Video Transcript
Assume that in rodents, the allele for black fur, uppercase B, is dominant to white fur, lowercase b, and the allele for smooth fur, uppercase F, is dominant to the allele for rough fur, lowercase f. Which of the following crosses will produce a phenotypic ratio of four to four to four to four? (A) Uppercase B lowercase b uppercase F lowercase f cross uppercase B lowercase b uppercase F lowercase f. (B) Lowercase b lowercase b lowercase f lowercase f cross lowercase b lowercase b uppercase F uppercase F. (C) Uppercase B uppercase B lowercase f lowercase f cross uppercase B lowercase b uppercase F lowercase f. (D) Uppercase B lowercase b lowercase f lowercase f cross lowercase b lowercase b uppercase F lowercase f.
This question is asking us about the inheritance of two different characteristics in rodents: fur color and texture. We are told that black fur is dominant to white fur and smooth fur is dominant to rough fur. Here are the four different phenotypes we’re looking for: black and smooth fur, black and rough fur, white and smooth fur, and white and rough fur.
Let’s look at the combinations needed for these phenotypes, starting with black and smooth fur. For the black fur, we need only a single copy of uppercase B and either an uppercase or lowercase b for the second allele, which we’ll indicate here as a blank space. The same is true for the smooth fur, since it is also a dominant trait. For black and rough fur, again we need only a single copy of the uppercase B allele to make black fur and two copies of the lowercase f allele for the rough fur phenotype, since rough fur is recessive. For white fur, we need two copies of the recessive lowercase b allele. And for smooth fur, we need a single copy of the dominant F allele. And finally, for white and rough fur, we need two copies of the recessive lowercase b and two copies of the recessive lowercase f allele.
We’re looking for the cross that will produce a four-to-four-to-four-to-four ratio of all these different phenotypes, which is the same as saying a one-to-one-to-one-to-one ratio. There are different ways of figuring this out, including drawing a four-by-four Punnett square and looking at the combinations that each cross will give us. However, that can be very time-consuming, so let’s try it a different way. Since each of these genes are inherited independently, we can look at the cross needed for each gene separately in a two-by-two Punnett square.
Let’s fill in the combinations for the B alleles that we want to see in the offspring. Since we want a one-to-one ratio, that’s the same as saying a two-to-two ratio as we see here.
We’ll start with the lowercase b allele. Notice how one parent needs to be homozygous recessive and the other parent needs at least one lowercase b allele to give this combination. And this other parent needs to have an uppercase B allele so this can be inherited. So we can fill in the blank space here. So we know that for the B gene, one parent needs to be heterozygote and the other needs to be homozygous recessive. We’ll write out what we know on the left in blue. But we still need to figure out how the F alleles are distributed in the parents, so we’ll leave these blank.
Now let’s work out the F allele parental contributions. Again, we’re looking for a one-to-one ratio, or two-to-two, as indicated here. This is the same as before, so we’ll just fill this in. So now we’ll fill this in on the left. Because of the way we did this, the F gene can be inherited as shown here, or it can be switched with the other parent. So these two crosses will give a one-to-one-to-one-to-one ratio of each of the indicated traits.
Let’s just confirm this by drawing out a four-by-four Punnett square. We’ll start with the first cross. Here we can see the different combinations that correspond to the phenotypes on the bottom left. Notice how we get the four-to-four-to-four-to-four ratio as directed by the question. Now let’s look at the second cross. The pattern in the Punnett square appears a bit different, but we still have the same four-to-four-to-four-to-four ratio.
Looking back at our answer choices, we can see the option that gives the four-to-four-to-four-to-four phenotypic ratio is given by answer choice (D): uppercase B lowercase b lowercase f lowercase f cross lowercase b lowercase b uppercase F lowercase f.
