Genotype vs. Phenotype

Our laboratory receives calls on a daily basis from parents of twins that don’t believe the zygosity determination of their twins. “I can’t believe my twins are identical, they have so many differences”. Or, “We can’t tell our twins apart how can they be fraternal”.

First, lets get the terminology correct. Twins are either from a single fertilized egg, which has split into two or more eggs with the same genotype. This is called monozygotic twinning, commonly referred to as identical. The other more common type of twinning is dizygotic commonly referred to as fraternal twins. Dizygotic twins are the result of two separate eggs fertilized by two separate sperm resulting in two separate embryos with different genotypes.
A genotype is the “internally coded, inheritable information” carried by all living organisms. The genotype is commonly referred to as the blueprint of living organisms. The genotype is written in genetic code, which provides cells with the instructions for formation.

A phenotype is the physical manifestation of the genotype. In other words the genotype is the instruction manual and the phenotype is the end result or observable structure or behavior. Phenotypic traits are what you can see in your twins. Examples of phenotypes are: hair color, eye color, shape of head, size of ears and so on. Even monozygotic twins can show phenotypic differences due to the environment in the uterus despite an identical genotypes. Sometimes these differences are subtle and sometimes very noticeable.