Figure 1

Note: The functional genetic units, genes, are composed of DNA that is arranged linearly on chromosomes located in the nuclei of cells.

source: Author.

second creates embryos (with identical nuclear DNA) from cells of embryos, fetuses, young animals, or adult cells.

Conceptually, the simplest approach would be to separate the two cells of a two-cell embryo so that two identical organisms form. This has been done repeatedly in one way or another, even occasionally resulting in identical quadruplets when dividing a four-cell embryo four ways. Success rates are quite high when aiming at identical twins, but become very low when dividing embryos into quadruplets, the practical limit of the technique. For technical reasons, this approach is much more practical at later stages of embryonic developmentā€”at the 100-cell stage, for example), when embryos can be bisected. This latter approach has been used to produce thousands of identical twins (and occasionally triplets) commercially, primarily with cattle (as illustrated by Timothy Williams and colleagues [1984]).

Surprisingly, the main reason for splitting embryos to produce demi-embryos is not to produce sets of identical copies, but rather because splitting embryos augments the general technology of embryo transfer, which is designed to increase the reproductive rates of agricultural (and other) females, much like artificial insemination increases the reproduction of males. To illustrate, pregnancy rates for whole bovine embryos are around 65 percent, whereas pregnancy rates for half embryos are around 50 percent. Thus, because there are twice as many demi-embryos after the splitting process, the net pregnancy rate is frequently over 100 percent. Identical twins and triplets produced by these methods make excellent experimental subjects because genetic variation can be controlled, and sometimes they are produced mainly for these purposes.

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