Professor Owen made an astute observation: In outbred cattle, dizygotic twins that share the same placenta occur occasionally. Indeed, if one examines peripheral blood samples from such twin pairs, each has blood cells derived from the other, indicating shared circulatory systems in utero. Surprisingly, even though they are not genetically identical, they accept tissue grafts from one another.

(a) In as much as these are dizygotic twins and thus are not genetically identical, does their mutual graft acceptance violate the "isografts succeed, allografts fail" rule?

(b) What does this experiment tell you about the underlying mechanism of that last property of the immune system - non-responsiveness to "self?"

When talking about dizygotic and not monozygotic twins, they share the uterus, but they do not share the bag, nor the nutritional sources nor the placenta, if they can share the same amniotic sac depending on how far one ovum separated from another.

Thus, being two different ovules and fertilized by two different sperm (unlike homozygotes, which is an ovum fertilized by a sperm that divides into two and share the same development medium) they present the same genetic load but not the expression of this or the same phenotype since they develop in two different environments, this was raised in certain investigations and it was presented that even the different location that they present in the uterus can influence the different phenotypic manifestations of the genome.

There are studies that maintain that immunosuppressive treatments in grafts between monozygotic twins do not make sense and should not be done since it was experimented in certain cases and the patients had no problem of rejection in allografts, since it is argued that they have the same genotype and this genotype. It is usually expressed in the same way, therefore it is rare that the immune receptors and the immune response differ and consider something foreign to a tissue that presents histocompatibility almost 100 percent.

In contrast, in dizygotes it is different, allografts are made but under protocol immunoregulatory treatments that prevent rejection of the allogeneic implant or graft. Thus, certain researchers argue that having developed other phenotypes between twins, this causes the histocompatibility complex to express itself differently and thus the immune response does not recognize it as its own.

Explanation:

With the wording then we can conclude that the dizygotic twins corrupt the myth that transplants of tissues or organs can be performed without immunomodulatory treatment, if or if drugs must be administered to avoid rejection of said tissue or organ.