Mapacha au ndugu pacha ni watu au wanyama waliozaliwa kwa pamoja kutoka tumboni mwa mama yao. Kwa binadamu mara nyingi huwa wawili. Kuna aina kuu mbili za mapacha ambao ni wale wanaofanana na wale wasiofanana. Tofauti ni kwamba wa kwanza wametokana na kijiyai kimoja kilichofikiwa na mbegu moja, kumbe wa pili wametokana na vijiyai viwili vilivyofikiwa na mbegu mbili. Hivyo wa kwanza wana jinsia ileile na kwa kawaida kabisa DNA ileile, kumbe wa pili DNA zao ni tofauti, ila zinafanana kiasi kama zile za ndugu wote wenye baba mmoja na mama mmoja. Kimataifa aina ya kwanza inatokea mara 3 katika ujauzito wa wanawake 1,000. MedicineNet > Definition of Twin Last Editorial Review: 19 June 2000 Michael R. Cummings "Human Heredity Principles and issues" p. 104. April Holladay (2001-05-09). What triggers twinning?. WonderQuest. Iliwekwa mnamo 2007-03-22. Bacon, Kate. Twins in Society: Parents, Bodies, Space, and Talk (Palgrave Macmillan; 2010) 221 pages; explores the experience of child twins, adult twins, and parents of twins, with a focus on Britain. Nieuwint, Aggie; Van Zalen-Sprock, Rieteke;