Published: April 27, 2026
Welcome to the last installment of the Sports and Science column, Letters on Sunspots. The title is inspired by Galileo Galilei’s book of the same name, in which he documents his observations of sunspots and argues that they were features on the Sun’s surface, not separate celestial objects like Aristotle thought. Like this 17th-century gem, this column will explore the observations of the world around us. However, these concepts will be explained as simply as possible for readers who are new to science.
Our topic for this issue is: How do twins form? Fraternal twins are influenced by genetics; however, the development of identical twins is entirely random. Fraternal twins, scientifically referred to as dizygotic twins, occur when two different eggs are fertilized by two different sperm within the same pregnancy. Unlike identical twins, fraternal twins don’t share the same genetics; rather, they are simply siblings born at the same time. In fact, fraternal twins only share about 50% of their genes.
In some families, the maternal line tends to hyperovulate. In other words, they release multiple eggs at the same time. This increases the likelihood that mothers with a family history of fraternal will conceive them. So when you hear someone ask if “twins run in the family,” this is why. However, this phrase does not apply to identical twins.
Identical twins, or monozygotic twins, occur when one egg is fertilized with one sperm, similar to a regular pregnancy. As the ‘zygote’, or fertilized egg develops, it can split, then create two separate embryos. The timing of the split determines whether or not they will share a placenta and amniotic sac. For example, if the split occurs within day 1-3 of pregnancy, it will create two placentas and two amniotic sacs; 4-8 days will result in a shared placenta but different amniotic sacs; 8-13 days will result in both a shared placenta and amniotic sac; and after 13 days, an incomplete split may occur resulting in conjoined twins. Because identical twins form from a single fertilized egg, they share the exact same genetic makeup!
Thank you for reading this column throughout the semester, and as Galileo Galilei said, “All truths are easy to understand once they are discovered; the point is to keep discovering them”.
