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 'instruction tape' 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.