Two alleles control a trait for pathogenicity in a species of bacteria. Those homozygous for allele A cause a virulent disease upon infection of a host, whereas those homozygous for allele B cause no symptoms whatsoever. Bacteria carrying both alleles cause a mild disease upon infection of a host. No other differences in phenotype were observed. If allele A produces a dose-responsive toxin and allele B produces no toxin, what kind of trait would this be?
A) Codominance
B) Incomplete dominance
C) Pleiotropy
D) Sex-linked
E) Epistatic

Question

Quizplus · Accepted Answer

Incomplete dominance is indicated because the heterozygous phenotype (mild disease) is intermediate between the two homozygous phenotypes (virulent disease and no symptoms), suggesting that neither allele is completely dominant over the other.

Two Alleles Control a Trait for Pathogenicity in a Species