312 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
without freezing. However, 70- to 80-percent success 
rates may be attainable within several years. It ap- 
pears that embryos can be stored indefinitely with 
little deterioration. 
Sex selection 
DEFINITION 
Tests to determine the sex of the unborn or deter- 
mination of sex at fertilization by separating x- bear- 
ing from y-bearing sperm. 
STATE OF THE ART 
Sexing of embryos.— Through karyotyping 
nearly two-thirds of embryos can be sexed. Tech- 
niques using identification of the condensed X chro- 
mosomes are unreliable. A third method, identifica- 
tion of sex-specific gene products, is under develop- 
ment. 
Sexing of sperm.— A 100-percent method has 
not been achieved in any mammalian species; and no 
standard protocol for farm species exists. 
FllTlIRE 
Before this technology can be applied commercial- 
ly, it must be simple, fast, inexpensive, reliable, and 
nonharmful for embryos. Such techniques could un- 
doubtedly be developed. There would be numerous 
medical and experimental applications. 
There is much interest in research in this area 
because of its use in understanding male fertility 
with AI in bumans, and in enhancing sperm survival 
after frozen storage. 
Twinning 
DEFINITION 
Artificial production of twins, either using embryo 
transfer or hormone treatments. 
STATE OF THE ART 
Currently, embryo transfer is tbe most effective 
method for inducing twin pregnancies in cattle, 
resulting in pregnancy rates of between 67 to 91 per- 
cent, of which 27 to 75 percent deliver twins. Other 
methods include transferring one embryo into a cow 
which has been artificially inseminated, and hor- 
monal induction of twinning, which is a modification 
of superovulation. This latter method is not reliable. 
ADVANTAGE 
The advantage of twinning in nonlitter-bearing 
species is the improved feed conversion ratio of pro- 
ducing the extra offspring. 
DISADVANTAGE 
The major disadvantage of twinning is intensive 
management necessary for periparturient conqtlica- 
tions, unpredictable gestation periods, depressed lac- 
tation, etc. 
FUTliRE 
Technical feasibility for twinning, in conjunction 
with embryo transfer, management adjustments, 
and selection for good recipients, can be predicted. \ 
reliable procedure for twinning in sheep can alst) be 
expected. The technologv' would most likely be first 
used in Europe and Japan, whei e there are shortage's 
of calves to fatten for beef. 
In vitro fertiliz.ation 
DEFINITION 
rhe union of egg and sperm outside the re[)i ()du( - 
tive tract. For some species, the technology includes 
successful developmetit of the embryo to gestation 
and birth. 
STATE OF 'HIE AR I' 
In vitro fertilization has been accomplished in 
several laboratory animal sjiecies, including the rab- 
bit, mouse, rat, hamster, and guinea pig and nine 
other mammalian nonlaboratorv species, including 
man, cat, dog, pig, shee|), and cow lloucver, norm.il 
development following in \ilro tertili/alion and cm 
bryo transfer has only been accom|)lished in the rab 
bit, mouse, rat, and human ( onsisteni .md lepe.il 
able success with in \ itro lertili/.alion in larm spei les 
has not yet been accomplished 
None of th(? ca.ses of reported siici ess ot in \ilm 
fertilization, embryo Iranster. and normal de\i*lop 
ment in man is well documented 
Most of the in vitro lerlili/.alion work to d.ile has 
concentrated on the development ol a leseaich tool 
so that the physiological and bioi hemical events m 
fertilization and early devi-lopment could be In-tter 
understood. More practic.il .ipplii .ition ol m vitro 
fertilization techni(|ues would iiu lode 
1. a means for as.sessing the lerlililv o! ovum 
and/or sperm: 
2. a means to overcome lem.ile inlet tilitv w ilh em 
bryo transfer into .i recipient .mim.il .ind 
;i. when coupled with ovum .md or embrvo situ 
age and transfer. .i me.ins to l.i< ilit.ile t ombm.i 
tion of selected ov a w ilh selet ted sperm loi pm 
duction of indiv iduals w ilb iiredicled < h.ii ,ii lei 
istics at an appropri.ile time 
