Studying Twins

Conjoined twins The Tocci twins, Giacomo and Giovanni. We sometimes say that two people are so close that they appear joined at the hip – metaphorically speaking, of course. But imagine this were a reality. (Indeed, think about how the phrase might have arisen.) What if you came from an egg that started to separate into identical twins but got stuck (or, as the alternative theory has it, were one of two embryos that became fused together) attaching you to your double for a lifetime? As we shall see in these seven cases from the late 19th and early 20th century, natures whim can join twins in the strangest of places – from the breastbone to the buttocks – making life difficult, to say the least. Given the state of medical knowledge at the time, surgical separation would have been out of the question in many cases, even if no organs were

Chang and Eng in later life. The Bunker twins died on January 17, 1874, after Chang fell ill with pneumonia and died in his sleep. Even though the surgery of the day may have been sufficient to finally separate the twins, Eng is said by some sources to have refused the operation and in any case died a few hours later. The Bunkers shared liver can be seen in Philadelphias Mütter Museum to this day.

A twin is one of two offspring produced in the same pregnancy. [1] Twins can either be monozygotic ("identical"), meaning that they develop from one zygote that splits and forms two embryos, or dizygotic ("fraternal") because they develop from two separate eggs that are fertilized by two separate sperm.offspringpregnancy [1]zygotesperm

Zygosity Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning. The three most common variations are all dizygotic (fraternal): Zygositygenome Male–female twins are the most common result, 50 percent of dizygotic twins and the most common grouping of twins. Female–female dizygotic twins (sometimes called "sororal twins") Male–male dizygotic twins The other two variations are monozygotic ("identical") twins: Female–female monozygotic twins Male–male monozygotic twins (less common) Male-female monozygotic twins (Very rare where no gender is originally determined and then chosen) Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female, [12] while it is 1.07 males/female in Italy. [13] However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins. [12] [13]

Fraternal or dizygotic (DZ) twins Fraternal or dizygotic (DZ) twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, in cases of females, sororal twins) usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular.fertilizedspermzygotes Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings, dizygotic twins may look similar, particularly given that they are the same age. However, dizygotic twins may also look very different from each other. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal and/or sororal twins are simply siblings who happen to be the same age.siblings Studies show that there is a genetic basis for dizygotic twinning. However, it is only their mother that has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries. [6]ovum [6] Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. [14] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. [14]

Actresses Mary-Kate and Ashley Olsen, well-known dizygotic twins.Mary-Kate and Ashley Olsen

Monozygotic ("identical") twins Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.placentaamniotic sacs Monozygotic (MZ) or identical twins occur when a single egg is fertilized to form one zygote (hence, "monozygotic") which then divides into two separate embryos.eggzygote embryos

Mechanism Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. [15] Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event. [14][16]blastocyst [15] [14][16] Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer. [17]IVFembryo transfer [17]

TypeDescriptionDay Dichorionic- Diamniotic Normally, twins have two separate (di- being a numerical prefix for two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or "DiDi". It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts [33] ), in 99.7% of all pregnancies, [34] and in 18–36% [35] (or around 25% [33] ) of monozygotic (identical) twins. DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. [36] numerical prefixchorionsamniotic sacs [33] [34] [35] [33]monozygotic [36] Dichorionic- Diamniotic twins form when splitting takes place by the third day after fertilization. [33] fertilization [33]