Two offspring produced in the same pregnancy. See also dizygotic and monozygotic twins (below).
- conjoined t's — fused, symmetrical twins formed by incomplete division of one embryo into two components. See also siamese twins.
- discordant t's — a particular trait is present in only one of the twins.
- dissimilar t. — dizygotic twins (below).
- dizygotic t's — those that develop from two separate ova fertilized at the same time. They may be of the same sex or of opposite sexes, and are no more similar than any other two offspring of the same parents. Called also fraternal, binovular, dichorial, dissimilar and unlike twins.
- fraternal t's — dizygotic twins.
- free symmetrical monozygotic t's — see identical twins (below).
- identical t's — derived initially from a single zygote; placentas may be shared or separate.
- impacted t's — twins so situated during delivery that pressure of one against the other produces simultaneous engagement in the birth canal of both.
- induced t. — produced experimentally in ruminants using PMSG or FSH, or by immunization against androstenedione.
- monovular t. — see monozygotic twins (below).
- monozygotic t's — those which develop from a single ovum that divides after fertilization. Because they share the same set of chromosomes, they are always of the same sex, and are remarkably similar in haircoat color and pattern, teeth, and other respects. Monozygotic twins have exactly the same blood type and can accept tissue or organ transplants from each other. May be free and symmetrical, free and asymmetrical, cojoined and symmetrical or cojoined and asymmetrical. Called also identical, enzygotic, monochorial, mono-ovular, similar, or true twins.
- Siamese t's — cojoined twins. See also siamese twins.
- similar t. — see monozygotic twins (above).
- symmetrical t's — twins of equal size, age and stage of development.
- true t. — see monozygotic twins (above).
- unequal t's — twins of which one is incompletely developed, called also asymmetric twins.
- unlike t. — see dizygotic twins (above).
