Are Identical Twins Truly Identical in Every Way?"There are two main factors that cause identical twins Not to be truly identical:
1. 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. 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.
2. 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."
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