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The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. [5] The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births, [6] [7] [8] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may ...
The word zygosity may also be used to describe the genetic similarity or dissimilarity of twins. [6] Identical twins are monozygotic, meaning that they develop from one zygote that splits and forms two embryos. Fraternal twins are dizygotic because they develop from two separate oocytes (egg cells) that are fertilized by two separate sperm.
The power of twin designs arises from the fact that twins may be either identical (monozygotic (MZ), i.e. developing from a single fertilized egg and therefore sharing all of their polymorphic alleles) or fraternal (dizygotic (DZ), i.e. developing from two fertilized eggs and therefore sharing on average 50% of their alleles, the same level of genetic similarity found in non-twin siblings).
There is no sign of any membrane between the fetuses. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right. Ultrasound is the only way to detect monoamniotic-monochorionic twins before birth. [4]
From twin studies is typically estimated at 0 because the correlation between monozygotic twins is at least twice the correlation for dizygotic twins. When using the Falconer variance decomposition ( 1.0 = a 2 + c 2 + e 2 {\displaystyle 1.0=a^{2}+c^{2}+e^{2}} ) this difference between monozygotic and dizygotic twin similarity results in an ...
However, there was no difference in the handedness frequency between monozygotic and dizygotic twins. [ 8 ] Currently, there is not much evidence to further prove the idea that monozygotic twins have a higher prevalence for left-handedness using the pathological left-handedness syndrome because of the improvements within medicine causing a ...
Studies of twins have shown that genetic traits of monozygotic twins are fully concordant, whereas in dizygotic twins, half of genetic traits are concordant, while the other half are discordant. Discordant rates that are higher than concordant rates express the influence of the environment on twin traits. [2]
Twin concordances for seven psychological traits (sample size shown inside bars), with DZ being fraternal and MZ being identical twins. Heritability for traits in humans is most frequently estimated by comparing resemblances between twins.