Video Transcript
The Punnett square shows the phenotypes resulting from a cross between mice that are heterozygous for fur color. What are the missing alleles?
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 allele, 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.
Here are the possible outcomes of this cross. We can see here that for this cross, three of the offspring carry the dominant phenotype, while one carries the recessive phenotype.
Now let’s look at a Punnett square for a cross involving two traits. This is called a dihybrid cross and is also what we see in our question on the left. In this example, parents one and two are heterozygous for both traits R and B. Therefore, they have a dominant and recessive allele for both traits. On either the top or side of the Punnett square are the different possible gametes produced by each parent. Here are the possible offspring genotypes for this dihybrid cross. Notice how one allele is contributed by each parent for each gene.
Now, looking at our question, we are looking for what alleles will be contributed by this parent in this column. If we look at this combination in this single square, we can see that an uppercase A and uppercase B are contributed by the first parent on the side as indicated by the blue arrows. So the second set of alleles, uppercase A and lowercase B, must be coming from the second parent as indicated by the green arrows. We can confirm this by looking at the other combinations in this column.
Therefore, the missing alleles as indicated in this Punnett square are given by uppercase A lowercase b.