Question Video: Calculating Phenotypic Ratios from Completed Punnett Square Biology

State the phenotypic ratio (dominant to recessive) for the Punnett square provided.


Video Transcript

State the phenotypic ratio, dominant to recessive, for the Punnett square provided.

To answer this question, we’re going to need to look at a few key terms. A phenotype is an organism’s observable characteristics. For example, suppose a pea plant produced two different colored pea pods, green and yellow. These two colors represent two different phenotypes. The phenotype is caused by the expression of certain genes. The set of genes that determine the phenotype is called the genotype. Pea plants are diploid and contain two sets of chromosomes. One set is inherited from each of the two biological parents.

Because there’s two sets of chromosomes, there’s two sets of genes that can be slightly different from each other, called alleles. One allele is said to be dominant because whenever it’s present, it expresses its phenotype. And one is said to be recessive because it needs two copies to express its phenotype. To be consistent with the Punnett square in this question, we’ll indicate the dominant green-color allele as an uppercase G and the recessive yellow-color allele using a lowercase g. We can use a Punnett square to determine the distribution of these alleles in the offspring.

Let’s redraw the Punnett square and color in the different alleles to simplify things. So first, we’ll write the individual alleles for the first parent, then for the second parent, and then the different combinations of these alleles or genotypes of the resulting offspring. Now let’s look at the different genotypes to see what the corresponding phenotypes will be. We’ll also clear these definitions so we can tally up the offspring that have the dominant green phenotype and the recessive yellow phenotype from this Punnett square. This will be needed to determine the phenotypic ratio that we’re being asked for in this question.

When the pea plant has two copies of the green allele, the phenotype is the green color. So let’s color this box green to indicate that this genotype corresponds to the green phenotype. And we’ll put a mark here to tally this green-color phenotype. The next genotype we’ll look at has two copies of the recessive allele. This produces the yellow phenotype. So what happens when we have a copy of the dominant green allele and a copy of the yellow recessive allele? If you’ll recall from earlier, it’s the presence of the dominant allele that determines the phenotype. So a pea plant with both the dominant and recessive allele will still show the dominant green-color phenotype. So let’s color in the rest of the boxes in the Punnett square and tally this up.

As we can see, there’s a total of three dominant green phenotypes and one recessive yellow phenotype. Therefore, the phenotypic ratio, dominant to recessive, for the Punnett square provided is three to one.

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