Blue flower color is produced in a species of morning glories when dominant alleles are present at two gene loci, A and B. (Plants with the genotype have blue flowers.) Purple flowers result when a dominant allele is present at only one of the two gene loci, A or B. (Plants with the genotypes and are purple.) Flowers are red when the plant is homozygous recessive for each gene (i.e., aabb).
Two pure-breeding purple strains are crossed, and all the F₁ plants have blue flowers. What are the genotypes of the parental plants?

Question

Blue flower color is produced in a species of morning glories when dominant alleles are present at two gene loci, A and B. (Plants with the genotype have blue flowers.) Purple flowers result when a dominant allele is present at only one of the two gene loci, A or B. (Plants with the genotypes and are purple.) Flowers are red when the plant is homozygous recessive for each gene (i.e., aabb).
Two pure-breeding purple strains are crossed, and all the F₁ plants have blue flowers. What are the genotypes of the parental plants?

Two pure-breeding purple strains are crossed, and all the F₁ plants have blue flowers. What are the genotypes of the parental plants?

Accepted Answer

Hey, everyone. Let's take a look at this question together. The shape of summer squash is classified as either long, spherical or disc shaped, disk shaped fruits are produced when dominant alleles are present at two gene loci A and B spherical shaped fruits are produced when a dominant allele is present at either the A or B gene loci and long shaped fruits are produced when the plant is homozygous, recessive for both genes A and B, two pure breeding plants that produce spherical shaped fruits are crossed and all the resultant F one progeny plants produce disk shaped fruits determine the genotypes of the parental plants. So we know from the information provided in the question that the parents produce spherical shaped fruits, which we know that the spherical shaped fruits are classified as having a genotype in which there is a dominant allele present at either the A or B gene loci. So their genotypes are either A as having that dominant allele or B Having the dominant allele and that dominant allele, it's only found at one or the other and not both. And we also know that the F1 progeny only produce the disc shaped fruits, which we know from the question that the disc shaped fruits are classified as having a genotype where the dominant alleles are present at two gene loci, both A and B. So that means that the genotypes of the F one progeny are classified as having a dominant allele for A and a dominant allele four B. So in order to produce both dominant, which are A N B, each parent must have one of the two dominant alleles. Meaning that if one parent has the dominant allele for A or for B, the other parent must have the remaining dominant allele. For example, parent, one can have either the dominant allele or A Or the dominant allele or b. and then parent two must have that remaining dominant allele. So if parent, one has the dominant allele for a parent, two must have the dominant allele for B. And if parent one has a dominant allele for B, parent, two must have the dominant allele for A. So looking at our answer choices, we have to find a combination in which one parent has one dominant allele and the other parent has the other dominant allele. So starting off with answer choice A, we see that one parent has a dominant allele for A but the other parent also has the dominant allele for a. So answer choice A is incorrect answer. Choice B has the dominant allele for A and the dominant allele for B. So answer choice B is a possible option. But let's double check with the other two. Just to make sure answer choice C, we have the dominant allele for B and then another dominant allele for B, which is incorrect because we need to have both dominant alleles A and B. And then lastly answer choice D has no dominant alleles making answer choice D incorrect. And leaving us with the only correct answer here, which is answer choice B because we know that the parents produce spherical shaped fruits. And their F one progeny only produce those disc shaped fruits which the parents producing the spherical shaped fruits need to have a dominant allele or A or a dominant allele for B and their F one progeny which produce those disk shaped fruits have to have both dominant alleles, one for A and one for B. And in order to produce both dominant alleles, each parent must have one of the two dominant alleles, meaning that whichever parent has A or B, the other parent must have the remaining dominant allele. So answer choice B is the only genotype combination for the parents that would produce this F one progeny that only produce the disk shaped fruits. So answer choice B is the correct answer. I hope you found this video to be helpful. Thank you and goodbye.

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Key Concepts

Dominant and Recessive Alleles

Gene Loci

Punnett Square and Genetic Crosses