Medical Genetics To determine the fraction of genes II-2 and ii-3 have in common one simply counts all of the pathways and their connecting lines through the common ancestors. There is one line from ii-2 to i-1, and a line from i-1 to ii- 3. That is one pathway with two lines of descentMedical Genetics To determine the fraction of genes II-2 and II-3 have in common one simply counts all of the pathways and their connecting lines through the common ancestors. There is one line from II-2 to I-1, and a line from I-1 to II- 3. That is one pathway with two lines of descent