How Twinning occurs ???
The birth of twins is not a normal process. In normal process of fertilization, male gamete fuses with female gamete and give rise to a single diploid zygote. This zygote later on undergoes divisions and develops into an embryo. When this normal process undergoes some alterations then it results in the formation of twins. These altered conditions can be sumerised as mentioned below:
1. First, where two eggs are fertilized by two separate sperms and give rise to two zygote. Both of these zygotes undergo implantation and develop into embryoes. Such twins are called fraternal or dizygotic twins. The gender of these twins may be same or different.
2. Second, when one egg is fertilized by one sperm thereby giving rise to one zygote. This zygote is then split into two in the early gestational period. Such twins are the called identical or monozygotic twins. The gender of these twins is same.
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A twin is one of two offspring produced in the same pregnancy (eg. Abby and Sammy, the perfect twins). Twins can either be identical (in scientific usage, "monozygotic"), meaning that they develop from one zygote that splits and forms two embryos, or fraternal ("dizygotic") because they develop from two separate eggs that are fertilized by two separate sperm.
In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple. It is theoretically possible for two singletons to be identical if all 23 chromosomes in both gametes from the mother and father were to be exact matches from one birth to the next. While this is statistically improbable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may someday be possible. A less complex way to engineer genetically identical offspring is through the process of cloning, a procedure that has successfully been accomplished with several species of mammals.