Twins are two babies born on the same day to the same mother. All twins are siblings – brothers or sisters – so they will always have some similarities in their appearance. However, only some sets of twins are identical.
Identical twins form from a single fertilised egg. After fertilisation, this splits into two and an embryo forms from each of the two cells formed. Every thing about the babies is therefore identical – they have the same genes and the same patterns of gene expression; they are like two clones of the same person.
Twins that are non-identical are produced from two separate eggs that are fertilised by two separate sperm. Both implant and grow in the uterus at the same time, but they are like an ordinary brother or sister; they are not genetically identical.
Whatever type the twins are, they are still completely individual separate people and their environment and experiences can shape them to be completely different, even if they look the same.
Identical twins are the result of one sperm fertilising one egg. Soon after fertilisation the embryo may divide into two (or more) genetically identical embryos. These become identical twins.
Identical twins share all of the same genetic information, but they may display different traits as a result of their environment (and the influence that has on gene expression). For instance, it is not uncommon for one identical twin to be right handed and for the other to be left-handed.
Fraternal twins result from two different sperm fertilising two different eggs (some women ovulate more than one egg per cycle). Fraternal twins share no more genes than any other pair of siblings.
A third type of rare twin is possible. When a woman ovulates, each egg is unique, no other egg in her body has exactly the same genes. But for each sperm, there are three others identical to it. In theory, a woman could ovulate two different eggs, fertilised by identical sperm. This type of twin has been called 'identical-fraternal'. No one knows how common such twins may be, but there are anecdotes of twins who always thought seemed to be identical, only to find out that they have different blood types or another physical difference known to be inherited.
The difference is that identical twins share the same placenta meaning that it was one egg that split in two. Non identical are two different eggs fertilized at the same time and have their own placenta. Hope that helps.
In identical twins, during implantation only one ovum is implanted which divided into two embryos. The twins have exactly the same chromosomes, resemblance and sex. Whereas in non identical twins two separate ovums are implanted. Each have different chromosomal structure.
Identical twins share the same water sack and fraternal means they are in two different sacks ...identical don't nessicearly mean they will look alike ....look at the Olson twins they are faternal twins and they look just alike.