Two or more children born together are called twins. Only 1.1% of natural births have twins and 35% of test tube births. Side effects of various fertility drugs are also responsible for many cases of twin births.
DNA sequence
It is natural to ask the question, do twins have exactly the same DNA sequence or are they different? Interestingly, DNA sequences can be similar, or different. There is no guarantee that twins will be all boys or all girls. There may also be a mixture of children. It may also be that on paper twins are born at the same time but in reality there is no more similarity than the genetic similarity between siblings. The type of twins depends on how the egg and sperm are fertilized. There are at least eight types of twins.
Fraternal Twins/Dizygotic/Non-Identical: Siblings have no more in common than their genetic similarities. Their blood group may be different, one may be a boy and another may be a girl. (Again, both boys or both girls.) Twins are born when two separate eggs are fertilized separately by two different sperm.
Maternal factors that increase the chance of having fraternal twins include:
Taking fertility drugs,
Weight and height above average weight and height,
family history,
fraternal twin itself,
History of past pregnancies,
of West African descent,
Age over 40.
Identical twins (Identical twins)/Monozygotic/Maternal: These twins have identical genes. Identical twins are born after an egg and a sperm are fertilized and divide. They have almost 100% genetic similarity. They will either be both boys or both girls. However, being both a boy or a girl does not mean identical twins, but can also be fraternal twins.
Conjoined twins: When the egg and sperm are separated more than 13 days after fertilization, the baby is born as a separate child, but the twins remain in some part of the body.
Mirror image twins (mirror image twins): Contrasts are seen between such twins like mirror images. For example one person's hair may be clockwise and another's counterclockwise. Or the birthmark of one on the right shoulder and the other on the left shoulder. Such twinning cannot be determined by any test, only by observation.
Half identical twins (Half identical twins): Half of the genes of such twins are identical and half are like siblings.
Mixed Chromosome Twins: Mixed Chromosome Twins are born when two eggs are fertilized by two separate sperm. Many of them have more than one type of red blood cell, many have both XX and XY chromosomes. Such cases are very rare.
Superfecundation: Formation of one embryo and development of another is called superfecundation.
Superfecundation: Twins may have separate fathers. An incident in 1810 is well-discussed. An American woman gave birth to twins, one black and one white. This happened due to being in a relationship with a black and a white partner at the same time.
