Answer: The correct answer for the fill in the blank is Anisogamy.
Anisogamy is also termed as heterogamy. It is a form of sexual reproduction in which fusion of two gametes that are different in size and/ or form takes place.
It explains the differences that occur between males and females. Due to the difference in the size of gametes, physiological and behavioral differences take place in two sexes that explains the concept of sexual dimorphism.