Identical twins share the same DNA as they come from the same egg. Fraternal twins are born from two separate eggs so, although they are siblings, they have separate DNA. Even though identical twins have the same DNA, they aren’t perfectly identical as they develop in different eggs. Environmental factors can play a part too.
When two eggs are fertilised by two separate sperms, fraternal twins are conceived. As the two eggs and sperms contain different DNA, these twins do not share the same DNA. Fraternal twins just happen to be siblings that share the same birthday.
Identical twins occur when one egg is fertilised by one sperm and splits. The two halves of the egg develop as two separate babies. As they have both come from the same source, they share the same DNA at conception.
The same but different
Despite their name, identical twins aren’t actually identical they’re just very similar.
DNA provides the building blocks to create a person but changes can happen after conception that alter their development. Environmental factors also play their part. As these twins develop separately, they are exposed to different external factors. For example, one may be more susceptible to disease, which slightly affects their similarity.
This explains why twins have different fingerprints and why mothers can usually tell their twin babies apart.
Growing up, identical twins may choose different lifestyles, which accounts for further differences.
It’s not unusual for twins to develop different personalities, with one being more extroverted than the other. Despite sharing their DNA, they may also age differently due to living in different climates, choosing different hobbies, being exposed to disease or working in separate jobs.