Gidji yes, you are basically right. Chimerism is when there are twin foals developing in utero fuse, very early on in the zygote phase. It's specifically caused by non-identical twins, as in the result of the mare releasing and having fertilize two eggs, as opposed to a single egg splitting, which created identical twins. It results with a single horse having two complete and separate strands of DNA. The chimeric horse can have 2 sets of the same sex DNA, or 2 opposite sex DNA (as in the case of the stallion, Dunbar's Gold, in the article I posted, who had both a full male strand of DNA as well as a full female strand of DNA, however the horse itself was physically a stallion. The brindle mare in the article Dunbar's Gold was bred two, Sharp One, had two complete and separate strands as well, however both were female.) Chimerism can wreak havoc with DNA typing in breeding horses and their get, if you don't know they are chimeric - it's how the link was discovered in the first place.