Smithsonian National Zoological Park l Friends of the National Zoo



Genetic Drift

Genes Are Lost in Small Populations
The Culprit: Genetic Drift

Just by chance, genes carried by the parents might not be passed on to their offspring. Each offspring gets one gene from its mother, the other from its father.Which gene is chosen is determined by chance.

In this example, the parents carry three different genes for a particular trait: a, A and B. By chance they pass on only a and A to their offspring. Gene B is lost due to random chance.

In this example, on the average:

  • a will be lost 6% of the time
  • A will be lost 25% of the time
  • B will be lost 25% of the time