The percent of the plants that have short horns in a cross between a purebred long-thorn (BB) plant and a hybrid medium-thorn (Bb) plant is 0% (Option d). It is a case of codominance.
Aleleles and codominance
An allele is a specific gene variant, diploid organisms inherit two alleles for each gene locus.
A heterozygous individual carries two different alleles for a given gene locus.
In codominance, both alleles are equally expressed in heterozygous individuals.
In this case, the long-thorn allele will be always inherited almost one time because one parent is BB.
Learn more about codominant alleles here:
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