Twin

[24] Regarding spontaneous or natural monozygotic twinning, a 2007 theory related to in vitro fertilization (IVF) proposes that monozygotic twins may be formed when a blastocyst contains two inner cell masses (ICM), each of which will lead to a separate fetus, rather than by the embryo splitting while hatching from the zona pellucida (the gelatinous protective coating around the blastocyst).

Identical twins do not have the same fingerprints however, because even within the confines of the womb, the fetuses touch different parts of their environment, giving rise to small variations in their corresponding prints and thus making them unique.

The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for 506,786 single-nucleotide polymorphisms known to occur in human populations.

Specifically, the study found that vegan mothers (who exclude dairy from their diets) are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation.

[54] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births (4.5%) for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.

[56] The Argentine historian Jorge Camarasa has put forward the theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area.

His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation.

[citation needed] In a study on the maternity records of 5750 Hausa women living in the Savannah zone of Nigeria, there were 40 twins and 2 triplets per 1000 births.

The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of the country.

A 15-year German study[67] of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.

Possibly the worldwide record for the duration of the time gap between the first and the second delivery was the birth of twins 97 days apart in Cologne, Germany, the first of which was born on November 17, 2018.

The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart, liver or lungs.

The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin coloration in human chimeras.

This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus.

Another abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated.

In their seminal publication, Gabbett, Fisk and colleagues documented a second case of sesquizygosis and presented molecular evidence of the phenomenon.

Mirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12.

This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (situs inversus).

[96] Non-human dizygotic twinning is a common phenomenon in multiple animal species, including cats, dogs, cattle, bats, chimpanzees, and deer.

This commonly repeated behavior in larger mammals evolved as a fixed, naturally-selected adaptation, resulting in a decreased twinning propensity in species such as giraffes, elephants, and hippopotami.

Despite this adaptation, a case of rare monozygotic twinning has been documented in two elephant calves at the Bandipur Tiger Reserve in Karnataka, India.

Additionally, the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment (Rickard, 2022, p. 2).

Through their studies on Vespertilionidae and Cebidae species, scientists Guilherme Siniciato Terra Garbino (2021) and Marco Varella (2018) have proven that smaller species experiencing infertility in old age and/or unstable habits as a result of increased predation or human interference can experience have undergone natural selection in gaining even higher twinning propensities.

In his study on the evolution of litter size in bats, Garbino discovered that the vespertilionidae genus has higher twinning propensities as a result of their high roosting habitats.

When tracked phylogenetically, scientists determined that the common ancestor of bats carried a higher twinning propensity which was then lost, and picked up again, eighteen times in evolutionary history.

The height and exposed nature of Vespertilionidae's roosting locations resulted in a sharp increase in species mortality rate.

This means that despite the family's high exposure to factors that would seemingly increase mortality rates, Vespertilionidae counteracts their environmental conditions through the evolutionary adaptation of dizygotic twins.

The smaller size of the cebidae genus also makes these species more susceptible to predators, thus triggering the heightened pace of birth, maturation, reproduction, and death.

Despite their access to resources, the cebidae genus has a high mortality rate attributed to their size, meaning that in order to "keep up" their quickened lifecycle, they must produce an excess of offspring in ensuring generational survival.

The positively-selected adaptation of twinning counteracts the genus's high mortality rate by giving older mothers the chance to produce more than one offspring.