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The orange is the fruit of the citrus species Citrus × sinensis in the family Rutaceae.[1] It is also called sweet orange, to distinguish it from the related Citrus × aurantium, referred to as bitter orange. The sweet orange reproduces asexually (apomixis through nucellar embryony); varieties of sweet orange arise through mutations.[2]

The orange is a hybrid between pomelo (Citrus maxima) and mandarin (Citrus reticulata). It has genes that are ~25% pomelo and ~75% mandarin;[2][3] however, it is not a simple backcrossed BC1 hybrid, but hybridized over multiple generations.[4] The chloroplast genes, and therefore the maternal line, seem to be pomelo.[2] The sweet orange has had its full genome sequenced.[2] Earlier estimates of the percentage of pomelo genes varying from ~50% to 6% have been reported.[3]

References:

1. "Citrus ×sinensis (L.) Osbeck (pro sp.) (maxima × reticulata) sweet orange". Plants.USDA.gov. Archived from the original on May 12, 2011.
2. Xu, Q; Chen, LL; Ruan, X; Chen, D; Zhu, A; Chen, C; Bertrand, D; Jiao, WB; Hao, BH; Lyon, MP; Chen, J; Gao, S; Xing, F; Lan, H; Chang, JW; Ge, X; Lei, Y; Hu, Q; Miao, Y; Wang, L; Xiao, S; Biswas, MK; Zeng, W; Guo, F; Cao, H; Yang, X; Xu, XW; Cheng, YJ; Xu, J; Liu, JH; Luo, OJ; Tang, Z; Guo, WW; Kuang, H; Zhang, HY; Roose, ML; Nagarajan, N; Deng, XX; Ruan, Y (Jan 2013). "The draft genome of sweet orange (Citrus sinensis)". Nature Genetics. 45: 59–66. doi:10.1038/ng.2472. PMID 23179022.
3. Andrés García Lor (2013). Organización de la diversidad genética de los cítricos (PDF) (Thesis). p. 79.
4. "A genealogy of the citrus family". Nature Biotechnology. 32: 640–642. doi:10.1038/nbt.2954.

Text from https://en.wikipedia.org/wiki/Orange_(fruit )