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ADVANTAGES1. Amplification of advantages of embryo transfer.2. Elimination of requirements for large recipientherds when embryo transfer is being used.3. Reduction of costs in animal transport.4. Control of genetic drift in animals over prolongedtime intervals.FUTUREAnticipated development of embryo culture tech-nology would be of significance in efforts toward invitro maturation of gametes, in vitro fertilization, sexdetermination, cloning, and genetic engineering, allof which involve prolonged manipulation of gametesand embryos outside of the reproductive tract.As freezing rates improve, nearly all embryos re-covered from cattle in North America will be frozen.Probably as many as half of the embryos will bedeep-frozen for 2 to 3 years. It is unlikely that suc-cess rates will ever approach 90 percent of those
312 ●Impacts of Applied Genetics—Micro-Organisms, Plants, and Animalswithout freezing. However, 70- to 80-percent successrates may be attainable within several years. It ap-pears that embryos can be stored indefinitely withlittle deterioration.Sex selectionDEFINITIONTests 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 ARTSexing of embryos.—Through karyotypingnearly 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 hasnot been achieved in any mammalian species; and nostandard protocol for farm species exists.FUTUREBefore this technology can be applied commercial-ly, it must be simple, fast, inexpensive, reliable, andnonharmful for embryos. Such techniques could un-doubtedly be developed. There would be numerousmedical and experimental applications.There is much interest in research in this areabecause of its use in understanding male fertilitywith AI in humans, and in enhancing sperm survivalafter frozen storage.TwinningDEFINITIONArtificial production of twins, either using embryotransfer or hormone treatments,STATE OF THE ARTCurrently, embryo transfer is the most effectivemethod 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. Othermethods include transferring one embryo into a cowwhich has been artificially inseminated, and hor-monal induction of twinning, which is a modificationof superovulation. This latter method is not reliable.ADVANTAGEThe advantage of twinning in nonlitter-bearingspecies is the improved feed conversion ratio of pro-ducing the extra offspring.DISADVANTAGEThe major disadvantage of twinning is intensivemanagement necessary for periparturient complica-tions, unpredictable gestation periods, depressed lac-tation, etc.