Even in a large sexual population, senescence can be caused by the cumulative effects of many rare deleterious alleles held in mutation-selection balance.

A gene's most common allele is called the wild type allele, and rare alleles are called mutants.

Therefore it is only rare hypomorphic alleles or allele combinations that allow the animals to survive to adult stage where increased lifespan can be observed.

The rare allele of the SNP rs642961 showed a significant association with cleft lip cases.

The inbreeding coefficient is unstable as the expected value approaches zero, and thus not useful for rare and very common alleles.

Usually favorable alleles are rare mutant alleles ( for example usually a resistant parent might be 1 out of 10000 genotypes).

It is also noteworthy that some of these rare or unique alleles or allele combinations that were selected by humans might never survive in the wild.

For example, for observing a rare allele at least twice (to eliminate the possibility is unique to an individual) a little less than 4-fold redundancy should be used, regardless of the sample size.

The only real way of investigating the level of gene flow will be an analysis of the geographical distribution of rare, electrophoretically identifiable alleles.

Some specialists believe they have discovered a link between CRMO with a rare allele of marker D18S60, resulting in a haplotype relative risk (HRR) of 18.