3.3 Genotype-by-Environment Interaction

When the extra-leg deformity is due to a combination of worms and genetics, the model of development looks like one of these two (again, I’m not saying which):

A pair of tables side-by-side, both with the same design. Top row is Genotype, with columns for f f, f f-prime, and f-prime f-prime. Bottom row is Phenotype, illustrated with cartoon frogs, text labels, and sometimes flow charts. In the first table, genotypes f f and f f-prime have a phenotype of 4 legs and genotype f-prime f-prime has a flow chart indicating that if the frog is uninfected it has phentype of 4 legs and if it is infected is has a phenotype 5 legs. In the second table, genotype f f has a phenotype of 4 legs and genotypes f f-prime and f-prime f-prime have a flow chart indicating that if the frog is uninfected it has phentype of 4 legs and if it is infected is has a phenotype 5 legs.

When the variation in leg number results from genotype-by-environment interaction, individuals that inherit two copies of a deleterious allele (left)—or one or two copies (right)—AND acquire worms as tadpoles develop five legs. All other individuals grow four legs.

The difference between ponds in the frequency of five legs is due to both a difference in the density of worms and a difference in the frequency of the f’ allele.