Dizygotic twinning can be hereditary and run in families. Also, most twins and multiples that result from fertility-enhancing treatments, such as drugs or procedures like in-vitro fertilization, are dizygotic rather than monozygotic.
Every rule has exceptions, of course. In this case, they are extremely rare exceptions, and it's not likely that the average person would ever encounter twins in this situation. Note that there have been a few reported cases of a genetic mutation in monozygotic male twins.
For some reason, after the zygote splits, one twin loses a Y chromosome and develops as a female. The female twin would have Turner syndrome, characterized by short stature and lack of ovarian development. It's extremely rare. Less than 10 cases have been confirmed. Given the odds, it's safe to assume that over While identical twins come from the same fertilized egg and share the same DNA combination, there are more possibilities for fraternal twins.
Besides being the offspring of two sperm and egg combinations, they may have different genetic donors. These include the following combinations. A woman may carry twins conceived in-vitro with donor eggs, which could come from two different women.
If they are fertilized by sperm from the same man, the twins would be half-siblings. When eggs from one woman are fertilized by sperm from two different men, fraternal twins may result. This can happen with unassisted conception and with in-vitro fertilization. In this case, the twins would be half-siblings. Donor eggs from two different women could be fertilized by sperm from two different men.
In this case, the twins may have no shared DNA unless the sperm and egg donors were related. They would be similar to step-siblings. You are likely to have to answer the same questions about your twins repeatedly. Armed with a few facts about twins, you'll be able just to grin and bear it or enlighten the curious about the biology of twins. Get diet and wellness tips to help your kids stay healthy and happy.
National Human Genome Research Project. Fraternal twins. Identical twins. National Library of Medicine. Is the probability of having twins genetic? American Academy of Pediatrics. The difference between identical and fraternal twins. Updated November 7, Impact of conception method on twin pregnancy course and outcome. Geburtshilfe Frauenheilkd. Monozygotic twins of opposite sex. Traditionally, the science around twins has taught that identical and fraternal are the only two types. But a third type might exist, called polar body or half-identical twins.
Though this has never been confirmed, a research review from suggested that a third twin type would explain why some fraternal twins look so similar.
After the ovaries release an egg , the egg can split into two halves, the smaller of which is called a polar body. This polar body contains all the chromosomes necessary to join with a sperm to create a baby. Meanwhile, the larger half of the original egg could also be fertilized by a separate sperm.
The result? Polar twins. Polar twins share the same chromosomes from their birthing parent, but they get different chromosomes from their non-birthing parent. For this reason, they may or may not be assigned the same sex at birth, and they may look very similar but not exactly identical. In a basic twin pregnancy, two embryos go their separate ways and develop into twin babies, whether identical or fraternal.
Some unique sets of twins follow a different path. Mirror twins are exactly what they sound like! These twins are actual mirror images of each other.
This means that:. What causes this phenomenon? In a typical identical twin pregnancy, an egg splits during its first week after fertilization. Conjoined twins are a rare twin type in which the two siblings are physically connected. Typically, conjoined twins are joined together at the chest or abdomen, but this varies. Some conjoined twins are connected to a greater extent than others. Most share at least one vital organ. Though physically attached to each other, conjoined twins are two individuals.
They have unique thoughts and their own personalities. Researchers are still trying to understand the origins of this type of twin birth. This happens when the egg divides 12 or more days after conception. Another theory is that the fertilized egg divides completely, but it later fuses back together. A conjoined twin pregnancy is high-risk, but breakthroughs in surgery have been able to improve outcomes.
Surgery can even allow conjoined twins to live independently. Although a small study found that just 7. Sometimes, as twins develop in utero, one twin becomes larger and more dominant. The other twin stops developing and begins to depend on their sibling. Known as parasitic twins , these twins are physically conjoined.
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