In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes. Semi-identical (sesquizygotic) twins
Monozygotic twins can develop differently, due to their genes being differently activated. More unusual are "semi-identical twins", also known as "sesquizygotic". As of 2019, only two cases have been reported. These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova, both of which are viable for fertilization. Both ova are then fertilized, each by a different sperm, and the coalesced zygotes undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, with different paternal genetic information and identical maternal genetic information. This results in a set of twins with identical gene sequence from the mothers side, but different sequences from the fathers side. Cells in each fetus carry chromosomes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded. In 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their fathers genetic makeup. The twins were both found to be chimeras. One was an intersex XX, and one a XY male.
These studies have converged on the notion that there is a greater rate of delay in language development in twins compared to their single-born counterparts. The reasons for this phenomenon are still in question; however, cryptophasia was thought to be the major cause. Idioglossia is defined as a private language that is usually invented by young children, specifically twins. Another term to describe what some people call "twin talk" is cryptophasia where a language is developed by twins that only they can understand. The increased focused communication between two twins may isolate them from the social environment surrounding them. Idioglossia has been found to be a rare occurrence and the attention of scientists has shifted away from this idea. However, there are researchers and scientists that say cryptophasia or idioglossia is not a rare phenomenon. Current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language. Animals
Twins are common in many mammal species, including cats, sheep, ferrets, giant pandas,Cow,dolphins, dogs, deer, marmosets, tamarins, and elephants. The incidence of twinning among cattle is about 1–4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. A female calf that is the twin of a bull becomes partially masculinized and is known as a freemartin. See also
References
Further reading
Bacon, Kate.