But in research from the University of New South Wales and the Queensland University of Technology published today in the New England Journal of Medicine, we learn of a very unique set of Australian twins.
A boy and a girl from Brisbane, aged four, have been identified as only the second set of semi-identical, or sesquizygotic twins, in the world. They are the first to have been observed in utero (in the womb).
This extremely rare phenomena is the result of two sperm fertilising the same egg.
Six TV sisters we absolutely love to watch:
How do twins occur?
The most common type of twins are non-identical twins, which can be the same or different sexes. Non-identical twins are also known as fraternal twins or dizygotic twins (from two zygotes, what we call the earliest embryo when the egg and sperm fuse).
This type of twinning occurs when more than one egg is released from the ovary at ovulation (normally just a single egg is selected and released), and both of the eggs are fertilised by different sperm. These twins are no more genetically similar than siblings born years apart.
The rates of non-identical twins differ between groups: it’s about eight in 1,000 in caucasian populations, 16 in 1,000 in African populations, and four in 1,000 in people of Asian decent. This suggests there is a genetic component to non-identical twins.
Identical twins (also known as monozygotic – originating from one zygote) are less common. They are the product of just one egg and one sperm, which originally form one embryo, but which break into two during the earliest stages of pregnancy. Scientists are continuing to explore how this occurs biologically, but the process remains a mystery.
The rate of identical twins is consistent across the globe at about four in 1,000 births. This suggests it’s probably just a random biological phenomena not influenced by genetics.