Here's why:
* Genetics: Guinea pig coat length is determined by a single gene with multiple alleles. One allele codes for short hair, while another allele codes for long hair.
* Dominant and Recessive: The short hair allele is dominant, meaning that even if a guinea pig has one short hair allele and one long hair allele, they will still have short hair. The long hair allele is recessive. This means that a guinea pig must have two long hair alleles to actually have long hair.
* Hidden Traits: Even if both parents have short hair, they could each carry one long hair allele. If they both pass down the long hair allele to their offspring, that offspring will have long hair.
To illustrate:
* Parent 1: Short hair (Ss) - has one short hair allele (S) and one long hair allele (s)
* Parent 2: Short hair (Ss) - also has one short hair allele (S) and one long hair allele (s)
If they both pass down their recessive 's' alleles to their offspring, the offspring will have the genotype 'ss' and will be long-haired.
Therefore, even though both parents have short hair, they can still produce long-haired offspring if they both carry the recessive long hair allele.