interesting question and one that has no doubt been
contemplated since ancient times. People have paid tribute to mythical
hybrids (the offspring of two animals or plants of different races,
breeds, varieties, or species) throughout history. The sphinx, chimera,
minotaur, and other mythological creatures all have their root in this
notion of crossbreeding. The general line of thinking being that if
humans and animals were to somehow produce successful offspring then the 
resultant progeny would exhibit a blend of human and animal physical and
mental characteristics. This theme still continues today in science
fiction and horror novels and movies. I remember a couple of years ago
the release of a modern motion picture update of H.G. Well's classic story
"The Island of Dr.Moreau" in which a colony of half-human and
half-animal beings were created via genetic engineering (that oh so
contemporary boogeyman) with disastrous effects.
     However, such human-animal hybrids currently only exist in the
realm of fiction. The science behind successful human and animal
crossbreeding would be extremely complex as there are several obstacles
that would have to be overcome to accomplish such a feat.
	"Species" is technically defined in the animal kingdom based on the
ability of organisms to mate (herein defined as fertilization between
the sperm and egg) and produce viable fertile offspring together. This
idea of reproductive isolation is really the basis for defining a
species. Even in rare cases where extremely similar members of different
species (but like genus) can mate and have offspring - the hybrid
offspring are sterile and therefore cannot propagate (e.g. donkeys and
horses begetting sterile mules).
    There are several obstacles that would interfere with crossing
species let alone organisms from a different genus. First, physical
differences called mating barriers would have to be overcome. These
barriers could be as significant as physical size or as fundamental as
anatomical incompatibilities. Also to be considered are differently
evolved states of breeding behavior between the two species that might
make them incompatible. If such barriers were overcome and copulation
where to occur then other mechanisms would become problematic. Special
molecules called receptors on the surface of the sperm and egg of the
two species would probably be so vitally different that they could not
properly join for fertilization to take place. If fertilization were
somehow to continue then genetic dissimilarity would stop things during
reproductive division. For example, even though chimpanzees are primates
and are 99% genetically similar to humans in both coding and non-coding
regions (possibly our closest animal match) - they have 24 pair of
chromosomes and humans only have 23 pair. This difference in number of
chromosomes would make normal cell division impossible during meiosis.
    As for "special labs" where such crossbreeding could be attempted - 
there are several issues to consider when contemplating such research.
Such a project would require millions of dollars as well as scientists'
valuable time to pursue. Modern scientists have certain moral standards
and social obligations to consider before attempting their research. So
much so that there are scientists and committees that spend the majority
of their time discussing scientific ethics and pondering the utility of
certain types of research. Of what benefit would such a project be to
humanity as a whole? What would be the ramifications on the natural
environment of creating an entirely new species? What rights would a
human-animal hybrid have if it displayed higher intelligence? Such
questions (and hundreds more) would have to be carefully weighed before
any scientific team would even consider tackling such a complex project.
	Great question Peter - if you're still interested in more information I'd
suggest picking up a general life science text (or see if your local
library has one) and look up the following: crossbreeding, meiosis,
reproductive isolation, hybridization, and fertilization.

Regards,
Todd