Reading 3

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 twinningTwin-to-Twin Transfusion Syndromemitochondrial 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 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 actually 'breaks' (or splits) into two instead of dividing into two.

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.


Opposite Sex Identical Twins

Identical Twins will almost always be of the same sex. However, there have been some 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.


1. What is the purpose of this article?

2. What does the word “they” in paragraph one refer to?

a) a parent    b) identical twins   c) DNA  d) Xerox


3. The word Xerox in paragraph one means:

a) a machine    b) a twin   c) an exact copy  d) a difference


4. The term “misnomer” is a noun that describes:

a) missing information   b) an inaccurate term   c) an identical twin d) a medical experiment


5. After reading the underlined sentence in paragraph 2, it can be inferred that the author is:

a) a doctor   b) a scientist   c) a woman        d) a man


6. How do identical twins happen?


7. What are “conjoined” twins? How does this happen?


8. The pronoun “this” in the last paragraph refers to:

a) Identical twins of the same sex    b) A set of identical boys    c)A case of opposite sex identical twins


Extra Credit:

9) After reading the last paragraph, it can be inferred that opposite sex twins does not happen in:

a) some countries       b)a set of male twins            c)  a set of female twins          d) 1 out of 2500 births

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