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Every parent of twins faces the question "Are your twins identical or fraternal? Many think that being identical twins refers to how twins look and not how they form. Take a look at the biology that determines the sex combinations that are possible with identical and fraternal twins. The terms identical and fraternal don't describe what the twins look like, but actually how they form. Identical monozygotic twins are always of the same sex because they form from a single zygote fertilized egg that contains either male XY or female XX sex chromosomes. Fraternal twins can be either two girls, two boys, or one of each.
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These Boy-Girl Twin Names Are the Perfect Match — & Twice the Cuteness

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Semi-identical twins 'identified for only the second time' - BBC News

Female twins who shared a womb with a brother tend to get less education, earn less money, and have fewer children than girls who shared a womb with another girl, according to an analysis of hundreds of thousands of births over more than a decade. Researchers suspect the cause is testosterone exposure during fetal development, though the exact mechanism remains a mystery. Still, she cautions, a lot more work needs to be done to establish a causal link. Fraternal twins, in which each of two eggs is fertilized by a different sperm cell, occur in about four of every births. About half of those result in male-female twin pairs. Scientists have known about this phenomenon for decades, and have been arguing for just as long over what effects, if any, it has on women later in life.
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'Extraordinarily Rare' Semi-Identical Twins Were Born in Australia

The majority of twins are fraternal twins twins that are more like siblings born at the same time than identical twins. Zygotic refers to the zygote, the egg fertilized by the sperm that will develop into an embryo and grow into a baby. Monozygotic twins come from a single egg and sperm that splits into two after conception. Because fraternal twins originate from separate conceptions, they can be boys, girls, or one of each just like a singleton baby.
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A pair of twins born in January in Australia share all of their mother's genes, but only 78 percent of their father's, according to a new case report published yesterday Feb. It's unclear how many other semi-identical, or "sesquizygotic," twins are out there, but it's likely "extraordinarily rare," said lead author Dr. Michael Gabbett, the diagnostic genomics course coordinator at the Institute of Health and Biomedical Innovation in Brisbane, Australia.
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