Skin color in a certain species of fish is inherited via a gene with four different alleles. Each allele confers a unit of color darkness such that having allele S1 confers one unit of color, S2 has two units, S3 has three units, and S4 confers four units. A fish of this type has the genotype S1, S3, and its mate has the genotype S2S4. What proportion of their offspring would be expected to have five units of color