One of the more complex forms of chimerism is tetragametic chimerism -- two separate sperm fertilize two separate ova. During the blastocyst or zygote stage, the cells then merge and fuse together. The resulting organism is a chimera -- a fusion of two non-identical twins. Due to this, the person with chimerism may be male, female, or possess intersex characteristics. It is also possible for this to occur in the case of potential identical twins, but the person's separate genetics would be too similar for them to qualify as a chimera.

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