Video Transcript
Assume that inheritance of fur color in mice is controlled by complementary genes. For the dominant phenotype to be expressed, there must be a dominant allele of each gene present in the genotype. The Punnett square provided shows the phenotypes resulting from a cross between mice that are heterozygous for fur color. Complete the Punnett square to state the ratio of dominant to recessive phenotypes.
Punnett squares are diagrams that show all the potential inheritance outcomes between two individuals for an allele or set of alleles. To start, we will look at a Punnett square for one gene, which is called a monohybrid cross. Parent one and parent two are both heterozygous for trait R. This means they have one dominant and one recessive allele, shown by the uppercase and lowercase letters. Either allele can be passed on to offspring, and each has an equal chance of doing so.
To complete the Punnett square, we take one allele from parent one and combine it with parent two. This is repeated for the remaining alleles as shown here. We can see here that for this cross, three of the offspring carry the dominant phenotype, while one carries the recessive phenotype. This gives us a phenotypic ratio of three to one. This was an example of a monohybrid cross.
Now, let’s turn our attention to the provided diagram, which is an example of a dihybrid cross. In this example, we’re crossing two heterozygotes for the two genes A and B. We’ll use blue and green colors here to represent the different parents. So parent one is in blue, and parent two is in green. Now let’s focus on this square here. For the A gene, we’ll get both an uppercase A from both parent one and parent two, so we’ll indicate that. And for the B gene, we’ll get a lowercase b from both parents. So overall the genotype is uppercase A uppercase A lowercase b lowercase b.
Now, let’s complete the rest of the Punnett square. Before we circle the genotypes to determine the phenotypic ratio, let’s remind ourselves what a complementary gene is as indicated in the question. Complementary genes complement each other and code for the same trait. So in this example, fur color is controlled by two different genes called A and B. These genes code for different enzymes that we’ll call enzyme A and enzyme B.
To get the dominant fur color, the mouse must have at least one functional and dominant copy of the allele for both genes A and B. Nonfunctional alleles are recessive, and if the mouse is homozygous recessive for either gene A or B, then the precursor won’t be converted. As an example, a mouse with the genotype uppercase A lowercase a lowercase b lowercase b does have a functional copy of the A gene. So enzyme A will be produced and precursor one will be converted into precursor two.
However, there are two copies of the recessive and nonfunctional B allele in this genotype. So a functional copy of enzyme B will not be produced, and precursor two will not be converted into the dominant fur color. With this in mind, let’s circle the possible genotypes on the left that have at least one functional copy of both genes A and B. We can see here that there are nine genotypes that have at least one dominant allele for both genes A and B, so these carry the dominant phenotype. And the remaining seven have at least one gene that is homozygous recessive for either gene A or B. So these carry the recessive phenotype.
Therefore, the ratio of dominant to recessive phenotypes is nine to seven.
