The clinal decrease in type b blood from east Asia to western Europe is probably the result of gene flow.
The movement of genes into or out of a population is referred to as gene flow. Such movement may result from the movement of gametes or from the migration of specific organisms that reproduce in their new populations.
Gene flow has the effect of reducing genetic diversity among populations, inhibiting or delaying the development of populations in various geographic locations into distinct species of the disease. That is why gene flow an important role in decreasing in type B blood from east Asia to western Europe.
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