Twin

thumb|Identical twins can be difficult to visually distinguish especially when young, as demonstrated by brothers Billy and Bobby Mauch. Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell. Since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. In very rare cas

single organism composed of two or more different populations of genetically distinct cells

Pleucadeuc (; ) is a commune in the Morbihan department of Brittany in north-western France.

municipality of Rio Grande do Sul state, Brazil

human genetic study conducted on twins

complication of disproportionate blood supply, resulting in high morbidity and mortality

Cryptophasia is the phenomenon of a language developed by twins (identical or fraternal) that only the two children can understand. The word has its roots from the Greek crypto-, meaning secret, and -phasia, meaning speech. Most linguists associate cryptophasia with idioglossia, which is any language used by only one, or very few, people. Cryptophasia differs from idioglossia on including mirrored actions like twin-walk and identical mannerisms.

thumb|A plate showing a "Free Martin" from the collected works of John Hunter. A freemartin or free-martin (sometimes martin heifer) is an infertile cow with masculinized behavior and non-functioning ovaries. Such a cow is born as one of a pair of twins, with a male calf as the other twin. Phenotypically, the animal appears female, but various aspects of her female reproductive development were altered due to acquisition of anti-Müllerian hormone from the male twin. Genetically, the animal is chimeric: karyotypy as a sample of cells shows XX/XY chromosomes. The animal originates as a female (X

one of two twins that never fully forms

incestuous relationship between twin siblings

twins appearing in mythical stories