Final answer:
Incomplete penetrance is when the genotype does not always result in the expected phenotype, such as in Huntington's disease where symptoms may not appear until later in life.
Step-by-step explanation:
Incomplete penetrance occurs when the genotype does not always produce the expected phenotype. Unlike incomplete dominance where the heterozygote exhibits a blending of phenotypes, or codominance where both alleles are simultaneously expressed, incomplete penetrance means that some individuals do not express the phenotype at all, despite having the genotype associated with that phenotype. One classic human example of incomplete penetrance is in the genetic condition of Huntington's disease, a dominant lethal inheritance pattern where individuals who are heterozygous for the Huntington allele might not show symptoms until well into adulthood, after they have potentially passed on the allele to their offspring.