Question

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) albino - recessive: black - recessive B) albino - dominant; black = incompletely dominant C) albino and black - codominant D) albino = recessive; black - dominant E) None of the answers listed is correct

Answer 1

Answer:

D) albino = recessive; black - dominant

Explanation:

The question involves a single gene coding for skin color in guinea pig. The question states that a cross between a black guinea pig and albino guinea pig gave rise to 5 black offsprings. A second cross between the same albino guinea pig and another black guinea pig resulted in a phenotypic ratio 4:3 of black to albino pigs.

This pattern of inheritance has been explained by Mendel when he discovered that one allele is capable of masking the expression of another allele in the same gene. The allele that masks or is expressed is called DOMINANT allele while the allele being masked, RECESSIVE allele. In the first cross, the black allele for skin colour gene covered up the expression of the albino allele in the offsprings. This means that the black allele must be dominant over the albino allele (recessive) and both parents must be in their homozygous state i.e. same allele.

In the second cross, the black guinea pig is in a heterozygous state i.e. combination of dominant and recessive alleles. Due to the dominance of the black allele over the albino, it masks its expression. During gamete formation, the recessive allele of the second guinea pig got separated and combined with a recessive allele of the albino to produce a recessive trait in the offsprings ( 3 albino offsprings).

Mendel performed this second cross, also known as test cross to determine the genotype of the dominant organism. He obtained a 1:1 ratio.