By replacement in the definition of the notion of "generation" by "meiosis". Since identical twins are not separated by meiosis, there are no "generations" between them, hence n=0 and r=1. See:Lancaster, F. M. (October 2005). "Calculation of the Coefficient of Relationship". Genetic and Quantitative Aspects of Genealogy. Archived from the original on 2007-06-29.
By replacement in the definition of the notion of "generation" by "meiosis". Since identical twins are not separated by meiosis, there are no "generations" between them, hence n=0 and r=1. See:Lancaster, F. M. (October 2005). "Calculation of the Coefficient of Relationship". Genetic and Quantitative Aspects of Genealogy. Archived from the original on 2007-06-29.