Identical twins known as monozygotic twins, meaning one fertilized egg. They occur when one egg is fertilized by one sperm as usual, but the egg splits in two shortly afterward. Each half then grows into a baby.
Twins originally came from the same egg and sperm, 100 percent of their chromosomes are identical. This means they’re the same sex and have the same genetic characteristics, like hair and eye color.
However, things in their environment, such as how much room they each had in the womb, can cause slight differences in their appearance.
MIRROW IMAGE TWINS
This is a subtype of identical twins that happens when the egg splits 7 to 12 days after fertilization, instead of during the first week. By this time, the growing embryo has already developed a left and right side.
These twins are identical but mirror images of each other.
For example, their hair may spiral in opposite directions, their teeth may start coming in on opposite sides of their mouth, and one may be right-handed while the other is left-handed. They may even cross their legs in opposite directions.
https://www.healthline.com/health/pregnancy/types-of-twins#risks
TRUE LIFE STORY
That reminds me of the female twins I met during my staying at Akure. I never knew they were twins intially untill I finally set my eyes on both of them staying together. Same dress, same hair styles. The very first day I collected Taiwo's number not knowing that they were using same phone, any time I called, Kehinde will be the one to answer. The first week I do not really understand because I can not really differentiate them. As time goes on, I started observing differences in them. I learnt that Twins maybe Identical but their behaviour can never be the same.
Fraternal twins
All pregnancies start when a sperm fertilises an egg. This fertilised egg is called a zygote.
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Sometimes a woman’s ovaries release two eggs, and two separate sperm fertilise each egg. This forms twins. These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins.
During pregnancy, the developing babies get oxygen and food from their mother through the placenta and umbilical cord. Fraternal twins have separate placentas and umbilical cords. The technical name for this is dichorionic.
Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different – for example, they might have different hair or eye colour. Occasionally they look quite similar.
Fraternal twins happen in about 70% of twin births in Australia.
Identical twins
Sometimes a fertilised egg splits within a few days of conception to produce genetically identical twins. Because these twins come from one zygote, they are also known as monozygotic. Identical twins are the same sex.
There are three types of identical twins.
About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas.
The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic.
In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac, called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins. They’re often called MoMo twins.
https://www.healthline.com/health/pregnancy/types-of-twins
Thanks for your time and thanks for reading.