Skin color in a certain species of fish is inherited via a single gene with four different alleles.

One fish of this type has alleles 1 and 3 (S1S

and its mate has alleles 2 and 4 (S2S4). If each allele confers a unit of color darkness such that S1 has one unit, S2 has two units, and so on, then what proportion of their offspring would be expected to have five units of color?

A) 1/4

B) 1/5

C) 1/8

D) 1/2

E) 0

The answer is D) 1/2.

If each allele confers a unit of color darkness, then only genotypes S2S3 and S1S4 will have 5 units of color darkness because in genotype S2S3 S2 will have 2 units, and S3 will have 3 units, which is 5 units in total. The similar math is for genotypes S1S4.

Now, let's cross S1S3 and S2S4:

Parents: S1S3 x S2S4

Offspring: S1S2 S1S4 S2S3 S3S4

The offspring with genotype S1S2 will have 3 units of color darkness. The offspring with genotype S1S4 will have 5 units, as well as the offspring with genotype S2S3. The offspring with genotype S3S4 will have 7 units.

It can be assumed that only 2 genotypes out of 4 will have 5 units:

2/4 = 1/2.