The Twins Univitelinos Are those that come from the same zygote, this way two babies are formed with the same DNA.
This type of siblings are always of the same sex and possess the same type of blood. Divisions of the ovule determine whether the twins will share the same placenta.
There are several types of twins: bicyclists and monozygotic twins. Grouping inside these monocoriales and bicoriales
Factors for having twins
Firstly, the term twin refers primarily to two individuals or to one of two individuals who have shared the same uterus and who are generally born the same day. That is to say, they are two children produced from the same pregnancy.
Also, these twins can be monozygotic also known as identical twins or bicigóticos although they are also known as fraternal twins.
In the case of identical or univiteline twins, the embryos develop from a zygote that divides and gives rise to two embryos. And in the case of bicigóticos develop from different ovules.
In this latter type, each twin is fertilized by its own sperm cell and has its own placenta and its own amniotic sac.
As a curiosity, bicigóticos twins or bivitelinos as any brother, have a very small possibility to have exactly the same chromosomal profile. This type of twins can be of a different sex or the same sex, and like all siblings, they share 50% of their DNA.
Two-thirds of all twin births result in fraternal twins of the same sex and one-third are fraternal twins of different sex. About two-thirds of all births of this type of child are fraternal.
Univiteline or monozygotic twins
Univiteline twins are those that come from the same zygote, this happens when a single ovule fertilized by a spermatozoon is divided into two identical halves, that is to say, two separate babies are formed but with the same DNA.
This type of twins are always the same sex and also have the same type of blood and almost always share the same placenta but sometimes they may have separate placentas. Divisions of the ovule determine whether or not identical siblings will share the same placenta.
In this type of pregnancy, the division is carried out almost always within the first few days. If the embryo divides within 2-3 days after fertilization, then the twins are bicorial and biamniotic, meaning each embryo grows within its own amniotic sac and feeds on its own placenta.
But if the embryo divides between 3-8 days, then they are monocorial twins Y Biamnióticos sharing the placenta but growing each one in its amniotic sac.
Finally, if the embryo divides between 8-13 days then the twins are monocorial and monoamniotic. However, the latter are the less frequent because they share both the placenta and the amniotic sac with the embryo being the only one to be divided.
Within this group we can include the Siamese twins whose division of the zygote takes place after the 13th, being carried out incompletely and being united by some part of the body.
Twins bivitelinos or bicigóticos
On the other hand, pregnancies of fraternal, bicigtic or biviteline twins usually occur when two fertilized eggs are implanted in the wall of the uterus at the same time.
But when this happens, the process is carried out independently since two different sperm cells are fertilized resulting in two fraternal twins.
Bicigtic twins are essentially two common siblings who are born at the same time as they arise from two separate ovules fertilized by two independent spermatozoa, in the same way as normal siblings. In general, this is considered the most frequent type of twins.
The bicyclic twins have an extremely small chance of having the same chromosomal profile as any other sibling and even if they do, they would always have the same different genetic material on each chromosome due to their crossing during the meiosis process.
The bicygic or fraternal twins may seem similar particularly considering that they are the same age. However they may also look very different from each other.
The pregnancies of fraternal twins are more likely the woman meets some of these characteristics:
She is using fertility drugs, is over 40 years old, the woman is of Western African ancestry, is greater than the height and the average weight, had several previous pregnancies. It is a bicyclic twin. She has a family history of bicigótico twinning, infertility treatments increase the number of fraternal twins and about 35% of pregnancies of in vitro fertilization.
Approximately 1 in 90 human births is the result of a twin pregnancy.
The rate of bicyclic or fraternal twins varies greatly among ethnic groups, reaching about 4.5% for the Yoruba African ethnic group and 10% for Linha São Pedro, a small settlement in Brazil that belongs to the city of CândidoGodói; There one in every five pregnancies gives rise to twins.
Renowned Argentine historian Jorge Camarasa has presented a theory about the experiments of the Nazi physician Josef Mengele, who could be responsible for the high proportion of twins in the area. His research was rejected by some Brazilian scientists who had studied twins who lived in that region.
A high rate of twin pregnancies has also been observed in other parts of the world such as Igbo Ora in Nigeria as well as in Kodinji and Mohammadpur in India.
In a study of the maternity records of 5,750 women living in the Savannah area of Nigeria, 40 twins and 2 triplets per 1,000 births were found, with 26% of them being monozygotic.
The incidence of multiple births that was approximately five times greater than that observed in any western population was significantly lower than that of other ethnic groups living in the hot, humid climate of the south of that country.
This incidence was related to the age of the mother and had no association with the climate or the prevalence of malaria.
On the other hand, the use of fertility drugs are responsible for the so-called hyperovulation that is no more than the stimulated release of multiple ovules by the woman.
This has caused what some call a " Epidemic of multiple births " . In 2001, for the first time in the United States, the birth rate of twins exceeded 3% of all births. However, the rate of monozygotic or univiteline twins remains about 1 in 333 worldwide.
Behavior and environmental influences
The identical twins or univitelinos can behave differently with respect to any other brother since they develop their own personalities so that they can be identified as individual individuals.
Many identical twins spend most of their time together especially as children, so people often assume that they will behave in the same way.
However, this is not the case. Twins are unique individuals who establish their own individual likes and dislikes and there are usually obvious signs of differences when observed together or apart.
Univiteline twins, in turn, have identical DNA but have different environmental influences throughout their lives. This causes genes that can be activated or deactivated to be affected. This phenomenon is known as epigenetic demodification.
A study of 80 pairs of twins aged between 3 and 74 years showed that younger twins have relatively few epigenetic differences, meaning that the number of differences between identical twins increases with age.
According to the observation, the 50-year-old twins had more than three times the epigenetic difference of the younger twins. However, it was also shown that twins who had spent their lives separated by causes such as adopting two different parent families made the biggest difference.
Study of twins
Twin studies are those that reveal the importance of environmental and genetic influences for phenotypes, disorders and traits; It is considered as a key tool in behavioral genetics and in the content fields of biology and psychology.
These studies have been used to track traits ranging from personal behavior to the presence of mental problems such as schizophrenia.
At the practical level, twins are a valuable source for observation because they allow the study of environmental influence and variable genetic composition.
Identical or univiteline twins, share almost 100% of their genes, this means that most of the differences between them like intelligence, depression or susceptibility to boredom, are due to experiences that one of them has but the other does not .
In contrast, fraternal or biviteline twins share only 50% of their genes in the same way as a normal sibling so they share many aspects of their environment such as parenting, education, wealth and culture because they are born within from the same family.
The presence of a genetic trait in only one member of a pair of identical twins provides a window into the study of environmental effects. This is known as discordance.
In this way, the twins are also useful to show the importance of the unique environment specific to one twin or the other, when studying the presentation of traits.
Changes in the single environment may come from an event or incident that has only affected a twin, which could range from a head injury or a birth defect that one twin has presented, the other remaining healthy.
The classic twin design, compares the similarity of identical twins and fraternal twins. If identical or univiteline twins are considerably more similar than fraternal or biviteline twins, this would imply that genes play an important role in these traits.
By comparing hundreds of families with twins, researchers can understand more about the functions of genetic effects such as shared environment and unique settings in behavioral settings.
Modern studies on twins have shown that almost all traits are partly influenced by genetic differences with some characteristics that show a stronger influence, such as height, personality traits and some more complex inheritances.
Other findings of the study are that different genes affect various aspects of the trait as in the case of autism.
For reasons not yet known, while the woman is older there are more likely to have a multiple birth naturally.
In this sense, there are hypotheses that reflect that this is due to the higher level of follicle-stimulating hormone that older women have.
Sometimes multiple pregnancies may involve more than two fetuses. If there are three are called the group of children are called triplets. If there are four quadruplets, if there are five they are called quintuplets and so on.
Before the advent of drugs to stimulate ovulation, triplet pregnancies were very rare, about 1 in 8,000 births. The births of more quantities were almost so strange that they were little known.
The average duration of pregnancy is about 37 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.
At present, there is no way to prevent twins from being born prematurely. In women with single-fetal pregnancies, drugs called betamimetics can relax the muscles of the uterus and delay birth.
In this line, giving this drug could give more time to provide steroids for the development of the baby's lung or to transfer the mother to a hospital with a special care unit.
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