Plans A & B correspond to alternative genotypes. Under any of a wide variety of Environments (1, 2, & 3), these genotypes will determine the course of a zygote in development to produce alternative phenotypes, corresponding to Organisms A & B. For example, persons who inherit certain alleles at the PAH locus [Plan B] will have little or no phenylalanine hydroxylase and tend to accumulate phenylalanine (Phenylketonuria), irrespective of pre-natal environment.

    Environments A & B include factors that predispose organisms one way or another. Depending on which particular environmental factors are present, the genetic program make allow any zygote to develop as alternative phenotypes, corresponding to Organisms A & B. For example, although genetic makeup enables humans to use language, persons born in Iceland [Environment B] are likely to grow up speaking Icelandic, regardless of their genetic background.

    Alternative Genotypes A & B interact with Environments I & II to produce a range of organismal phenotypes, such that their ranking is Organism AI  >  BI  >  AII  >  BII.  Note that Environment I always produce a superior phenotype than does II, and within either environment Genotype A always produces a better phenotype than B. However, Genotype B in Environment I is superior to A in II (BI > AII). Prediction of the phenotype requires simultaneous knowledge of both genetics and environment.

Homework: Is Phenylketonuria "Genetic" or "Environmental" ?
                        (i) PKU is treatable by modification of diet. Draw an interaction model based on the above diagrams that shows the effect of this environmental modification.
                        (ii) Children born to mothers who have been treated for PKU may sometimes be born with PKU. Draw an interaction model that explains this maternal PKU.

Figures ©2002 by Griffiths et al.; all text material ©2012 by Steven M. Carr