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Gemelli monozigoti: chi sono e differenze con dizigoti

gemelli monozigoti

Gemelli monozigoti

Si definiscono gemelli gli individui nati dalla stessa gravidanza. Avere una gravidanza gemellare sembra essere legato a fattori sia ereditari (le probabilità aumentano se nella famiglia materna ci sono stati altri casi di parti gemellari), alla razza (i gemelli nascono più frequentemente tra le persone di colore, raramente tra quelle orientali) e all’età della madre (in età avanzata aumenta il livello di ormone follicolo stimolante e il numero di ovuli da fecondare). Negli ultimi anni è stato registrato un aumento di gravidanza gemellari, forse proprio a causa dell’innalzamento dell’età media delle mamme e al ricorso alla fecondazione assistita e a cure per la fertilità. Sebbene spesso si pensi che i gemelli siano sempre uguali, non è così. Vi sono, infatti, due categorie di gemelli, ovvero:

  • gemelli monozigoti (anche detti gemelli omozigoti, monocoriali o monoovulari),
  • gemelli dizigoti (detti anche gemelli eterozigoti, bizigoti, bicoriali o biovoulari).

I gemelli monozigoti sono quelli identici e condividono lo stesso patrimonio genetico. Essi sono più rari dei gemelli dizigoti, in quanto costituiscono solo il 25-35% delle gravidanze gemellari.

Gemelli monozigoti monocoriali

I gemelli monozigoti, anche detti omozigoti o monocoriali, derivano da un unico zigote (la cellula derivata dall’unione tra cellula uovo e spermatozoo), che in seguito si divide in due parti o anche più. Durante il concepimento, quindi, viene fecondata una sola cellula uovo, che in seguito si divide formando due diversi zigoti. Dal momento in cui la cellula uovo di divide dipende la condivisione della placenta e del sacco amniotico: se si divide entro poco tempo dalla fecondazione, ognuno dei gemelli avrà la sua placenta, se invece si divide dopo 4-7 giorni dalla fecondazione, i gemelli condivideranno la placenta. Nel raro caso in cui lo zigote non si divida completamente, si ha la formazione dei gemelli siamesi, che si presentano uniti e che condividono uno o più organi. I gemelli monozigoti hanno lo stesso DNA, per questo motivo sono identici e dello stesso sesso. I gemelli monozigoti maschio e femmina sono quindi impossibili, a differenza di quanto accade nel caso dei gemelli eterozigoti.

gemelli monozigoti monocoriali

Gemelli monozigoti e dizigoti: differenze

La differenza tra gemelli omozigoti ed eterozigoti sta nel DNA: i gemelli dizigoti hanno diverso DNA, mentre, come abbiamo visto, in quelli monozigoti il patrimonio genetico è identico. I gemelli dizigoti, quindi, sono semplicemente due fratelli, che nascono dalla stessa gravidanza, durante la quale hanno condiviso l’utero. Essi però non nascono dallo stesso zigote, ma a partire da due o più ovuli distinti, fecondati da due o più spermatozoi diversi quasi nello stesso momento. Per questo motivo, i gemelli dizigoti o eterozigoti non sono identici, ma si somigliano come dei normali fratelli. Inoltre, i gemelli dizigoti possono essere maschio e femmina, cosa impossibile nel caso dei gemelli monozigoti. In rarissimi casi, è possibile anche che due gemelli dizigoti abbiano padri diversi. Questo accade quando, in caso di rapporti con due uomini diversi a breve distanza, vengono fecondati due ovociti distinti da spermatozoi diversi.

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