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Superfetation (also spelled superfoetation – see fetus) is the simultaneous occurrence of more than one stage of developing offspring in the same animal. [1] [2] [3]In mammals, it manifests as the formation of an embryo from a subsequent menstrual cycle, while another embryo or fetus is already present in the uterus.
Heteropaternal superfecundation is common in animals such as cats and dogs. Stray dogs can produce litters in which every puppy has a different sire. Though rare in humans, cases have been documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents had been involved in paternity suits. [6]
Twins are by far the most common form of multiple births in humans. The U.S. Centers for Disease Control and Prevention report more than 132,000 sets of twins out of 3.9 million births of all kinds each year, about 3.4%, or 1 in 30. [5] Compared to other multiple births, twin births account for 97% of them in the US. [6]
Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos. [2] In humans, there are two forms of twinning that can lead to two heads being supported by a single torso ...
The twin reversed arterial perfusion, or T.R.A.P. sequence, results in an 'acardiac twin', a parasitic twin that fails to develop a head, arms and a heart.The parasitic twin, little more than a torso with or without legs, receives its blood supply from the host twin by means of an umbilical cord-like structure, much like a fetus in fetu, except the acardiac twin is outside the autosite's body.
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).
In monozygotic twinning for conjoined twins such as ischiopagi, the twins form by the splitting of a bi-laminar embryonic disc after the formation of the inner cell masses. Thus, making the twins occupy the same amnion which can lead to a conjoining of the twins as a result of the twins not separating properly during the twinning process.
) and the interpolation problem consists of yielding values at arbitrary points (,,, … ) {\displaystyle (x,y,z,\dots )} . Multivariate interpolation is particularly important in geostatistics , where it is used to create a digital elevation model from a set of points on the Earth's surface (for example, spot heights in a topographic survey or ...