Why do you have to pull two alleles (two straws) from the bag to represent one fish?

For most organism's any trait is usually determined by two alleles/"genetic codes" for that trait (one from each parent). For example, your eye or hair color results from a combination of your mother and father's genes (one allele from each for hair/eye color). So, a specific trait for a fish follows the same process by which two alleles (one from each parent) are needed to represent the fish.