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According to classical mendelism, recessive traits affect one quarter of the offspring of apparently unaffected carrier parents. In genetic terms recessive traits appear only when both alleles of a genetic pair carry the mutant gene. That is, the individual must be homozygous for the defect in question. The presence of one normal gene in a pair, the heterozygous condition, suffices to repress manifestation of the defect.
That this general formulation is not entirely true has become increasingly evident in the past few years, and much current research is directed toward the very practical problem of detecting the carrier or heterozygous individuals.
In a few instances the heterozygotes present a disease process that is simply a mild form of that of the homozygotes. Thus the abnormality of red blood cells in sickle cell disease does not become manifest in the heterozygote until adult life and then causes relatively benign disturbances such
C. DG. Heterozygosity. Arch Ophthalmol. 1964;72(1):1-3. doi:10.1001/archopht.1964.00970020003001