<P> It is even possible that in any one generation no marbles of a particular colour are chosen, meaning they have no offspring . In this example, if no red marbles are selected, the jar representing the new generation contains only blue offspring . If this happens, the red allele has been lost permanently in the population, while the remaining blue allele has become fixed: all future generations are entirely blue . In small populations, fixation can occur in just a few generations . </P> <P> The mechanisms of genetic drift can be illustrated with a simplified example . Consider a very large colony of bacteria isolated in a drop of solution . The bacteria are genetically identical except for a single gene with two alleles labeled A and B. A and B are neutral alleles meaning that they do not affect the bacteria's ability to survive and reproduce; all bacteria in this colony are equally likely to survive and reproduce . Suppose that half the bacteria have allele A and the other half have allele B. Thus A and B each have allele frequency 1 / 2 . </P> <P> The drop of solution then shrinks until it has only enough food to sustain four bacteria . All other bacteria die without reproducing . Among the four who survive, there are sixteen possible combinations for the A and B alleles: </P> <P> (A-A-A-A), (B-A-A-A), (A-B-A-A), (B-B-A-A), (A-A-B-A), (B-A-B-A), (A-B-B-A), (B-B-B-A), (A-A-A-B), (B-A-A-B), (A-B-A-B), (B-B-A-B), (A-A-B-B), (B-A-B-B), (A-B-B-B), (B-B-B-B). </P>

When gene flow occurs it’s effect is the opposite of genetic drift because