penetrance /pen″ətrəns/ [L, penetrare, to penetrate] , the regularity with which an allele is expressed in a person who carries it. If an allele always produces its effect on the phenotype, it is fully penetrant. Achondroplasia is caused by a fully penetrant allele; if the allele is present, achondroplasia results. If an allele produces its effect less frequently than 100% of the time, it is not fully penetrant. Retinoblastoma develops in 90% of the children carrying the allele for the disease; in 10% of children the allele is nonpenetrant. −penetrant, adj.
