A number of mutations have been linked to diseases for which the underlying mechanisms are poorly understood. An example is Timothy Syndrome, a multi-system disorder that includes severe cardiac arrhythmias. Here we employ theoretical simulations to examine the effects of a Timothy Syndrome (TS) mutation in the L-type Ca²⁺ channel on cardiac dynamics over multiple scales: from a gene mutation to protein, cell, tissue and finally to the ECG, to connect a defective Ca²⁺ channel to arrhythmia susceptibility. Our results indicate that: 1) The TS mutation disrupts the rate-dependent dynamics in a single cardiac cell and promotes the development of alternans. 2) In coupled tissue concordant alternans is observed at slower heart rates in mutant tissue than in normal tissue, and once initiated, rapidly degenerates into discordant alternans and conduction block. 3) The ECG computed from mutant simulated tissue exhibits prolonged QT intervals at physiological rates, and with small increases in pacing rate, T-wave alternans and alternating T-wave inversion. At the cellular level enhanced Ca²⁺ influx due to the TS mutation causes electrical instabilities. In tissue, interplay between faulty Ca²⁺ influx and steep action potential duration restitution causes arrhythmogenic discordant alternans. Prolongation of action potentials causes spatial dispersion of Na⁺ channel excitability, leading to inhomogeneous conduction velocity and large action potential spatial gradients. Our model simulations are consistent with the ECG patterns from TS patients, suggest that the TS mutation is sufficient to cause the clinical phenotype and allows for the revelation of the complex interactions of currents underlying it.