The muscle cells of people with muscular dystrophy lack a protein, called dystrophin, that maintain the sarcoplasmic reticulum of muscle cells. as a result, the sarcoplasmic reticulum allows calcium ions to leak out. why would this condition affect muscle function?

It will affect muscle contraction because it is needed for the formation of muscle contraction cross-bridge.

Explanation: During muscle contraction calcium binds to troponin, a protein complex that regulates muscle contraction by exposing the binding site of actin filament. Actin filament active site is where myosin head binds to, creating a cross-bridge using enzyme ATPase to speed the reaction. Calcium ion are needed to activate troponin movement, otherwise it stays in its inactive form blocking the active site of actin filament. Without calcium ions, troponin will not move and there will be no muscle contractions.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).