defined: |
![](https://webarchive.library.unt.edu/web/20090509025845im_/https://www.genome.gov/Images/spacer2.gif) |
The nucleus of a sperm or an egg prior to fertilization. Sperm and egg cells carry half the number of chromosomes of other nonreproductive cells. When the pronucleus of a sperm fuses with the pronucleus of an egg, their chromosomes combine and become part of a single nucleus in the resulting embryo, containing a full set of chromosomes.
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