Another analogy that works well to help students understand the relationship of genotype (genetic coding) to phenotype (expression of coding) is to compare a computer to a computer program or a Nintendo machine to a Nintendo game cartridge.
For years my son (neighbor, nephew etc.) had two Nintendo games that Uncle Fred had sent him for his birthday. There was only one "small" problem. He had a couple of cartridges, each with coded information for one game (DNA molecules coding for one gene) but was only able to play (express) them when they were in the environment of a machine (cell) that could use the code. If the game cartridge was placed in an incompatible machine (a Nintendo cartridge in a Sega machine) the game cannot be played.
Phenotype can be equated to the expression of the game that shows up on the screen. (If more than one game is found on one cartridge, each game represents a gene on a chromosome.) The game program represents the genotype. Other cartridges (chromosomes) contain other games (genes) which are played (expressed) when placed in the machine and accessed (turned on by cell).