What is incomplete penetrance and what causes it? incomplete penetrance means the heterozygote has an intermediate phenotype between the two homozygous phenotypes because of interactions with environmental factors. incomplete penetrance occurs in individuals where the genotype does not produce the expected phenotype because of interactions with environmental factors or the genetic background of the individual. incomplete penetrance occurs when dominant alleles remain rare in a population because the dominant phenotype confers some disadvantages in survival and reproduction. incomplete penetrance is the failure of a dominant allele to produce a dominant phenotype because it is partially blocked by another codominant allele in a heterozygote. incomplete penetrance is when a heterozygote shows a mosaic phenotype, with some parts of the body showing the recessive phenotype, because the dominant allele is inactivated in some cells?