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. This is because the smaller twin is usually missing vital organs or a fully developed brain or heart. In fact, you might not even recognize the smaller twin as a separate individual. A milder version of this is called twin to twin transfusion syndrome TTTS. Acardiac twins experience a more extreme form of TTTS that may result in developmental issues for the fetuses. Treatments like surgery can improve outcomes, according to research.
In semi-identical twins, two separate sperm fertilize one egg. The fertilized egg then splits in two. So, semi-identical twins share all the same chromosomes from their birthing parent, but only about 50 percent from their non-birthing parent. Sometimes identical twins can be assigned the sex of male and female at birth. These twins start off as identical males with XY sex chromosomes. But shortly after the egg divides, a genetic mutation called Turner syndrome occurs, leaving one twin with the chromosomes X0.
This twin will be assigned female at birth but may have developmental issues, as well as difficulties with fertility later in life, according to the National Health Service.
Once a person becomes pregnant, their body stops releasing new eggs for potential fertilization — except in some rare cases. A phenomenon known as superfetation can occur when a second egg gets released and fertilized after a person is already pregnant.
In this case, both fertilized eggs will develop, but one twin will be slightly older than the other. This is known as heteropaternal superfecundation — a common occurrence in animals but very rare in people. Twin pregnancies often come with an increased risk of developing some medical conditions. These can include:. A twin pregnancy has risks of complications, so parents should make sure to receive good prenatal care and seek medical attention for any concerns.
Identical twins are the same in so many ways, but does that include having the same fingerprints? There's conflicting information out there so we look…. The chances of having twins are certainly higher than they were 20 years ago, but why? We'll tell you what's changed and what you may be able to do to…. Do twins run in families?
Can twins skip a generation? 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. Cytogenet Cell Genet. PMID: Y Chromosome in Tumer syndrome; a case report. Images Paediatr Cardiol. Monochorionic-diamniotic twins discordant in gender from a naturally conceived pregnancy through postzygotic sex chromosome loss in a 47,XXY zygote. Prenat Diagn. Your Privacy Rights. To change or withdraw your consent choices for VerywellFamily. At any time, you can update your settings through the "EU Privacy" link at the bottom of any page.
These choices will be signaled globally to our partners and will not affect browsing data. We and our partners process data to: Actively scan device characteristics for identification. I Accept Show Purposes. Table of Contents View All. Table of Contents. Identical Twins. Fraternal Twins. Exception to the Rule. Genetically Dissimilar. Was this page helpful? Thanks for your feedback! Sign Up. Normally, a human's DNA comes from two sources: one set of chromosomes comes from the mother's egg and one set comes from the father's sperm.
But in the semi-identical twins, one set of chromosomes came from the egg, and the second set was made up of chromosomes from two separate sperm, Gabbett told Live Science. But how does this happen? Gabbett hypothesizes that the mother's egg was fertilized by two separate sperm, each carrying its own set of chromosomes —— one set of chromosomes from the mother ended up combining with two different sets of chromosomes from the father. Once an egg is fertilized, it becomes impenetrable to other sperm, so Gabbett believes that the two sperm must have arrived at the egg at the same time.
Any other explanation, such as two sperm fertilizing two eggs with the exact same genetic makeup, "is biologically implausible," he added. Next, the three sets of chromosomes would've been divvied up into three separate cells: One cell received chromosomes from the mother and first sperm cell, the other from the mother and second sperm cell and the third from both sperm cells.
Because a cell needs chromosomes from both the mother and father to survive, that last one would have eventually died. The surviving cells would go on to combine together and then divide again into the two twins. In addition, the girl and boy each have male and female sex chromosomes — in other words, each twin has some cells that carry an XX pair female and some that carry an XY pair male.
Having both male and female pairs of sex chromosomes throughout various cells in the body is linked to certain developmental problems in the reproductive organs as well as cancer, Gabbett said.
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