What Is Biotechnology? - Biotechnology and Your Health: Pharmaceutical Applications - page 19

Figure 1.6 Early development of an embryo is illustrated here. The human blastocyst
develops by about 4 1/2 days. Embryonic stem cells are formed from inner cell mass.
treated to separate those that have successfully taken up the new
genetic material from those that have not. The successfully trans-
formed ES cells are injected into the hollow center of a blastocyst,
so that the genetically modified ES cells mix with the small number
of ES cells present. The embryo is allowed to develop in the
laboratory for a short time, and is then placed into the uterus of
a female. Success is still not certain: About 10% of the live mouse
pups will have the new gene, the transgene. Only one of two
chromosomes of the transgenic mice will carry the transgene, so
the mice will have to be bred to produce animals with two copies
of the new gene.
ES cells can also be used to generate an animal genetically
identical to a living animal, a process termed reproductive cloning .
Reproductive cloning has been used in some species, and has been
hotly debated, particularly regarding its potential use in humans.