The effect of migration on inbreeding is modelled for small populations with immigrants from a large unrelated population. Different migration rates and numbers for the two sexes are assumed, and a general recursion equation for inbreeding progress derived, which can be shown to lead to an equilibrium inbreeding coefficient where the effects of genetic drift and migration balance each other. For small migration rates and large numbers of breeding animals it is shown that migration of only the scarcer sex will minimize the equilibrium inbreeding. Migration from only one sex will also be an advantage in small populations with large migration rates. In small populations with large migration rates fewer migrants are necessary for a given equilibrium inbreeding coefficient than in large populations with small migration rates. Finally, an equation is derived for situations where the number of females is so large that their contribution to inbreeding can be ignored. Simple tables are given for the equilibrium inbreeding coefficients where the number of migrants and herd sizes are taken into consideration. The general impression from these tables is that, for equal numbers of the two sexes, the provision of 2-4 migrants to a population should stabilize inbreeding. In populations with low male to female ratios, where only the inbreeding from the male side is important, one or two male migrants should stabilize the inbreeding.
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