Blue flower color is produced in a species of morning gl...

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).

If an F₁ plant is backcrossed to one of the pure-breeding parental plants, what is the expected ratio of phenotypes among progeny? Why is the phenotype ratio the same regardless of which parental strain is selected for the backcross?

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 or 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. If the F one progeny was self fertilized, determine the pheno ratio of the F two progeny. Is it answer choice A, the ratio of 9 to 3 to 3 to 1 answer. Choice B the ratio of 15 to 1 answer choice C, the ratio of 9 to 7 or answer choice D the ratio of 9 to 6 to one. Let's work on this problem together to try to figure out which of the following answer choices best represents the phenotypic ratio of the F two progeny. So from the question, we know that we have two E breeding plants which produce spherical shaped fruits are being cropped and all the resultant f one progeny plants produce disk shaped fruits, which we know that those spherical shaped fruits are the ones that have a dominant allele present at either the A or B gene low. However, in order for them to produce disk shaped fruits, their offspring must have a dominant allele present at both the A and B gene loci. And in order for that to happen, our parents must include one parent that is homozygous dominant or A and homozygous recessive four B and one parent that is homozygous recessive four A and Homozygous dominant four B. Because as a result of that cross, our F one genotype has a dominant allele at the two gene loci A and B, which means that our F one genotype is heterozygous four A and heterozygous four B. And so now we have our F one ross and since the F one progeny was self fertilized, we are crossing that heterozygote B genotype with another heterozygous A heterozygous B genotype. And let's go ahead and set up our puit square to see what the F one cross would result in. And so now that we have our puit square set up, we can go ahead and do the cross for our F one generation to find out what the progeny of that self fertilized. F one cross would be to figure out the phenotypic ratio of the F two progeny. So now that we have the cross of the F two progeny, we can see what all of the different genotypes are to determine the phenotypic ratio which from the plane square, we are able to see that the F two generation as three different phenotypes. And those three phenotypes come from four different genotypes. But because we can only have long spherical or disk shaped plants, two of the genotypes are used to make the same phenotype. So that is why we have three different genotypes. And our three phenotypes are the disk shaped fruits, which are represented by having dominant alleles present at two gene loci, which are represented by the genotypes that have at least one dominant allele for A and at least one dominant allele for B. And we know that we also have those homozygous recessive or A and homozygous recessive four B which represent the long shaped fruits. And lastly, we have the genotypes that produce the sphere shaped fruits that are characterized by having one dominant allele that is present at either the A or B G. So we have our homozygous recessive A with at least one dominant allele for B or we have at least one dominant allele for A, followed by the homozygous recessive or B. And so we have our three phenotypes where we have our disc shaped fruits, our long shaped fruits and our spherical shaped fruits. And so looking at our punt square, we can go ahead and calculate what the phenotypic ratio is for each of the following phenotypes based on the genotypes that are present in the Punt square. So looking at our disk shaped genotypes, we can see we have 123456789, because those are all the genotypes where we have both dominant alleles present at the two gene loci, which means that we have nine out of 16. And next are the spherical genotypes which are characterized by having one dominant allele present at either of the two gene loci, which we have 123456. So we have six out of 16. And lastly, we have our long phenotype, which is characterized by a homozygous recessive genotype for both genes, which is present in only one genotype, which means that we have one out of 16. And our ratio is as followed 9 to 6 to one ratio, which means that answer choice d the ratio of 9 to 6 to one is the correct answer. Because when we have that F one progeny that is self fertilized, we end up with the following punt square that you can see on the screen. And the phenotypic ratio of the F two progeny is characterized as that 9 to 6 to one phenotypic ratio. Since we have nine of 16 progeny, having that disk shaped fruit, and we have six out of 16 progeny being the spherical shaped fruit. And lastly one out of 16 progeny are characterized by that long shaped fruit. So answer choice D is the correct answer. I hope you found this video to be helpful. Thank you and goodbye.

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