Thursday, November 17, 2011

Forum Thread: Monozygotic ("Identical") Twins

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Definition of "Identical Twin" from Wikipedia:
Monozygotic ("identical") twins Monozygotic (MZ) occur when a single egg is fertilized to form one zygote (hence, "monozygotic") which then divides into two separate embryos.

Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo.

Eventually, two separate fetuses develop. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.
Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.

The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world. This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India[16] and up to over 20 in some Central African countries. The exact cause for the splitting of a zygote or embryo is unknown.

In-vitro fertilization techniques are more likely to create dizygotic [fraternal] twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.
Genetic and epigenetic similarity
Monozygotic twins are genetically identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test as half-siblings (or full siblings, if monozygotic twin sisters reproduce with monozygotic twin brothers), rather than first cousins. On rare occasions, monozygotic twins may express different phenotypes, normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally from an XXY Klinefelter's syndrome zygote splitting unevenly.

Monozygotic twins actually have only nearly identical DNA, and differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification.

A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between "identical" twins increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.
Phenotype similarity
Contrary to common opinion, monozygotic twins are not always of the same phenotypical sex. There have been described cases where monozygocity resulted in 46,XO (i.e. female with Turner syndrome) and 46,XY (i.e. male). This is thought to be due to unequal distribution of zygotic protoplasm. However, as a rule, traits and physical appearances of MZ twins are very similar.

Monozygotic twins do generally look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, monozygotic twins often look less alike or more alike because of lifestyle choices or external influences.

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