Homopaternal superfecundation is fertilization of two separate ova from the same father, leading to fraternal twins, while heteropaternal superfecundation is a form of atypical twinning where, genetically, the twins are half siblings – sharing the same mother, but with different fathers.
Fraternal – or dizygotic – twins form from two eggs that have been fertilized by two of the father's sperm, producing two genetically unique siblings. They share 50% of their DNA. But “semi-identical” twins are so rare, experts say they have only identified two cases – ever.
Half-identical twins are also called polar body twins.
In this case of semi-identical, or sesquizygotic, twins, the egg is thought to have been fertilised simultaneously by two sperm before it divided. If one egg is fertilised by two sperm, it results in three sets of chromosomes, rather than the standard two - one from the mother and two from the father.
Monoamniotic-monochorionic Twins
These types of twins share a chorion, placenta, and an amniotic sac. This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords.
Such twins, known scientifically as 'MoMo', an abbreviation for monoamniotic-monochorionic, are some of the rarest types of twins, making up less than one percent of all births in the United States, noted the statement. It also stressed that such MoMo twin pregnancies have a high risk of fetal complications.
Even identical twins, who are meant to have identical DNA, differ somewhat in appearance and are not the same in terms of height, physical makeup, disease profile and the like. Why is this so? A. Because differences arise from their DNA sequences and the way the DNA is folded.
Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different. For example, they might have different hair or eye colour. Occasionally they look quite similar.
The only way to be 100% sure is to do DNA testing on each twin and see what genetic information they share. This can be done in the early neonatal period by taking a swab of tissue from inside each baby's cheek. This is painless and non-invasive. The other way to check is to take a blood test and do blood grouping.
Half-identical twins
But when an ultrasound showed they had different genitalia, more testing was done. Scientists say the twins share 100% of the DNA from their biological mother and 78% of DNA from their biological father. The theory is that two different sperm fertilized the egg, and then the zygote split.
Fraternal twins (also called dizygotic twins) result from the fertilization of two separate eggs with two different sperm during the same pregnancy. Fraternal twins may not have the same sex or appearance. They share half their genomes, just like any other siblings.
Therefore, children with a spacing of less than 12 months became known as Irish twins (also known as “Catholic twins” or “Dutch twins”).
Twins with different birthdays are rare, and the chance of twins being born in different years is 1 in 2 million, according to the news release by Natividad. Though the incident is rare, it's not the first time twins have been born in separate years.
In rare cases, fraternal twins can be born from two different fathers in a phenomenon called heteropaternal superfecundation. Although uncommon, rare cases have been documented where a woman is pregnant by two different men at the same time.
Homopaternal superfecundation is fertilization of two separate ova from the same father, leading to fraternal twins, while heteropaternal superfecundation is a form of atypical twinning where, genetically, the twins are half siblings – sharing the same mother, but with different fathers.
When two zygotes do not undergo fusion but exchange cells and genetic material during development, two individuals, or twin chimeras, one or both of whom contain two genetically distinct cell populations, are produced. The most widely known examples of twin chimerism are blood chimeras.
Septuplets (7)
The Frustaci septuplets (born 21 May 1985, in Orange, California) are the first septuplets to be born in the United States. Born at 28 weeks, only two boys and one girl survived; one daughter was stillborn and three died within 19 days of birth.
The DNA of monozygotic twins tends not to be 100% identical, and epigenetic and environmental differences further widen the gap between twin pairs. It's not nature or nurture; it's a complex interaction between our genes, our environment, and our epigenetic markers that shape who we are and what illnesses befall us.
Because identical twins develop from a single fertilized egg, they have the same genome. So any differences between twins are due to their environments, not genetics. Recent studies have shown that many environmentally induced differences are reflected in the epigenome.
The longest interval between the birth of twins is 90 days, in the case of Molly and Benjamin West, dizygotic (fraternal) twins born in Baltimore, Maryland, USA to parents Lesa and David West (all USA) on 1 January and 30 March 1996.
Ivory Coast registered a rate of 24.9 twins for every 1,000 deliveries in the period 2010–15, according to a study of data from 135 countries by the University of Oxford's Christiaan Monden and his colleagues published in 2020. The rate of twinning was highest overall in Africa.
The heaviest birth weight for surviving twins on record goes to Mrs. J.P. Haskin of Arkansas, on February 20 1924, whose twins weighed in at an eye-watering 14lbs & 13lbs 12oz, totalling 27lb 12oz.