Fertilization is fusion of gametes. Fusion of two haploid gametes restores the diploid state in the inheritance model used for Mendelian segregation.
In the common angiosperm arrangement, one sperm fuses with the ovum to form a diploid zygote; another fuses with the central biological cell's two polar cell nuclei, initiating endosperm with three chromosome sets. The zygote develops into a plant embryo, and the ovule develops into a seed.[ref]
For an original counting example, stipulate a plant with chromosomes and the common two-polar-nucleus arrangement. Each haploid sperm and ovum contributes . The zygote therefore has chromosomes, while the initiating endosperm cell nucleus has . The maternal seed coat remains derived from plant tissue; the whole seed has no single uniform chromosome count. This is bookkeeping for a stipulated example, not identification of a particular species. It follows by counting the chromosome sets entering each fusion, rather than assigning one count to everything enclosed by a seed coat.
The biological cell formed by fusion of gametes.
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