What precisely does the word 'identical' mean in the case of identical twins?

Question

Their are a couple of twins I know and a few of my friends and I were discussing them.

I know that if two twins came from seperate eggs that were seperately fertilised then they are just 'siblings sharing a womb'.

Now, if they are both from a single fertilised egg that subsequently seperated they have 'identical' DNA. Is it possible that they may still look slightly different?

The twins I know look too much alike to be non identical but they can just about be individually recognised; especially when they are next to one another.

So are they still identical?

Also - while I'm here - some of my friends maintained that identical twins could be different sexes. I just need conformation on this. If two people have identical DNA then they must also both have XX or XY chromosomes so therefore they MUST be the same sex. Am I right?

Answer 1

Don't forget that lifestyle choices will make identical twins look different as well. You're right that they must be the same sex.

Interesting side note - I had a friend who was a triplet from a boy/boy/girl set. But the brothers looked so much alike into their 20's (I dated one and had trouble telling them apart once in a while...) I thought they must have been identical twins who happened to have a fraternal twin sister too!

Answer 2

Most identical twins will look exactly the same, because they share genes that will express the same phenotype, or physical traits. However, siblings, as you may have seen, can look very similar too, even though they are not identical. You should ask your twin friends if you want a clear cut answer.

Identical twins cannot be different sexes. Identical twins occur when on fertilized egg is split into two, therefore they have identical chromosomes, and as you said, this includes the sex chromosomes. They must both be XX or both XY.

Answer 3

Identical twins only come from one egg that has split to make 2 identical embryo's. They are the same sex always & will look exactly the same.. (NOTE, their might be slight difference's a person who knows them well can see such as a habit they each do, if they hold their head different, tip their mouth different, make expressions different, but technically speaking they are identical.

Faternal twins are twins but each do not share the same egg, they were created from 2 completely separate eggs. They can be of different sex & can look as different as day & night. They can possibly look the same like identical twins, but not be identical technically.

Answer 4

Identical twins have identical DNA, because they came from the same fertilized egg. Yes, they may look very slightly different, but unless they make a point of it (such as wearing different color clothes) or have acquired differences (such as a scar) only people who know them well can tell them apart. Identical twins are often mirror images of each other, so one may be left-handed and the other right-handed. They will always be the same gender.

Answer 5

It is not possible to have "identical" twins that are not the same sex. That I do know for a fact. Because, let's face it, if they are NOT the same sex, how can they be identical. It's like saying an apple and an orange are the same! So yes, you are right.

Answer 6

Identical is best defined by deriving from a single egg. Environmental factors may shape appearance. Identical twins will be the same sex.

Answer 7

Identical twins do have identical DNA, so its impossible for them to be different genders. The twins that have different dna are called fraternal.

Answer 8

slightly but not much