eineiigen

Eineiigen is a term used in biology to describe an organism or pair of organisms that originate from a single fertilized egg. In practice, it is most often applied to monozygotic twins, commonly called identical twins, who arise when a zygote splits into two embryos early in development. The result is two individuals with nearly identical genetic material.

Formation and genetics: A single fertilized egg divides and can separate into two embryos. If the split occurs early, two separate placentas form (dichorionic diamniotic). If the split happens later, the twins may share a placenta (monochorionic) and possibly a single amniotic sac. In humans, monozygotic twinning occurs spontaneously in about 0.3–0.4% of pregnancies, with variation by population.

Genetics and phenotype: Monozygotic twins typically share nearly identical DNA sequences, but post-zygotic mutations and epigenetic differences can produce subtle phenotypic differences. Environmental factors in utero can also influence development.

Clinical and research relevance: Because they share most of their genetic material, einiigen twins are frequently studied to understand heredity, development, and the effects of environment. They can present unique medical considerations, such as twin-twin transfusion syndrome in monochorionic pregnancies.

Etymology and usage: The term is linked to languages such as German, where eineiige Zwillinge literally means one-egg twins; in some contexts, einiigen is used as a shorthand. In English, monozygotic or identical twins are the more common terms.