- Also called:
- fraternal twin or nonidentical twin
- Related Topics:
- twin
dizygotic twin
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What are dizygotic twins?
Dizygotic twins are siblings that develop from two separate eggs, released at the same time, that are fertilized by two separate sperm. They develop in the uterus separately, each with its own chorion, placenta, and amniotic sac.
Because dizygotic twins come from two separate zygotes, their genetic makeup differs, and they are not identical. They share about half of their genes, like any other siblings, and may be of the same or different sexes. In rare cases, they may even have different fathers.
Dizygotic twins are more common in older mothers, especially those over 35. The rate of dizygotic twins has also increased with the use of assisted reproductive technologies. Other factors that may increase the likelihood of having dizygotic twins include a family history of dizygotic twinning, being taller and heavier, recent discontinuation of oral birth control, emotional stress, and diet.
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How are dizygotic twins different from identical (monozygotic) twins?
Dizygotic twins, commonly called fraternal twins, occur when two separate eggs are released from an ovary at the same time and are fertilized by two separate sperm. Because they come from two distinct zygotes, dizygotic twins are not genetically identical and share only about half of their genes, similar to any other siblings. They can be of the same sex or different sexes.
Monozygotic, or identical, twins, result from a single egg fertilized by a single sperm. The zygote splits into two separate cell masses, which then develop into embryos. Monozygotic twins are genetically identical and are always the same sex.
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How are dizygotic twins formed during pregnancy?
Dizygotic twins, commonly known as fraternal twins, occur when two separate eggs are released from the ovary during the same menstrual cycle and are fertilized by two separate sperm. Each egg develops individually, has its own chorion (outer sac), and usually forms its own placenta.
Because dizygotic twins come from two distinct zygotes, their genetic makeup differs, and they share only about half of their genes, similar to any other siblings. They can be of the same sex or different sexes. The likelihood of having dizygotic twins is higher in older mothers and can be influenced by family history or fertility treatments.
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Why do dizygotic twins only share about 50% of their genes?
Dizygotic twins, also known as fraternal twins, occur when two separate eggs are fertilized by two separate sperm. Because they arise from two distinct zygotes, dizygotic twins are no more genetically similar than any other siblings, sharing only about 50% of their genes on average.
Monozygotic, or identical, twins, originate from a single fertilized egg that splits into two, resulting in nearly identical genetic material. Dizygotic twins, however, can be the same sex or different sexes, further illustrating their genetic dissimilarity.
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What factors increase the chances of having dizygotic twins?
Dizygotic twins result from two separate eggs being fertilized by two separate sperm. Several factors can increase the likelihood of having dizygotic twins.
These factors include:
* Older mothers The rate of dizygotic twins is highest among mothers over the age of 35.
* Family history A family history of dizygotic twinning can increase the chances.
* Assisted reproductive technologies Procedures such as in vitro fertilization (IVF) and the use of fertility drugs can lead to multiple ovulations, increasing the chance of dizygotic twins.
* Other factors Height and weight, recent discontinuation of oral birth control, emotional stress, and diet may also play a role.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How can scientists tell if a pair of twins are dizygotic or monozygotic?
To determine whether twins are dizygotic (fraternal) or monozygotic (identical), scientists focus on genetic similarity. Since the initial event in the mother’s body (either splitting of a single egg or two separate fertilizations) is not observed directly, inferential means are employed for diagnosing a set of twins as fraternal or identical.
Dizygotic twins arise from two separate eggs fertilized by two separate sperm, making them no more genetically alike than any siblings. Monozygotic twins, however, develop from a single fertilized egg that splits, resulting in genetically identical individuals.
The most reliable method involves analyzing multiple genetic markers with minimal environmental influence, such as eye color and blood types. Examining highly polymorphic HLA antigens can also determine twin type with high confidence.
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dizygotic twin, two siblings who come from separate ova, or eggs, that are released at the same time from an ovary and are fertilized by separate sperm. The term originates from di, meaning “two,” and zygote, “egg.” The rate of dizygotic twinning varies considerably worldwide. For example, parts of central and western Africa have very high twinning rates; studies in Nigeria have reported rates of more than 45 sets of twins per 1,000 births. By comparison, the rate in South and Southeast Asia appears to be as low as 6 to 9 per 1,000 births. While these figures concern all twin births (monozygotic and dizygotic), dizygotic twinning accounts for the majority—at least two-thirds—of them.
Dizygotic twins develop in the uterus separately. Each zygote develops with its own chorion (or outer sac). The chorion is connected to the placenta, which is the protective membrane that surrounds the developing fetus. The placenta lines the uterine wall, partially envelops the fetus, and is attached to the umbilical cord. The placenta exchanges nutrients, wastes, and gases between maternal and fetal blood. Each zygote also has its own inner sac that contains amniotic fluid, and each develops its own placenta. However, if the two zygotes implant in the uterus close together, the two placentas may appear as one on an ultrasound.
Because of their being two separate zygotes, the genetic makeup of dyzygotic twins is different, and thus they are not identical. On average, these twins will share half of their genes, just like any other pair of siblings. Furthermore, they may be the same sex or be a male and female pair. Dizygotic twins may have different fathers, meaning that one egg is fertilized by a sperm from one father and the other egg is fertilized by a different sperm from a different father. These cases are rare and few have been documented.
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Dizygotic twins are most common for older mothers, with the highest rates found among mothers over age 35. With the advent of technologies and techniques to assist women in becoming pregnant, the rate of dizygotic twins has increased markedly. In some cases, family history of dizygotic twinning is an important factor. Other possible predispositions to dizygotic twins include mothers who are taller and heavier, the recent discontinuation of oral birth control, emotional stress, and diet. There has been speculation that diet may play a pronounced role in African mothers, particularly among the Yoruba in Nigeria, where a high frequency of dizygotic twinning is coincident with the consumption of large quantities of phytoestrogen-rich yams. Environmental stress from chemical agents in highly polluted areas may have negative impacts on the frequency of dizygotic twinning. Thus, while certain background factors likely are important, the occurrence of dizygotic twins may also be influenced by a complex mix of events, many of which are yet to be fully understood.