The New England Journal of Medicine has reported only the second known case of “semi-identical” twins.
The boy and the girl, now four, from Brisbane, in Australia, are identical on their mother’s side. They only share a proportion of their father’s DNA. From a genetic point of view, this places them between fraternal and identical twins. Embryos such as these often do not survive. Experts say the phenomenon is very rare.
A team made the discovery during a routine pregnancy scan in 2014. It is the first time semi-identical twins have been identified this way. The first-time mother was 28 at the time and had conceived naturally, doctors said.
The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs. This indicated she was expecting identical twins. Yet, an ultrasound at 14 weeks showed the twins were male and female. This is not possible for identical twins.
How did this happen
Identical, or monozygotic, twins occur when a single egg is fertilised by a single sperm. This divides and makes two babies. These twins will be the same sex and share the same genes and physical features.
Non-identical, or dizygotic, twins occur when two separate eggs are fertilised, each by a different sperm. The foetus then develop in the womb at the same time.
Despite being born together, These twins can be the same or different sexes and are no more alike than any brother or sister.
In this case of semi-identical, or sesquizygotic, twins, fertilisation has occurred at the same time by two sperm before it divided. This results in three sets of chromosomes, rather than the standard two. One from the mother and two from the father.
According to researchers, three sets of chromosomes are “typically incompatible with life and embryos do not usually survive”.
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