In this section, we have emphasized that a gene can not determine the phenotype in isolation, but its effect on phenotype is controlled both by other genes and by the environment. These factors controlling the expression of a gene are complex and their precise nature can not be established easily. In order to describe this situation, the two useful terms often used in genetics are penetrance and expressivity.
In human pedigree analysis, variable penetrance and expressivity often create problems. For instance, if an allele causing a disease does not have 100% penetrance, an individual's phenotype in the pedigree may be disease free, but the individual may still carry the disease genotype leading to wrong classification (Fig. 3.5).
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