Semi-identical twins

Fraternal twins occur when 2 eggs are fertilized by 2 sperm and carried to term. Identical twins arise when one egg is fertilized by one sperm and the embryo splits at a later stage. Scientists had previously hypothesized and recently discovered a new type of twinning: semi-identical twins. In this case the twins are chimeras, containing populations of cells that are either XX or XY. The proportion of cells of each type varies from tissue to tissue. Researchers believe that this is the result of a single egg being fertilized by two different sperm and each sperm has contributed genes to each child. While one of the twins is a true hermaphrodite, both babies' growth and mental development seem normal.