30 
If the secondary recipient is another microorganism, the considera- 
tions described in IV-C-l-a apply. If the secondary recipient is 
one of the cells of an animal or plant, different considerations apply. 
The possible effects include alterations of normal cellular control 
mechanisms, synthesis of a foreign protein (such as a hormone), 
and insertion of genes involved in cancer production (if, for example, 
the foreign DNA were derived from a cancer-producing virus). 
It should be pointed out that the likelihood of inheritable changes 
in the offspring of complex organisms being caused by such a mechanism 
is extemely low because of the protection afforded germ -line cells 
(eggs and sperm) by their location. Thus, it is highly improbable or 
perhaps impossible for recombined foreign DNA to reach germ -line 
cells at a time in their life when secondary recombination can occur 
and affect an offspring. With one -celled organisms, plants, or simple 
multicellular organisms, the probability of heritable change resulting 
from secondary recombination is higher. 
What is the probability of secondary recombination between 
prokaryotes and eukaryotes in nature? It is generally held that recom- 
bination in nature is more likely if similar or identical sequences of 
bases (rungs in the DNA ladder) occur in the two recombining DNAs (2). 
The greater the degree of similar sequences, the more likely is recom- 
bination. In general, the more closely two species are related, the 
more likely it is that similar sequences will be found in their DNAs. 
Thus, DNA from primates has more DNA sequences in common with 
human DNA than does DNA from mice, or fish, or plants. Recom- 
bination may also occur between DNAs not sharing sequences but at 
lower frequencies. 
It is possible that the capacity for interspecies recombination 
between distantly related species exists in nature. For example, 
bacteria in animal intestines are constantly exposed to fragments 
of animal DNA released from dead intestinal cells. Significant 
recombination, however, would require the uptake of intact segments 
of animal DNA and their subsequent incorporation into the bacterial 
DNA. The frequency of such events, if they occur at all, is unknown. 
There are very few available data permitting assessment of 
the reverse process--namely, the incorporation of bacterial DNA 
into the cells, or DNA, of more complex organisms. Although there 
are reports of experiments in which bacterial DNA was inserted 
into animal and plant species and production of the bacterial protein 
followed, the process is very inefficient and many investigators have 
been unable to repeat these experiments (17-19). 
