However, males are also more susceptible than females to die in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.Zygosity is the degree of identity in the genome of twins. Dizygotic (fraternal) 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 develop in the womb together and who are 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, will practically always have different sequences on each chromosome, due to chromosomal crossover during meiosis. Dizygotic twins share only 50 percent of each others genes, which resemble amongst siblings that are conceived and born at different times. Like any other siblings, dizygotic twins may look similar, particularly as they are the same age. However, dizygotic twins may also look very different from each other (for example, be of opposite sexes). Studies show that there is a genetic proclivity for dizygotic twinning.
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. However, certain characteristics become more alike as twins age, such as IQ and personality.In January 2021, new research from a team of researchers in Iceland was published in the journal Nature Genetics suggesting that identical twins may not be quite as identical as previously thought. The four-year study of monozygotic (identical) twins and their extended families revealed that these twins have genetic differences that begin in the early stages of embryonic development. Polar body and semi-identical twins
A 1981 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. However, a study in 2012 found that it is possible for a polar body to result in a healthy fetus.In 2003, a study argued that many cases of triploidy arise from sesquizygotic (semi-identical) twinning. Sesquizygotic twin type
See Semi-identical (sesquizygotic) twins below, under Unusual twinnings. Degree of separation
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.