Which scenario applies to the production of chimeras?

• A. Ova are harvested shortly before fertilization.
• B. Oocytes are harvested shortly after fertilization.
• C. A DNA fragment is inserted in the pronuclei.
• D. Embryos are harvested at the blastocyst stage.

Answer: (D)

Chimeras arise when two genetically distinct cell populations are combined inside one developing embryo. The blastocyst stage is the right window for this because the embryo is still early and totipotent, with the inner cell mass able to incorporate donor cells. By injecting donor embryonic stem cells into a host blastocyst, those donor cells can contribute to most tissues of the developing animal, producing a true chimera after development. Other approaches—such as collecting gametes before or after fertilization, or inserting DNA into pronuclei—don’t create a mosaic organism from two cell lineages; they yield either a normal embryo or a transgenic organism, respectively. Thus, using embryos harvested at the blastocyst stage best supports chimera production.