I was watching a show on Discovery Channel that said all identical twins are not the same sex?

Question

I think the show was called "Multiples". Where can I find more information on the subject, as I was always taught identical had to be the same sex?

The show said that after fertilization, and after the egg had split into two, minute differences in environment, such as temperature, could cause one half to develop as male and the other as female.

Answer 1

If temperature was cited as a factor then perhaps these were reptile twins (not a joke). Crocodiles don't use genetics to choose sex, its based on nest temperature. This sort of show has been aired on discovery so perhaps you are confusing things here. Human sex is not controlled by temperature, since every womb should be the same, body temperature. Identical human twins (one egg splitting) must have the same sex chromosomes and hence the same sex. However two eggs and two separate fertilizations can also happen at the same time (two eggs), these fraternal twins can be different sex.

Answer 2

Identical twins must be the same gender. Identical twins are from one fertilized egg that might happen to split into two embryos, making their genetic make up identical. Twins that are of different gender must be fraternal twins, that is, there are two eggs within the female at the same time, and they are both fertilized by different sperm cells.

However, identical twins _will_ have genetic differences, especially as their life progresses, due to environmental factors which can turn on/turn off genes. Don't really personally know how that works, so I'll just give you a link to wikipedia if you don't mind: http://en.wikipedia.org/wiki/Epigenetics

Answer 3

Identical twins occur when a single fertilized egg splits into two embryos, hence identical twins are genetically identical, and could not be of different sexes unless chromosomal damage occurred (for example, both embryos being XXY), in which case the chances of both embryos surviving to birth AND showing different sexual characteristics would be almost 0. In short, identical twins should always be the same sex, I've never heard of different-sex identical twins, but there's a remote possibility it could happen if something went wrong during development. However, at least one of the twins would be a genetic aberration, and probably sterile.

FRATERNAL twins occur when the ovaries accidentally release two eggs into the uterus at once, in which case the embryos are not genetically identical, and could certainly be of different sexes. It happens surprisingly often.

Answer 4

Identical means that you have the same set of genes. There are some conditions where an individual may have a pair of XY chromosomes but the Y does not become expressed. If the Y is expressed in one twin but not the other the one expressing Y will be a male and the other female.

Answer 5

Identical twins are not identical twins if they are not genetically identical! I don't know how that could happen... I know they can look differently because of having identical genes that can go one way or the other, like my twin and I where I have a narrower face and she has a rounder face, but I don't think gender works like that.

Answer 6

Identical means the same in appearance, right down to the sex. Anything else would be fraternal twins (who, by the way, can look fairly close and be mistaken for identical twins).

Answer 7

In placental mammals (including us), all identical twins are the same sex.

You can get different sex identical twins from some reptiles and some fish which have environmental and not genetic sex determination.

Answer 8

Identicals DO have to be the same sex. Either the show was wrong, or you misheard it.