Splicing: In
genetic engineering, a set of foreign genes
is spliced - inserted - into the middle of the DNA 'code words'
(see DNA
to find 
out
about the 'instructions': the genetic code). This splicing can
mess up the normal coded instructions in the DNA. And that can
go on to mess up how the cell works. No-one can know in advance
what might happen and whether it might be hazardous. It
is unpredictable. The insertion or splice could make the chromosome
behave in a quite unexpected way. This does not happen in normal
mating because the arrangement of the coded instructions does
not change when the chromosomes of the father and mother combine.
So when people claim that GE is more or less the same as natural
mating (sexual reproduction), they are wrong. For a brief comparison
of mating and genetic engineering, check out this PSRAST
page.