To know what happened here you need to start analyzing the male and female guinea pig that this girl brought at the store.
Male with smooth black fur
Female with rough white fur
So, you have two characteristics: color and style of fur.
Those are the F0, the start of the bread.
The crossing made a litter of eight contained only rough black animals, this told you the dominance of the alleles:
Black (BB) over white (ww) and rough (RR) over smooth (ss).
Parents are BBss x wwRR = BwRs, BwRs,BwRs, and BwRs
So, the F1 or firs offspring will all have the BwRs alleles and the phenotype will be black rough.
When this F1 is crossed, you had a variety of coats and colors.
BwRs x BwRs =BBRR, BwRs, wBsR, wwss
This explains the variety of colors and coats.
