The Genetics of Twins

Being a twin is a double-edged sword: a twin can either be your best friend, or your biggest competition. Some people wish for twins to grow up as best friends, while others dread the idea of having two infants at one time and having to spend twice as much money than expected. Most people know at least somebody that is a twin and always wonder what it is like. However, I am more interested in why twins run in families. How does having twins get passed on from generation to generation? 

Primarily, it is important to understand that there are two types of twins. Twins can either be fraternal (dizygotic) or identical (monozygotic). Fraternal twins occur when two egg cells are fertilized by two different sperm cells within the same menstrual cycle. Some believe this could be more likely for women who release more than one egg in a single menstrual cycle due to hyperovulation. It also is more common for women to have fraternal twins if they conceive through in vitro fertilization (IVF). On the other hand, identical twins occur when a single egg is fertilized by a single sperm. Then, early in development, this zygote splits into two zygotes. With 1 in about every 250 births being twins, having twins is fairly common. But not all twins can pass on the “twin” gene to their offspring.

Only fraternal twins have been found to pass on their twin genes to their prodigy. For instance, women who have a sibling that is a fraternal twin are 2.5 times more likely to have twins themselves. Scientists gathered this by analyzing case studies and finding this trend, but no genes have been explicitly linked to the likelihood of a woman having twins until recently. In a more recent study, scientists from eight countries have found two SNPs (single nucleotide polymorphism) that increase a woman’s chance of having twins. One SNP is close to the FSHB gene which affects FSH levels. High FSH levels can lead to women releasing two eggs in the same menstrual cycle which would increase the chance of fraternal twins. Additionally, a second SNP called SMAD3. Scientists have not quite determined how this SNP helps increase the chance of women having twins, but they did conclude that women who have both these SNPs are 29% more likely to have fraternal twins.

 

Given that DNA sequencing is still relatively new, there is still much to study and learn about twins and genetics. Now that we are heading towards a world where designer babies are becoming more and more attainable, I also wonder if people will be able to pay for twins as well. 

 

 Shannon Gray (5)

https://medlineplus.gov/genetics/understanding/traits/twins/