Monozygotic (identical) twins Twin

1 monozygotic (identical) twins

1.1 mechanism
1.2 incidence
1.3 genetic , epigenetic similarity

monozygotic (identical) twins

comparison of zygote development in monozygotic , dizygotic twins. in uterus, majority of monozygotic twins (60–70%) share same placenta have separate amniotic sacs. in 18–30% of monozygotic twins each fetus has separate placenta , separate amniotic sac. small number (1–2%) of monozygotic twins share same placenta , amniotic sac. fraternal twins each have own placenta , own amniotic sac.

monozygotic (mz) or identical twins occur when single egg fertilized form 1 zygote (hence, monozygotic ) divides 2 separate embryos.

mechanism

regarding spontaneous or natural monozygotic twinning, recent theory proposes monozygotic twins formed after blastocyst collapses, splitting progenitor cells (those contain body s fundamental genetic material) in half, leaving same genetic material divided in 2 on opposite sides of embryo. eventually, 2 separate fetuses develop. spontaneous division of zygote 2 embryos not considered hereditary trait, rather spontaneous , random event.

monozygotic twins may created artificially embryo splitting. can used expansion of in vitro fertilization (ivf) increase number of available embryos embryo transfer.

incidence

monozygotic twinning occurs in birthing @ rate of 3 in every 1000 deliveries worldwide. (about 0.3% of world population).

the likelihood of single fertilization resulting in monozygotic twins uniformly distributed in populations around world. in marked contrast dizygotic twinning, ranges 6 per thousand births in japan (almost similar rate of identical twins, around 4–5) 15 , more per thousand in parts of india , on 20 in central african countries. exact cause splitting of zygote or embryo unknown.

ivf techniques more create dizygotic twins. ivf deliveries, there 21 pairs of twins every 1,000.

genetic , epigenetic similarity

monozygotic twins genetically identical , same sex unless there has been mutation during development. children of monozygotic twins test genetically half-siblings (or full siblings, if pair of monozygotic twins reproduces pair or same person), rather first cousins. identical twins not have same fingerprints, because in confines of womb, fetuses touch different parts of environment, gives small variations in same fingerprint, making them unique.

monozygotic twins have different phenotypes. due environmental factor or deactivation of different x chromosomes in female monozygotic twins, , in extremely rare cases, due aneuploidy, twins may express different sexual phenotypes, xxy klinefelter syndrome zygote splitting unevenly.

monozygotic twins, although genetically similar, not genetically same. dna in white blood cells of 66 pairs of monozygotic twins analyzed 506,786 single nucleotide polymorphisms known occur in human populations. polymorphisms appeared in 2 of 33 million comparisons, leading researchers extrapolate blood cells of monozygotic twins may have on order of 1 dna-sequence difference every 1.2 x 10 nucleotides, imply hundreds of differences across entire genome. mutations producing differences detected in study have occurred during embryonic cell-division (after point of fertilization). if occur in fetal development, present in large proportion of body cells.

another cause of difference between monozygotic twins epigenetic modification, caused differing environmental influences throughout lives. epigenetics refers level of activity of particular gene. gene may become switched on, switched off, or become partially switched on or off in individual. epigenetic modification triggered environmental events. monozygotic twins can have markedly different epigenetic profiles. study of 80 pairs of monozygotic twins ranging in age 3 74 showed youngest twins have relatively few epigenetic differences. number of epigenetic differences increases age. fifty-year-old twins had on 3 times epigenetic difference of three-year-old twins. twins had spent lives apart (such adopted 2 different sets of parents @ birth) had greatest difference. however, characteristics become more alike twins age, such iq , personality.