Twins, in biology (remaining in the case of the human species), are individuals born during the same childbirth, even if not at the same instant. The incidence of twin pregnancies for women is about 1-2% of the total because the woman usually has single pregnancies. The case of multiple pregnancies beyond twins is even rarer, and usually occurs in particular conditions.
Monovular twins and biovular twins
Twins differ in their origin, i.e. in the ways of fertilization first and then of the cell division of the zygote, and can be of two types: monozygotic twins (monovular twins) and dizygotic twins (biovular twins, that is simple brothers born at the same time ).
Monozygotic (or monovular) twins
Monozygotic twins (also called monovular or simply identical twins) arise from a single egg cell fertilized by a sperm. During the first phases of cell multiplication, the separation of the only mass of cells present at that moment takes place in two separate masses, called morules. Monovular twins can share the same placenta (monochorionic monovular twins), or they can develop two distinct placentas (monovular bicoronal twins).
The twins are of the same sex and strongly similar in appearance, since the individuals who originate from this random event have the same genetic heritage and are therefore identical. They are of the same sex (excluding cases in which there is mosaicism: for example when during the division of the egg the Y chromosome is missing in one of the two originally male twins, thus giving an individual X0, or a female affected by Turner syndrome with the same DNA as the twin; or when conditions are created in which one of the twins has a trisomy in mosaic and the other a monosomy resulting in two different pathological phenotypes depending on the proportion of cells with this disorder), the same eyes, the same hair, the same blood group, that is, the same somatic characters.
These cases are studied very carefully, representing the only case of natural human genetic identity; that is, this is the only case of equality, between distinct individuals, of genotype and DNA. This situation allows us to understand how much the environment influences the development of an individual's behavior with respect to the genetic heritage.
The only differences we can find are the following:
somatic mutations: during life they accumulate mutations that depend both on the environment surrounding each twin and on random (sporadic) mutations;
dermatoglyphs: they differ in the crests of the fingerprints;
X chromosome inactivation (females): female twins can deactivate one or the other X chromosome differently.
Dizygotic (or biovular) twins
Dizygotic twins are more common, accounting for about 2/3 of all twin births. They result from the fertilization of two different egg cells by two different spermatozoa and therefore originate from two different zygotes.
The dizygotic twins are therefore siblings who share the mother's womb during the same period of pregnancy. They can also diverge considerably from each other, similarly to what happens between common brothers, which can be very different from each other for all genetic and phenotypic characters, including sex.
The dizygotic twins can have different fathers in rare cases in which the mother has had close sexual intercourse with more than one man; in this case it may happen that the two different eggs are fertilized by two spermatozoa each belonging to a different father.
Fertilization occurs due to the simultaneous presence of two eggs in the same tube, which are therefore fertilized by two different spermatozoa. The belief that the twin who comes out of the mother's womb first is anagr