Sentence:  [P]reembryo cells from different parents can combine and grow into a <WSD>chimera</WSD> (an individual with cells from two or more zygotes)—in this case, an entity containing genetic material from four parents! Spontaneous <WSD>chimeras</WSD> ... occur rarely in our species. Recent examples include a woman who resulted from the merger of two zygotes or the early fusion of two genetically distinct embryos. 
Meaning of the word marked with <WSD> tags:  An organism with genetically distinct cells originating from two or more zygotes.