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Any parent of identical twins will tell you - there is no such thing as identical. Each twin has his or her own separate personality, and many physical differences. They share the same DNA, but they are not Xerox or carbon copies of each other, and that can make all the difference!
The term "identical" is a misnomer that many medical experts do not use. The preferred term is "monozygotic", which simply translated means "from one egg". Experts - and many parents of identical twins - argue that "identical" twins are not identical twins and should not be labeled as such; the term simply generates too much confusion. There are many 'self-professed' experts on the web (those with no actual training or experience with twins of any type) that proclaim that since the twins started as a single egg, they are identical, and the only differences that will be seen between the two are physical changes that happen after the babies are born. As a mom of identical twin boys (verified by DNA testing) who are not identical, I can tell you that these people are wrong, in small part because they do not take into account mirror twinning, Twin-to-Twin Transfusion Syndrome, mitochondrial DNA, the difference between developmental and genetic characteristics, and how genes in the DNA express themselves once the one egg divides into two.
Identical (or monozygotic) twins happen when a fertilized egg splits into two separate babies - usually within the first twelve days of Mom's pregnancy. After twelve days, the egg may not split entirely and result in conjoined twins. Some people theorize that the later in the pregnancy the egg splits (between days 8 and 12), the more likely the twins will have similar characteristics since they had the additional time to lay "identical" genetic groundwork.
As far as scientist can determine, there is no reason why the egg splits - it's a spontaneous and random occurrence. One theory is the egg ovulated by maternally mature women (Moms over 35) is not as flexible as it is in younger moms. When a fertilized egg begins its natural division process in creating a baby, the egg actual 'breaks' (or splits) into two instead of dividing.
Identical twins have almost identical brain wave patterns. They also can share very similar physical characteristics; for instance, if one twin's tooth doesn't grow in, his or her identical twin will most likely miss the tooth as well. Mirror twins are identical twins with mirror characteristics - one may have a dimple on the left side, and the other a dimple on the right.
There are two main factors that cause identical twins not to be truly identical:
I don't want to get to complicated with the explanation, but DNA differences of identical twins revolve around the mitochondrial DNA (DNA that is not located in the nucleus of a cell).
Mitochondrial is DNA passed on to the child by Mom. A fertilized egg will have Mom's mitochondrial DNA, half of Mom's nuclear DNA, and half of Dad's nuclear DNA. When the fertilized egg splits into two, the twins will each have identical nuclear DNA, but not EXACTLY identical mitochondrial DNA. Physical differences in identical twins are contributed, in part, to how much and how similar the mitochondrial DNA each twin inherited from Mom expresses itself.
Identical Twins will almost always be of the same sex. There have been 3 - 5 documented cases of opposite sex identical twins. This can happen in a set of identical boys, when one twin "drops" or "looses" a "Y" chromosome, resulting in a single chromosome, or an "XO" chromosome. Without a "Y" chromosome, the twin becomes a girl with a single chromosome, and will have Turner's Syndrome. Turner's Syndrome itself is not that uncommon, occurring in about 1 out of 2500 live female births. For more information about Turner's Syndrome, visit http://www.turner-syndrome-us.org.
Experienced ultrasound techs are very good at giving a best guess during appointments as to whether twins are identical or fraternal. Testing can also been done on the placenta after the babies are born to determine if twins are fraternal or identical. However, since identical twins can look different, and fraternal twins can look very much alike, in some cases the only way to truly determine if twins are identical or fraternal is by DNA testing.