14) Mutations in the tobacco spotty (spt) gene cause dark spots on leaves. But, the phenotype of spt mutations is incompletely penetrant and varies with allelic combinations. For $spt^A$ homozygotes, penetrance is 70%, for $spt^B$ homozygotes, penetrance is 25%, and for $spt^A/spt^B$, penetrance is 50%. If a non-penetrant $spt^A/spt^B$ hybrid is crossed with a $spt^A$ heterozygote, what fraction of the resulting plants would be expected to show the spotty phenotype?
A) 0
B) 0.25
C) 0.3
D) 0.35
E) 0.5
Answer: C
