Penrose,1 in a lecture delivered at University College in the University of London, emphasized the fact that the biologic units of inheritance are genes and not the visible characters. In the case of a rare recessive character the correspondence between presence of the gene and presence of the character is weak. The offending genes are maintained in the population almost entirely by the transmission through normal heterozygotes or carriers. These people carry the gene, but its presence is not manifest.
Penrose has chosen phenylketonuria as a practical example of a eugenic problem and its relation to natural selection. Phenylketonuria is a rare disease, first observed in 1934 by the Norwegian biochemist Fölling. The essential feature is the urinary excretion of about 1 Gm. daily of phenyl pyruvic acid, a ketonic acid with the formula C6H5.CH2.CO.COOH. The excretion is usually continuous throughout life and
EUGENICS AND PHENYLKETONURIA. JAMA. 1946;132(3):149. doi:10.1001/jama.1946.02870380031011