In guinea pigs, the gene for black coat color W dominates over the allele w, which causes white color. Short-haired is determined by the dominant gene L, and long-haired by its recessive allele l. Genes for color and coat length are inherited independently. Homozygous black short-haired animal was crossed with homozygous white long-haired.
According to the introduced conventions, let us write down the genotypes of the parent individuals.
The homozygous black shorthair will be WW LL. It will produce WL germ cells.
A homozygous white long-haired animal will be wwll, it will produce the same type of germ cells wl.
Individuals F1 will be homogeneous both geno- and phenotypically – WwLl. The germ cells produced by the resulting individuals will look like this: WL, wl, wL, Wl.
When crossing offspring with a WWLL guinea pig, the offspring will look like this:
WWLL, WwLl, WwLL, WWLl – cubs with short black hair 100%;
When crossed with a long-haired white guinea pig, the offspring of F1 hybrids will look like this:
cubs with black short hair (WwLl) – 25%;
cubs with white long hair (wwll) – 25%;
cubs with white short hair (wwLl) – 25%;
cubs with black long hair (Wwll) – 25%.