Question

Under what circumstances can twins be born in humans?

Answer 1

Under following circumstances twins can be born in humans:
(i) The zygote or fertilised egg is a single cell. Sometimes, a single fertilised egg or zygote splits into two, each half developing into an embryo. This results in identical twins.
(ii) Sometimes, two eggs (instead of one egg) are released from the ovary. This may result into fraternal twins, whereby two eggs are fertilised by two sperms. As a result, two zygotes are formed, which separately develops into two embryos. Fraternal twins are also known as non-identical twins because they are genetically different and may not be of same sex.