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Question Video: Determining the Phenotypic Ratio of a Dihybrid Cross with Dominant Complementary Genes Biology

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 9 : 3 phenotypic ratio. [B] A dihybrid cross of heterozygous individuals for both genes gives a 9 : 7 phenotypic ratio. [C] A dihybrid cross of heterozygous individuals for both genes gives a 3 : 1 phenotypic ratio. [D] A dihybrid cross of heterozygous individuals for both genes gives a 9 : 1 phenotypic ratio.

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Video Transcript

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 phenotypic ratio.

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