Question

In mice, coat color is determined by 2 epistatically interacting genes. Mice with genotype aa for the first gene are always albino; otherwise, they can produce melanin. If they produce melanin (have at least 1 A allele), then their coat color is determined by a second gene. In particular, if their genotype at this second locus is bb, then they are black. Otherwise, they are agouti (brown). For example, a mouse with genotype Aabb is black, but one with genotype aabb is albino; genotype AaBb produces an agouti mouse. What is the phenotypic ratio of the following cross: aaBb x Aabb?

Answer 1

The phenotypic ratio is 7:4:5

Step-by-step explanation:

To discover the phenotypic ratio between the aaBb and Aabb crossing, it is necessary to fill the Punnet Square for dihybrid crosses, which has 16 houses. Dihybrid crosses are those that present a genetic inheritance confirmed by two characteristics, as is case of the mouse coat shown above. In this case, instead of two alleles per characteristic, we will have four alleles.

The Punnet Square for the crossing AaBb x AaBb, is shown in the image shown below. To form the first row and the first column of the square, we must determine the pair of alleles that each parent has. These pairs are formed by the first "A" with the first "B," the first "A" with the second "B," the second "A" with the first "B" and the second "A" with the second "B" . " For example, in parent AaBb we will form pairs of alleles AB, Ab, aB and ab. We will do this with the two parents. Then we combine the pairs to form the offspring, as shown in the figure.

In order to determine the phenotypic ratio of the offspring, we must pay attention to the information presented in the question. The question states that the Agouti genotype is determined by the presence of the Allelo B (uppercase), for this reason, we must count all the members of the offspring that have at least 1 letter B (uppercase) in their genotype. At the end of the count, you will notice that there are 7 individuals of the offspring with agouti fur.

The black coat is determined by the presence of allele A (uppercase) and absence of allele B (uppercase). In this case, we must count all the individuals who have the Allele A and have the allele b (lower case) in place of B. Upon finishing the count we will notice that there are 4 individuals with the black coat.

The white coat is only seen in individuals who have the aabb alleles, since this color is the recessive characteristic. In this case, we must count all individuals who have this genotype. At the end of the count, we will notice that there are 5 individuals with white coats.

In this case, the phenotypic ratio refers to the number of individuals of each color that is 7: 4: 5.