Video Transcript
Shorthorn cows show codominance in their coat color. They can be red, white, or roan, a mixture of the two, which is pictured here. Which of the following Punnett squares shows a correct cross when a cow homozygous for a white coat and a bull homozygous for a red coat breed?
Let’s take a look at some of the key terms in the question before we tackle answering it. And to do this, we’re going to take away the answer options for now. The question tells us that shorthorn cows show codominance in their coat color. Codominance is an example of when an allele does not have complete dominance over another. So an organism that is heterozygous for a trait, which means that they have one of each allele, displays both alleles in their phenotype, which is their physical appearance. And these two alleles are expressed simultaneously without blending.
In this example, there are two alleles that determine coat color in shorthorn cows: one that gives a red coat like this bull and one that gives a white coat like this cow. If a shorthorn cow has two copies of a CR allele, it will have a red coat. This is the homozygous red bull’s genotype. The uppercase C that precedes each superscript letter stands for color. And this notation shows that this pattern of inheritance of coat color does not show complete dominance.
The superscript R stands for red, which represents the color of the coat itself. So it will differ depending on which color is inherited. If a shorthorn cow has two copies of the allele that gives a white coat, written as CW, where the W now stands for the color white, it will have a white coat, giving it the genotype CWCW. If a cow inherits one allele for a red coat and one allele for a white coat, it expresses both, giving it a phenotype overall of a roan coat, as we can see in this picture.
The question asks us to determine which Punnett square shows correct cross between the two parents, one with a red coat and one with a white coat. So let’s put the information that we’ve just worked out about their genotypes into a Punnett square of our own. We’ve separated each parent’s genotype into each gamete, or sex cell, they might provide their offspring with. The gametes of the red bull are in the top row, while the gametes of the white cow are in the left column.
We need to complete the cells of the Punnett square by taking one allele from the column head and one from the row head for each cell. For example, the first blank cell will contain one CR allele from the red bull father and one CW allele from the white cow mother. Let’s do this for the rest of the cells. The cells we’ve just filled in represent the potential genotypes of the offspring that could be produced by a cross between these two parents. And you might have noticed that they are all heterozygous, CRCW. This means that all four of the offspring in this cross will have one allele coding for a red coat and one allele coding for a white coat. So all of the offspring will have a roan phenotype.
Let’s review our answer options to see which matches this Punnett square. In both of these options, the parents’ gametes are incorrect. The question tells us that both parents are homozygous for their respective coat color traits, one for the red coat and one for the white coat. But this parent is heterozygous for the coat color trait, as it produces gametes with two different alleles. And in this option, both the parents are heterozygous for the coat color traits. Therefore, both of these options are incorrect.
In this option, the offspring’s potential genotypes have been filled in incorrectly. For example, this offspring should have one CR allele from the red bull father. But it is shown to contain two CW alleles in its genotype. So this option is also incorrect. This Punnett square matches our one on the left. So we can deduce that this is the correct answer. All of the offspring are heterozygous for the coat color trait and would have a roan phenotype.
