Concept:In complete dominance, a recessive trait is expressed only when the organism has two identical recessive alleles.
Explanation:Let black coat colour be dominant (
B) and white coat colour be recessive (
b).
A white mouse must have the genotype
bb, so it is definitely homozygous.
A black mouse can be either
BB (homozygous) or
Bb (heterozygous), so black mice are not necessarily homozygous.
In the given cross, offspring S shows the recessive white coat colour.
Hence, S must be homozygous.
Answer:S, which is Option C.
