What can you conclude if the protein with an apparent mass of 45 kda is not detected in individuals who lack the dominant allele? that protein is probably coded for by the recessive allele that protein is probably coded for by the dominant allele that protein is probably expressed only when the recessive and dominant alleles are both present?

The answer is the second one. 

Dominance in hereditary qualities is a connection between alleles of one quality, in which the impact on the phenotype of one allele covers the commitment of a moment allele at a similar locus. The main allele is overwhelming and the second allele is passive. 
Consequently, I closed if the body has an absence of Dominance Allele, the protein is most likely coded for by the Dominance allele.