What does penetrance refer to in genetics?

Penetrance is a key concept in genetics that describes the proportion of individuals with a specific genotype that actually express the corresponding phenotype. In simpler terms, it quantifies how many individuals with a certain genetic makeup exhibit the traits that are expected based on that genotype.

This concept is particularly important when dealing with traits that are influenced by single genes or genetic variants. For example, if a certain genetic condition has 80% penetrance, it means that out of 100 individuals who have the genotype associated with that condition, 80 will show symptoms of the condition, while 20 will not, even though they possess the same genetic makeup.

Understanding penetrance helps geneticists and researchers gauge the reliability of genotype-phenotype relationships and can elucidate reasons for variable expression of certain traits within populations. The other choices relate to different concepts in genetics such as expressivity, the number of alleles, and allele interactions, but do not define penetrance correctly.