Final answer:
An allele may be lost from the gene pool due to genetic drift, which involves random events that affect allele frequencies in a population, particularly in small populations or after disasters like the bottleneck effect.
Step-by-step explanation:
An allele can be completely lost from the gene pool as a result of genetic drift, which is a change in the frequency of an allele in a population due to random sampling and chance events.
This phenomenon tends to occur more rapidly in small populations and can be exacerbated by natural disasters, such as storms or events like the bottleneck effect where a significant proportion of the population is wiped out, affecting the genetic structure of the survivors.
An example is if, in a population of rabbits with brown and white fur color alleles (B and b), a disaster kills all rabbits with the white allele, effectively removing it from the gene pool.
In addition to random events, genetic drift can also occur when only a few individuals reproduce, leading to a sampling of alleles that may not be representative of the entire population's gene pool. If these few reproducers do not carry a particular allele, it can be lost in the next generation.
An example of this is if in a population where the alleles B and b were initially present in equal frequency, by chance, only the B allele is passed on to the next generation, ultimately leading to the loss of the b allele.