Answer/Explanation:
Let's say the allele for short hair is S and the allele for long hair is s. Since long hair is recessive, an individual would need to have two copies of that allele to show the genotype (h0m0zygous - ss). That means they must inherit one from each parent.
A guinea pig with short hair could be h0m0zygous (SS) or heterozygous (Ss)
Since a short haired male and a short haired female produced some long haired offspring, they must each have a copy of the long haired allele (s). Otherwise there is no way their offspring could inherit two copies of the s allele. The offspring must have got one s allele from each parent.
Therefore, the parents must both be heter0zygous (Ss)
