Which of the following is true for a case in which a particular phenotype is
controlled by the complementation of two dominant genes? (A) A dihybrid cross of heterozygous individuals for both genes gives a nine-to-three
phenotypic ratio. (B) A dihybrid cross of heterozygous individuals for both genes gives a nine-to-seven
phenotypic ratio. (C) A dihybrid cross of heterozygous individuals for both genes gives a three-to-one
phenotypic ratio. Or (D) a dihybrid cross of heterozygous individuals for both genes gives a
nine-to-one phenotypic ratio.
This question asks us about complementary genes. We know that genes are sections of DNA that contain the instructions to produce a
functional unit, for example, a protein. This might be a protein that controls a visible characteristic, such as eye color in
humans or flower color in plants.
The expression of some proteins are controlled by more than one gene, as occurs in
the case of complementary genes. Complementary genes are different genes that work together to produce a particular
phenotype. Our answer choices all ask us to determine the phenotypic ratio resulting from a
dihybrid cross. Let’s visualize this cross using a Punnett square.
The Punnett square provided shows a dihybrid cross of two heterozygous
individuals. If an individual is heterozygous, it means that they possess one dominant and one
recessive allele for each gene. The letters B and F represent the alleles for the genes. Dominant alleles are usually represented by uppercase letters and recessive alleles
by lowercase letters. By combining the letters from the column and row headers, we can determine the
possible combination of alleles that the offspring of these parents could
inherit. Let’s use this method to complete the Punnett square.
We know that for a phenotype controlled by complementary genes, we need at least one
copy of the dominant allele for each gene present in the genotype. Here, we have circled all the genotypes to which this applies. Counting them up, we can see that there are nine possible genotypes that give the
phenotype controlled by these complementary genes and seven which do not. We can write this as a phenotype ratio of nine to seven.
So our correct answer is option (B). A dihybrid cross of heterozygous individuals for both genes gives a nine-to-seven