6 Types of Twins That Are Extremely Rare

Summary

Twinning is more complex than just identical or fraternal twins. The video explores the rarity and diversity of twin types, from mirror-image and conjoined twins to fetuses inside fetuses and chimeric twins. It delves into scientific debates about twinning mechanisms, including theories of incomplete splitting (fission) and embryo fusion (fusion theory). These rare twin variations offer valuable insights into human development, embryonic programming, and even genetic issues like chimerism. By studying these unique cases, researchers aim to better understand developmental biology and improve medical treatments.

Takeaways

• 😀 Twinning is rare, with only about 3% of live births involving multiple fetuses.
• 😀 Most twins are dizygotic (fraternal), arising from two separate eggs fertilized by two sperm.
• 😀 Monozygotic (identical) twins come from a single fertilized egg that splits into two embryos.
• 😀 Mirror-image twins are a rare form of identical twins where physical traits like handedness are reversed.
• 😀 The split of the embryo during development is believed to occur within a few weeks after fertilization, but it's not fully understood when and why it results in mirror-image traits.
• 😀 Conjoined twins may arise from incomplete splitting (fission theory) or from the fusion of two embryos (fusion theory).
• 😀 Conjoined twins often share organs, such as a heart, due to their early developmental connection.
• 😀 External heteropagus twins occur when one twin does not fully develop and relies on the other twin for survival.
• 😀 Fetuses in fetu are extremely rare, where one twin is enveloped or absorbed by the other, and some believe they may result from fusion rather than incomplete splitting.
• 😀 Vanishing twin syndrome occurs when one twin dies early and its cells are absorbed by the surviving twin, potentially creating a chimeric individual with two distinct genomes.

Q & A

What is the percentage of live births that involve multiple fetuses?

About 3% of live births involve multiple fetuses.

What are the two main types of twins, and how do they differ?

The two main types of twins are fraternal (dizygotic) and identical (monozygotic). Fraternal twins come from two separate eggs fertilized by two different sperm, while identical twins come from a single fertilized egg that splits into two embryos.

What are mirror-image twins?

Mirror-image twins are a type of monozygotic twin where the two individuals are physical mirror images of each other, such as having opposite handedness or even opposite placements of internal organs.

How does left-right asymmetry play a role in twinning?

Left-right asymmetry occurs during the early stages of embryonic development, determining which side of the body will develop certain organs. This asymmetry is thought to influence whether a twin will have mirrored physical traits or not.

What is the fission theory of conjoined twins?

The fission theory suggests that conjoined twins occur when an embryo splits late in development but doesn't completely separate, leading to the twins being physically attached to each other.

What is the fusion theory of conjoined twins?

The fusion theory posits that conjoined twins occur when two embryos that split early on physically collide and merge as they develop, causing them to remain attached in certain areas.

What is a heteropagus twin?

A heteropagus twin is a rare type of conjoined twin where one twin is underdeveloped and relies on the other twin for survival. The undeveloped twin may look more like a mutation or a part of the other twin's body.

What is the concept of a fetus in fetu?

A fetus in fetu refers to a rare condition where one twin is found inside the other twin’s body, typically as a partially developed fetus. There is debate as to whether this condition is a true twin or a highly developed tumor called a teratoma.

What is vanishing twin syndrome?

Vanishing twin syndrome occurs when one twin dies early in pregnancy and is absorbed by the surviving twin. This results in the surviving twin having two genomes, making them a chimera.

What are sesquizygotic twins and how do they differ from other types of twins?

Sesquizygotic twins are a rare type of twin that share between 50% and 100% of their DNA. They are more genetically similar than dizygotic twins but less similar than monozygotic twins. The exact mechanism behind sesquizygotic twinning is still unclear.