Assume that a black guinea pig crossed with an albino guinea pig produced 5 black offspring. When the albino was crossed with a second black guinea pig, 4 black and 3 albino offspring were produced. What genetic explanation would apply to these data?
A) Both albino and black= recessive
B) Albino= dominant, black= incompletely dominant
C) Albino and black= codominant
D) Albino= recessive, black= dominant
E) None of the above

According to Mendel's Law of Dominance, of the two alleles which represent a gene, one can express itself even when in the presence of the other allele, that is in heterozygous state. This is called the Dominant allele. The other allele can express itself only in homozygous state. This is called the Recessive Allele.

When a black guinea pig is crossed with an albino guinea pig, all black offspring (5) are obtained in the F1 generation.
This indicates that the Black colour is dominant over the albino colour, as it masks the expressions of the albino phenotype.
This also indicates that the genotype of the parent black guinea pig is homozygous dominant, as if it would have been heterozygous dominant then in the F1 generation there was a chance of formation of both black and albino offspring in 1 : 1 ratio.
When a black guinea pig from the F1 generation is crossed with the albino guinea pig it gives both black (4) and albino (3) guinea pigs, roughly in the 1 : 1 ratio.
This indicates that black guinea pig from the F1 generation was heterozygous dominant. This is why the recessive allele giving rise to the albino phenotype could express itself in the second cross.
The second cross between F1 offspring and recessive parent is called the Test Cross.