And we can look in particular at three types of siblings: identical twins, twins that actually share 100 percent of their genetic information and shared the same intrauterine environment, versus fraternal twins, twins that actually share 50 percent of their genetic information, versus regular siblings, brother-sister, sister-sister, also sharing 50 percent of their genetic information, yet not sharing the same intrauterine environment.
Podemos analizar tres casos concretos de irmáns: xemelgos univitelinos, xemelgos que comparten  o 100 % da información xenética e compartiron o mesmo medio intrauterino, fronte aos xemelgos bivitelinos, xemelgos que comparten  o 50 % da información xenética, fronte a irmáns comúns, irmán-irmá, irmá-irmá, que tamén comparten o 50 % da información xenética pero non compartiron o  mesmo medio intrauterino.