Genetic Linkage

Determining Linkage Distances

• By definition, one map unit is equal to one percent recombinant gametes or phenotypes
• In honor of Morgan, one map unit is also called one centimorgan (cM)
• To determine the distance between two genes, divide the number of recombinant gametes by the total number of gametes

Coupling Data

F1 Gamete	Testcross Distribution	Gamete Type
pr+ vg+	1339	Parental
pr+ vg	151	Recombinant
pr vg+	154	Recombinant
pr vg	1195	Parental

pr vg distance = (151 +154)/2839 = 10.7 m.u = 10.7 cM

Repulsion Data

F1 Gamete	Testcross Distribution	Gamete Type
pr+ vg+	157	Recombinant
pr+ vg	965	Parental
pr vg+	1067	Parental
pr vg	146	Recombinant

pr vg distance = (157 + 146)/2335 = 13.0 cM

• Remember these are estimates; the differences between the two estimates reflect random deviation
• Neither estimate is incorrect; repeated experimentation would give a more accurate estimate