Which would have a bigger effect on an organism, an error during transcription or a missense mutation?

Both have a big impact on the production of an organism. but I would have to say an error during transcription because then it leads to the mutation which would in a sense be a side effect of the error.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.