Now we see that both copies of the sheet music are unreadable. In this case (homozygous for the recessive loss-of-function allele) the performance has to stop. In an organism, being homozygous for a loss-of-function allele can lead to disease or death.
In the final image we see that, yet again, the sheet music printer has made a mistake. However, the nature of the mistake is different than in the previous two examples. Our musician is frantically trying to play all the extra notes this misprint has created. If you were in the audience for this performance, what would you hear? Would you hear the musician playing the correct (wildtype) part? No! You would hear the musician working frantically to play all those extra notes. In this case the mistake has created a gain-of-function allele. Gain-of-function alleles tend to be dominant to wildtype and this illustration helps to establish why that is. (One note: students often hear "gain" and think "better"...it should be clear from these illustrations that, compared to wildtype, both loss-of-function and gain-of-function mutation have negative consequences.)