178 • Impacts of Applied Genetics— Micro-Organisms, Plants, and Animals 
tion has been very rewarding, while selection 
for twinning in other species has not received 
much attention. 
Twinning in nonlitter-bearing species would 
greatly improve the feed conversion ratio of 
producing an extra offspring. The most impor- 
tant barriers, besides the high cost of embryo 
transfer techniques, include extra attention 
needed for the dam during gestation, parturi- 
tion, and lactation. 
More speculative technologies 
IN VITRO FERTILIZATION 
The manual joining of egg and sperm outside 
the reproductive tract has, for some species, 
been followed by successful development of the 
embryo through gestation to birth. The species 
include, at this writing, the rabbit, mouse, rat, 
and human. Consistent and repeatable success 
with in vitro fertilization in farm species has not 
yet been accomplished. The cases of reported 
success of in vitro fertilization, embryo reim- 
plantation, and normal development in man are 
beginning to be documented in the scientific 
literature. 
The in vitro work to date has attempted to de- 
velop a research tool so that the physiological 
and biochemical events of fertilization could he 
better understood. Despite the wide public at- 
tention it has received in the recent past, the 
technology is not perfected and will have little 
practical, commercial effect in producing in- 
dividuals of any species in the near future. 
Practical applications would include: a means 
of assessing the fertility of ovum and sperm; a 
means of overcoming female infertility by em- 
bryo transfer into a recipient animal; and, when 
coupled with storage and transfer, a means of 
facilitating the union of specific ova and sperm 
for production of individual animals with pre- 
dicted characteristics. 
Many of the practical applications should h(!- 
come available within the next 10 to 20 years. 
Further development, along with the storage of 
gametes, should allow fertilization of desired 
crosses. This technology may he combined with 
genetic engineering and sperm sexing in the 
more distant future. 
PARTHENOGENESIS 
Parthenogenesis, or “virgin birth,” is th<> ini- 
tiation of dex elopment in the absence of s|)ei'm. 
It has not been demonstrated or descrilu'd lor 
mammalian species, and the best a\ ailahle infor- 
mation indicates that the maintenance of pai'- 
thenogenetic de\elo|)ment to [)i'oduce normal 
offspring in mammals is pi'esently im|)ossihl(‘. 
CIAINING 
The possibility of protliu'ing gcMU'tically iden- 
tical indixiduals has fasc'inatc'd l)oth scientists 
and the general public. .As tar as liwstock are 
concerned, theix^ ai'(? scnc’ial ways to obtain 
genetically identical animals. rlu> natui'al way is 
through identical tw ins, although these are rai l* 
in species other than cattk’, sh(>ep, ami pri- 
mates. Foi’ pi'actic'al purposes, highly inbred 
lines of soim? mammals are ali ('ady considered 
genetically identical; first generation crosses ol 
these lines are also considered genetically iden- 
tical and do not suffer Irom the depressive el- 
fect of inhrc'eding. 
hahoratory imnhods for producing clones in- 
clude div iding early embryos. I he results ol re- 
cent e.\|)('rim('nts in the production of identical 
offspring using these* t(*chni(|ues are shown in 
table! 32. 
,\noth('i' methodology involves the insertion 
of the* nucleus of one cell into another, either 
liefore! oi’ alter the original genetic complement 
of the! "re!e'e*iv e*r” e e*ll is deslroyeef Iteseare hers 
have! feuind in ce'itain amjihihia that nucle.ir 
trans|)lantation fiom a body cell ol an embryo 
into a zvgote e an le*ael to the elev elopmeni ol a 
sexually mature* I reig. 
Table 32. — Experimental Production 
of Identical Offspring 
Methodology 
Result 
Dividing 2-cell embryo in 
half 
1 pair identical mouse twins 
Dividing morulae® in half 
8 pairs of identical mouse 
twins 
Dividing 2-cell embryos in 
5 pairs of identical sheep 
half 
twins 
Dividing 4-cell embryos in 
1 set identical sheep 
four parts 
quadruplets 
^An embryo wilh 16 lo 50 cells; resembii--. a muitxf', 
SOURCE. Beniamin G Bracken. School ol Volenna,, Mr ' , r ,> ,,c -. • 
Pennsylvania. Kennell Souarc Pa 
