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For example, the sex ratio of birth in the US is 1. Zygosity is the degree of identity in the genome of twins. Dizygotic DZ or fraternal twins also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case 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.
Fraternal twins are, essentially, two ordinary siblings who happen to be born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like ordinary siblings.
This is the most common type of twin. Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile.
Even if they happen to have the same chromosome profile, they will always have different genetic material on each chromosome, due to chromosomal crossover during meiosis.
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.
Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who 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.
Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 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.
Eventually, two separate fetuses develop. Monozygotic twins may also be created artificially by embryo splitting.
It can be used as an expansion of in vitro fertilization IVF to increase the number of available embryos for embryo transfer.
Monozygotic twinning occurs in birthing at a rate of about 3 in every deliveries worldwide. The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world.
IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1, Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development.
The children of monozygotic twins test genetically as half-siblings or full siblings, if a pair of monozygotic twins reproduces with another pair or with the same person , rather than first cousins.
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.
Monozygotic twins always have the same phenotype. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy , twins may express different sexual phenotypes , normally from an XXY Klinefelter syndrome zygote splitting unevenly.
Monozygotic twins, although genetically very similar, are not genetically exactly the same. The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for , single-nucleotide polymorphisms known to occur in human populations.
Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 1.
If they occur early in fetal development, they will be present in a very large proportion of body cells.
Another cause of difference between monozygotic twins is epigenetic modification , caused by differing environmental influences throughout their lives.
Epigenetics refers to the level of activity of any particular gene. A gene may become switched on, switched off, or could become partially switched on or off in an individual.
This epigenetic modification is triggered by environmental events. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences.
The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins.
Twins who had spent their lives apart such as those adopted by two different sets of parents at birth had the greatest difference.
A study of a deceased triploid XXX twin fetus without a heart showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was probably a polar body twin.
The authors were unable to predict whether a healthy fetus could result from a polar body twinning. In , a study argued that many cases of triploidity arise from semi-identical twinning.
In , a study reported a case of a pair of living twins, one intersex and one a phenotypical male. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.
The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes.
Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity the number of placentae and amniocity the number of sacs of the pregnancy.
Dichorionic twins either never divided i. Monoamnionic twins divide after the first week. In very rare cases, twins become conjoined twins.
Non-conjoined monozygotic twins form up to day 14 of embryonic development, but when twinning occurs after 14 days, the twins will likely be conjoined.
It is a common misconception that two placentas means twins are dizygotic. But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.
DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons.
Monochorionic twins share the same placenta , and thus have a risk of twin-to-twin transfusion syndrome. Monoamniotic twins are always monozygotic.
Monoamniotic twins, as with diamniotic monochorionic twins, have a risk of twin-to-twin transfusion syndrome.
Also, the two umbilical cords have an increased chance of being tangled around the babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from cerebral palsy due to lack of oxygen.
Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation as part of IVF.
The twin at left in the image is shown in the sagittal plane with the head pointing towards upper left. The twin at right in the image is shown in the coronal plane with the head pointing rightwards.
Abdominal ultrasonography of monoamniotic twins at a gestational age of 15 weeks. There is no sign of any membrane between the fetuses.
A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right.
A study has found that insulin-like growth factor present in dairy products may increase the chance of dizygotic twinning.
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.
It may also be linked to the increase of growth hormones in food. About 1 in 90 human births 1. The widespread use of fertility drugs causing hyperovulation stimulated release of multiple eggs by the mother has caused what some call an "epidemic of multiple births ".
Nevertheless, the rate of monozygotic twins remains at about 1 in across the globe. In a study on the maternity records of Hausa women living in the Savannah zone of Nigeria , there were 40 twins and 2 triplets per births.
Twenty-six percent of twins were monozygotic. 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.
The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:.
Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births.
The risk of twin birth can vary depending on what types of fertility treatments are used. With in vitro fertilisation IVF , this is primarily due to the insertion of multiple embryos into the uterus.
Ovarian hyperstimulation without IVF has a very high risk of multiple birth. Reversal of anovulation with clomifene trade names including Clomid has a relatively less but yet significant risk of multiple pregnancy.
A year German study  of 8, vaginally delivered twins that is, 4, pregnancies in Hesse yielded a mean delivery time interval of The study stated that the occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering the second twin.
Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other fetus has died very early in the pregnancy and has not been detected or recorded.
There are several reasons for the "vanishing" fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as vanishing twin syndrome.
Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus.
Conjoined twins or the once-commonly used term "siamese" are monozygotic twins whose bodies are joined together during pregnancy.
This occurs when the zygote starts to split after day 12  following fertilization and fails to separate completely.
This condition occurs in about 1 in 50, human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies.
The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain , heart or liver.
A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother.
A chimera may arise either from monozygotic twin fetuses where it would be impossible to detect , or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body.
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.
A chimera may be intersex , composed of cells from a male twin and a female twin. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin.
One fetus acts as a parasite towards the other. Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be treated medically.
A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or molar.
Typically, this results when one twin has either triploidy or complete paternal uniparental disomy , resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.
Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term.
This occurrence is similar to the vanishing twin syndrome, but typically occurs later, as the twin is not reabsorbed.
It is very common for twins to be born at a low birth weight. More than half of twins are born weighing less than 5. Premature birth and low birth weights, especially when under 3.
Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin.
The lives of both twins are endangered by this condition. Stillbirths occurs when a fetus dies after 20 weeks of gestation. There are two types of stillbirth, including intrauterine death and intrapartum death.
Intrauterine death occurs when a baby dies during late pregnancy. Since , more than 1 million bicycles were produced in Portugal. They also sell bike accessories and parts for budget prices.
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