A pedigree is an illustration of a trait possessed by individuals across generations of a family. This illustration uses shapes for to show inidividuals: squares for males and circles for females. The shaded shape means that the individual shows the trait in question. Since this illustration simply shows the individuals showing the trait, it is more like a phenotypic background of the family through the generations.
In the following pedigre, what does the double line between individuals 4 and 5 of generation IV indicate?
A. identical twins
B. dizygotic twins