Twins Biology
You shout out "twins" when you see two people looking exactly alike and maybe even better if they are dressed alike and have a good "twin" look.
Actually there are two types of twins the identical twins called monozygotic and the fraternal twins called dizygotic. So the term twins just indicates two children born together. A twin birth occurs every 100 normal births. In all cases of twins there is a genetical predisposition: in some families twin births are particularly frequent.
Identical Twins "monozygotic"
Identical twins are born from the same egg which spilts in two after conception. This is a rarer case that occurs in 30% of twins, these twins have the same Dna and therefore they are physically identical and must be of the same blood group and sex. In this case usually the two foetuses share the same placenta but have different amnios in some cases they can have even a separate placenta or share both the placenta and the amnios, only in this last case malformations such as siamese twins can occur.
Fraternal Twins "dizygotic"
Fraternal twins are born from two different eggs and develop separately in different amnios and different placentas, sometimes in the same placenta but the amnios is always different; they have different Dna so they can be of different sexes and can have different blood groups. Fraternal twins are more common and occur in 70% of twins. Fraternal twins are commonly also called "false twins" as from a genetical point of view they are like brothers or sisters, only they are born at the same time.
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