Final answer:
Incomplete penetrance is the term used to describe when not all individuals carrying a deleterious gene express the associated trait or condition. Carriers of a recessive disorder, heterozygous for the gene, can pass the allele to their offspring but do not exhibit symptoms themselves. Full expression is seen only in those who are homozygous for the recessive allele, which can lead to disease or death in conditions like Tay-Sachs disease.
Step-by-step explanation:
A form of penetrance in which not all individuals carrying a deleterious gene express the associated trait or condition is incomplete penetrance.
Penetrance refers to the proportion of individuals in a population who carry a particular gene and also express the associated phenotype. If penetrance is incomplete, it means that not every individual with the gene shows the trait. For instance, a carrier of an autosomal recessive disorder is heterozygous for the condition, having one normal allele and one deleterious allele, but does not exhibit symptoms of the disorder. These carriers can still pass the deleterious gene to their offspring, who may be affected if they inherit the deleterious allele from both parents.
In recessive lethal inheritance patterns, a child who is born to two heterozygous parents and who inherits the faulty allele from both would not survive. This also illustrates incomplete penetrance in that the heterozygous carriers show no symptoms, while homozygosity for the recessive allele results in disease or death, such as in the case of Tay-Sachs disease.