1 types of twins , zygosity
1.1 dizygotic (fraternal) twins
1.2 monozygotic (identical) twins
1.2.1 mechanism
1.2.2 incidence
1.2.3 genetic , epigenetic similarity
1.3 polar body , semi-identical twins
1.4 unknown twin type
types of twins , zygosity
the vast majority of twins either dizygotic (fraternal) or monozygotic (identical). less common variants discussed further down article.
twins can be:
female-female—sometimes called sororal twins
male-male twins—no special name
male–female twins—most common pairing; half of fraternal (dizygotic) twins male-female
among non-twin births, male singletons (about 5 percent) more common female singletons. rates singletons vary country. example, sex ratio of birth in 1.05 males/female, while 1.07 males/female in italy. however, males more susceptible females die in utero, , since death rate in utero higher twins, leads female twins being more common male twins.
zygosity degree of identity in genome of twins.
dizygotic (fraternal) twins
eight-month-old sororal twins napping
dizygotic (dz) or fraternal twins (also referred non-identical twins , dissimilar twins , biovular twins , and, informally in case of females, sororal twins ) occur when 2 fertilized eggs implanted in uterus wall @ same time. when 2 eggs independently fertilized 2 different sperm cells, fraternal twins result. 2 eggs, or ova, form 2 zygotes, hence terms dizygotic , biovular. fraternal twins are, essentially, 2 ordinary siblings happen born @ same time, since arise 2 separate eggs fertilized 2 separate sperm, ordinary siblings.
this common type of twin.
dizygotic twins, other siblings, have extremely small chance of having same chromosome profile. if happen have same chromosome profile, have different genetic material on each chromosome, due chromosomal crossover during meiosis. other siblings, dizygotic twins may similar, particularly given same age. however, dizygotic twins may different each other.
studies show there genetic proclivity dizygotic twinning. however, mother has effect on chances of having such twins; there no known mechanism father cause release of more 1 ovum. dizygotic twinning ranges 6 per thousand births in japan (similar rate of monozygotic twins) 14 , more per thousand in african countries.
dizygotic twins more common older mothers, twinning rates doubling in mothers on age of 35. advent of technologies , techniques assist women in getting pregnant, rate of fraternals has increased markedly.
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.
polar body , semi-identical twins
a 1981 study of deceased triploid xxx twin fetus without heart showed although fetal development suggested identical twin, shared placenta healthy twin, tests revealed polar body twin. authors unable predict whether healthy fetus result polar body twinning.
in 2003, study argued many cases of triploidity arise semi-identical twinning.
unknown twin type
in 2007, study reported case of pair of living twins, 1 intersex , 1 phenotypical male. twins both found chimeras , share of maternal dna half of father s dna. exact mechanism of fertilization not determined study stated unlikely case of polar body twinning.
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