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.
Cervical and postocciptal lymphadenopathy can also be seen, but this generally presents 2–4 days after the onset of the febrile phase.In rare cases, HHV-6 can become active in an adult previously infected during childhood and can show signs of mononucleosis. Cause
There are nine known human herpesviruses. Of these, roseola has been linked to two: human herpesvirus 6 (HHV-6) and human herpesvirus 7 (HHV-7), which are sometimes referred to collectively as Roseolovirus. These viruses are of the Herpesviridae family and the Betaherpesvirinae subfamily, underwhich Cytomegalovirus is also classified. HHV-6 has been further classified into HHV-6A and HHV-6B, two distinct viruses which share 88% of the same DNA makeup, with HHV-6B the most common cause of roseola.After infection, these viruses enter a latent phase. Roseola caused by HHV-7 has been linked to the ability of HHV-7 infection to reactivate latent HHV-6. Spread
After exposure to roseola, the causative virus becomes latent in its host but is still present in saliva, skin, and lungs. HHV-6 is thought to be transmitted from previously exposed or infected adults to young children by the shedding of virus through saliva. Even so, most cases of roseola are transmitted without known exposure. Diagnosis
The diagnosis of roseola is made clinically based on the presence of the two phases: fever and rash. Laboratory testing is seldom used as the results do not alter management of the disease.