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Facts about Identical Twins

What Does Identical Mean?

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.

How Identical Twins Happen

Identical (or monozygotic) twins happen when afertilized 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 randomoccurrence. 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

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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.

How Identical Twins are Not Identical

There are two main factors that cause identical twins not to be truly identical:

Identical twins share the same DNA - but do not have identical DNA. When the egg splits into two halves

to form identical twins, the DNA may not divide equally between the two cells. The basic concept is

similar to when you cut an apple in half, the two halves may not look the same.

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.

Environmental factors both prenatal (for instance, one twin having more room in the womb, viruses,

genetic missteps, Twin-to-Twin Transfusion Syndrome, etc.) and after the babies are born (from

something as minor as appetite to illness or accidents). Prenatal developmental differences can mean

that one twin didn't quite "finish" a developmental stage that his or her twin did. For instance, an

undescended testes is not uncommon in baby boys born early - about 50% of boys are born with an

undescended testicle. In boy/boy twins it is not uncommon for one boy to have one or both testicles

undescended, while his brother has both descended. It is also not uncommon for one twin to have no

physical problems, and his or her twin having a heart or lung defect that is caused because the heart

Opposite Sex Identical Twins

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.

How Do You Know Your Twins are Identical?

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.

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Environmental Differences

While identical twins form with the same set of genes, human development is not just genetic. The

environment also has an impact. So, beginning in the early environment of the womb, external

influences can change the appearance of twins. For example, some monozygotic twins share a placenta.

One twin may have a more advantageous connection to the placenta, receiving the first run of nutrients.This situation can cause a size discrepancy between the babies, a physical difference that continues as

they grow up. Twin-to-Twin Transfusion Syndrome (TTTS) is another condition that affects twins in the

womb, and can impact their development.

While most twins grow up in the same home environment, there are many circumstances that create

differences in the childrens' appearances, personalities, and interests. As the twins approach the teen

years, they may even seek to establish dissimilar qualities in order to establish individual identities.

Epigenetic Differences

Scientists have offered a new explanation for the differences between identical twins. Epigenome refersto natural chemical modifications within a person's genome (genetic material). As an article in the New

York Times explains, they "act on a gene like a gas pedal or a brake, marking it for higher or lower

activity."

A study conducted by a team of researchers at the Spanish National Cancer Center in Madrid concluded

that, while identical twins are born with the same epigenome, their epigentic profiles begin to diverge as

they age. The differences increase as twins live longer and spend more time apart. The scientists offered

two theories to explain this phenomenon. First, that epigentic marks are removed randomly as people

age. Secondly, environmental influences change the pattern of epigentic marks.

In a Washington Post article Dr. Manel Esteller, lead researcher, said that "small epigenetic events

before birth probably account for many of the minor distinguishing differences in the appearance,

personality and general health of young twins."

The research is significant because changes in the epigenome may be responsible for the development

of disease, like cancer. It's hoped that further study of the epigenome in identical twins will help

researchers pinpoint factors that contribute to cancer.

Further Research: Not Actually Identical

A study published in the March 2008 issue of The American Journal of Human Genetics offers further

explanation, even challenging the accepted notion that identical twins have identical genetic profiles.

The research found changes in the DNA sequence between identical twins, reflected in Copy Number

Variations (when a gene exists in multiple copies.) The researchdid not confirm whether these changes

occur during fetal development or as twins age.