Heterozygous twins exist at the time of conception when two eggs are joined with two separate sperms, and the provider of the sperms may be one father or there may be two fathers present. Heterozygous twins are also known as twins that develop from two fertilized eggs. They develop from the union of two egg cells with two different sperm at the same time or in succession. If a woman ovulates with two eggs, she is prone to this type of twinning if she has sex at that time. This type of twinning can also occur in women who do not have an unclean sex life. If there exists a woman who has sex with different men within the period of ovulation, then fertilization may originate from the sperms of two different men, resulting in the formation of heterozygous twins with a single pregnancy from the same mother and father. But the chances of twins with different sperm like this are very small.