Mendelian Link Practice Test: Practice Exam & Study Guide

In a test cross involving one parent that is heterozygous for two traits (in this case, SsYy) and another that is homozygous recessive for both traits (ssyy), the expected offspring ratios can be determined by looking at the alleles contributed by each parent.

The heterozygous parent (SsYy) can produce gametes with the following combinations of alleles: SY, Sy, sY, and sy. The homozygous recessive parent (ssyy) can only produce gametes with the combination sy. Therefore, when these gametes combine, the progeny can exhibit the following genotypes:

1. SsYy (from SY)

2. Ssyy (from Sy)

3. ssYy (from sY)

4. ssyy (from sy)

Each of these genotypes corresponds to a unique phenotype based on the traits being analyzed (S for one trait and Y for the other). Given that S and Y are dominant traits, the phenotypes can be grouped into four categories based on the combinations of alleles in the offspring:

1. Dominant for both traits (SsYy)

2. Dominant for the first trait and recessive for the second