The Sameness of Twins: A Critique of Literature

The root word of twinning is twine, suggesting a double thread. This is acknowledged as global human trait and is intrinsically part of what is inherited. The incidence of twinning differs, ranging from 1 out of 30 births among Nigerians, to as rare as 1 in 150 births among Japanese. The distinction between monozygous and dizygous twins lies in the fact that monozygous twins arise from the same ovum, while dizygous twins hail from separate ova (Medawar & Medawar, 1983; Thompson & Thompson, 1980).

Figure 1. Formation of identical and fraternal twins.


It has been widely acknowledged in biological sciences (Medawar & Medawar, 1983), genetics (King, 1968), and immunology (Humphrey & White, 1970) that monozygous twins are “genetically identical”. However, the current paper now comes to question where this identicality ceases. Their differences are attested to by the physical and pathologic distinctions found across empirical studies (Milne, 1976; Osborne & De George, 1959;  Schimke, 1978; Stern,1973).  Schimke (1978) asserts that these distinctions come from “environmental differences, cytoplasmic differences, equations of heritability versus variability, non-penetrance of the heritable mutant gene, gonadal mutation, and premutation.”

Distinguishing the Different Twin Types                                                                      

There are two primary twin types, namely identical and non-identical. The latter is also tagged as fraternal twins. The following section briefly describes and distinguishes the various twin types.

Identical twins. This type develops when one fertilized ovum divides into two individual entities during the initial phases of cell division, causing DNA and other genetic material to be common. In effect, they have similar genotypes and phenotypes; that is, they have the exact physical and genetic make-ups. They always have the same sex.

Siamese or conjoined twins. One modification of identical twins is Siamese or conjoined twins, brought about by the partial split of the single fertilized ovum during the early stages of conception. They share the same characteristics of identical twins, except for the fact that they are conjoined at some bodily points. In some instances, they may also share certain organs. Conventionally, the bodily points where they are joined encompass the head, chest, stomach, and hips.  

Figure 2a. Conjoined twins.

Figure 2b.  Craniopagus twins.

Figure 2c. Radiographic of craniopagus twins.

Figure 2d. Radiographic of craniopagus twins who faced in opposite directions.

Mirror twins. Yet another modification of identical twins is mirror twins, which are markedly rare. This twin pair has organs on opposite sides; for example, one twin will have the heart on the right side instead of the left and will be opposite handed to the other twin.”

Fraternal twins. The more usual twin type are fraternal twins, who are distinct both in genotype and phenotype. They may be likened to any other ordinary sibling pair. They develop when two individual ova are fertilized by two distinct sperm cells, sharing a common intrauterine area and environment. They may be unique both in terms of sex and physical makeup. (). While identical twins exhibit full likeness genetically; fraternal twins are only alike 50%. Identical twins have a common exact genetic structure since they have started from one ovum. During the initial days of conception, the egg is divided and develops into two distinct individuals. The rare case of Siamese twins transpires when such division transpires incompletely. Identical twins are contrasted against fraternal twins, who develop when two eggs are released during ovulation, ad both undergo fertilization. These individual fertilizations permit any gender combination to ensue from the fertilized ova (Plourde, 1986).

Figure 3. Formation of fraternal twins.



Twin Studies Across Decades

There has been significant and ample research attention dedicated to twin studies over the past decades, proven by the burgeoning number of centers devoted to conducting twin studies (Blakeslee, 1978). It has been implicitly believed that these researches involved identical individuals, whose differences were caused by environmental determinants.  The following description from Blakeslee (1978) attests to the proliferation of twin studies:

”That the registry, in a single country, of the health data of 50,000 pairs of MZ twins between 1870 and 1930, and, in another country of the medical histories of 23,000 living MZ pairs born between 1886 and 1973, and many such registries elsewhere have merely piled statistics upon statistics and theories upon theories, without throwing light on any single problem, bears testimony to the fundamental point that has been missed: MZ twins are highly similar, but not identical” (p. 33)

Distinguishing Between Monozygotic and Dizygotic Twins

Determining whether a twin pair is of a monozygotic or a dizygotic nature remains an issue of contention in twin research (Osborne & De George, 1959). To undertake this, an analysis of the sameness of phenotypes is done and increased similarity is taken to mean that the pair is monozygous (Stern, 1973). However, this undermines the fact that even monozygous twins do have marked differences as is reflected by Osborne & De George’s (1959) assertion: "the within-pair differences of monozygotic twins are frequently found to have a wide and continuous range from near identity to great dissimilarity." Moreover, they state that Siamese twins which are assumed to be monozygous, exhibit even stronger distinction than their dizygotic counterparts (Osborne & De George, 1959).

Dubos (1968) has emphasized the uniqueness of each individual, and does not present monozygotic twins as exceptions. For instance, the acid test of the possibility of transplanting intrapair tissue supports this proposition (Burnet, 1971). The principle that such tissue compatibility of twins is brought about by genotype identicality ought to be revisited in lieu of the fact that such may also be observed in binovular, genetically distinct twin calves (Humphrey & White, 1970). These were further supported by empirical studies by Medawar et al among cattle twins (Medawar et al 1977) and among humans (Dunsford et al, 1953). The success of transplantability must not be readily attributed to genotype identicality; instead, it is explained by the “tolerance through clonal elimination” caused by apt and timely exposure to the other’s cells (Simpson, 1973). The popular tissue compatibility between monozygous twins is completely reliant on their “mutual chimerism” by virtue of their placental connection – a condition that is also observed among binovular twins with different genotypes. Out of 3 twin human being sets, 2 are inclined to become monoplacental while one has the tendency to be biplacental (Fox, 1978).  Monoplacental monozygous twins mediate chimerism while biplacental monozygous twins reject it. Thus, those twin pairs who are dichorial do not have such tissue compatibility, and tend to exhibit rejection similar to everyone else. The time of splitting of the monozygote determines whether it shall be monoplacental or biplacental. If this occurs not later than the third day of conception, then dichorial placentation shall ensue. On the other hand, if this transpires between the 3rd to the 8th day, this results in a monochorial placenta. Ranging from the 8th to the 13th day, the placenta remains monochorial and the amnion, too. Following the 13th day, the monochorial and monoamniotic placenta may be related to Siamese twins (Kothari & Mehta,  1985).

Figure 4. Early splitting vs. late splitting of the zygote.


This is the logical explanation behind the tissue compatibility among monozygous twins; that is, they attain such tolerance for transplantation to develop as chimeras because of their usual monoplacentability. Such chimerism has yet to be empirically investigated through contemporary cytogenetics. Moreover, dizygotic twins may share a placenta in contrast with monozygous twins; thus, giving the former greater tissue compatibility (Osborne & De George, 1959; Stern, 1973). Medawar’s (1977) description of strongly inbred animals as highly pure organisms, who cease to have genetic distinctions; thus resembling twins. This is not validated; in fact, intrastrain grafts of strongly inbred animals do result in rejections. The same may apply for monozygous twins. While such pairs do exhibit strikingly high similarities in both genotype that their subtle differences easily go unnoticed (Humphrey &White, 1970; Osborne & De George, 1959). The foregoing arguments effectively explain why monozygotic twins do exhibit tissue compatibility, and yet are not exempt from rejection (Kothari & Mehta, 1985).  

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In contemporary times, twin research still remains popular, providing means for comprehending the most profound genetic determinants of human behavior (Plourde, 1986).

Distinctions between Twins: Outcomes of Twin Research

Fraternal twins are also known as dizygotic twins, and they hold exactly 50% of their genes with each other. While they are not as similar as identical twins for understanding genetic factors, they do result in interesting findings in comparison with identical twins. Fraternals may be likened to first-degree kin, minus the similarity in age as do identical twins. Twin research are dependent on both fraternals and identicals. These investigations ...

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