What is twin zygosity?
Zygosity is the genetic relationship between two twins. With respect to genetic similarities or dissimilarities, twins are classified as identical or fraternal. When one egg is fertilized by a single sperm cell and then splits, two identical or monozygous twins will result. These twins are referred to as identical since the sequence of the twins’ DNA is identical. Identical twins must be of the same sex. Importantly, the term “identical” often does not apply to the actual physical or behavioral similarities between identical twins. However, identical twins tend to be very similar for traits such as hair color, hair texture, eye color, height, and weight. When two eggs are independently fertilized by two sperm cells, fraternal or dizygous twins result. Fraternal twins do not have identical DNA but will have half of their DNA in common, just like siblings of the same parents that are born at different times. Fraternal twins can either be of the same sex or different sex.
