What Are Semi-Identical Twins?
Most people are familiar with the two common types of twins:
Identical (monozygotic) twins, which occur when a single fertilized egg splits into two embryos, resulting in two genetically identical individuals of the same gender.
Fraternal (dizygotic) twins, which occur when two separate eggs are fertilized by two different sperm, leading to siblings that share about 50% of their genetic material—just like regular siblings born at different times.
However, semi-identical (sesquizygotic) twins are an extremely rare third category. These twins develop when a single egg is fertilized simultaneously by two sperm. This leads to a unique genetic situation where the twins share 100% of their maternal DNA but only a portion of their paternal DNA.
How Are Semi-Identical Twins Formed?
For semi-identical twins to develop, an unusual fertilization event must occur:
A single egg is fertilized by two sperm at nearly the same time.
This initially results in an embryo with three sets of chromosomes—one from the mother and two from the father.
Normally, embryos with three sets of chromosomes (triploidy) are incompatible with life and do not survive.
However, in rare cases, the fertilized egg divides its three sets of chromosomes into two groups of cells, each containing different paternal DNA contributions.
These two groups of cells then develop into separate embryos, creating semi-identical twins.
A Rare Discovery: The Brisbane Semi-Identical Twins
The first confirmed case of semi-identical twins was reported in the United States in 2007. These twins came to doctors' attention when one was born with ambiguous genitalia, leading to a genetic investigation that revealed they shared identical maternal DNA but only 50% of their paternal DNA.
In 2014, a second case was identified during pregnancy in Brisbane, Australia, making it the first set of semi-identical twins diagnosed in utero.
"It is likely the mother's egg was fertilized simultaneously by two of the father's sperm before dividing," explained Professor Nicholas Fisk, who led the fetal medicine team at Royal Brisbane and Women’s Hospital.
Doctors initially suspected the mother was carrying identical twins when they saw a single placenta and the positioning of amniotic sacs at six weeks. However, at 14 weeks, an ultrasound revealed that the twins were a boy and a girl, which is impossible for identical twins.
Dr. Michael Gabbett, a geneticist involved in the study, explained:
"Three sets of chromosomes are typically incompatible with life and embryos do not usually survive. In this case, the fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins."
The Brisbane twins share all of their maternal DNA but only a portion of their paternal DNA, confirming them as semi-identical.
How Rare Are Semi-Identical Twins?
Semi-identical twins are extremely rare, and researchers believe they are likely underreported due to misclassification as either identical or fraternal twins.
"We at first questioned whether there were perhaps other cases which had been wrongly classified or not reported, so we examined genetic data from 968 fraternal twins and their parents," said Professor Fisk."However, we found no other sesquizygotic twins in these data, nor any case of semi-identical twins in large global twin studies."
This suggests that semi-identical twinning is an exceptional occurrence, making routine genetic testing unnecessary in most twin pregnancies.
Implications of Semi-Identical Twinning
While semi-identical twins do not present immediate health risks, their unique genetic makeup may provide new insights into human reproduction and embryonic development. Some potential implications include:
1. Advancing Our Understanding of Fertilization
Studying cases of sesquizygotic twinning could help scientists understand how embryos regulate chromosomal division, particularly in cases where triploidy (three sets of chromosomes) occurs.
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2. Medical and Genetic Counseling
Because semi-identical twins share more genetic material than fraternal twins but less than identical twins, it raises new considerations for genetic inheritance, medical conditions, and family planning.
3. Twinning and Fertility Research
This rare twinning phenomenon challenges existing scientific knowledge about how eggs and sperm interact, potentially informing fertility treatments and reproductive medicine.
Conclusion
Semi-identical twins are a rare and fascinating example of human reproductive diversity. They are formed when a single egg is fertilized by two sperm, resulting in twins that share all of their maternal DNA but only a portion of their paternal DNA.
With only two confirmed cases worldwide, sesquizygotic twins remain an exceptional biological event. While they pose no known health risks, their study offers valuable insights into embryonic development, fertility, and genetic inheritance. As medical technology continues to advance, researchers may discover more cases of semi-identical twinning, further expanding our understanding of this remarkable phenomenon.
This content includes text that has been generated with the assistance of AI. Lab Manager’s AI policy can be found here.
