In some plants a red pigment is synthesized from a colorless...

In some plants, a red pigment is synthesized from a colorless precursor by a single enzyme coded for by gene A. The red pigment can be changed into a purple pigment by another enzyme coded for by gene B. A cross between pure-breeding purple plants and white plants resulted in the F2 generation with 94 purple, 31 red, and 43 white plants. A. Genetic hypothesis for inheritance of flower color from these results: The inheritance of flower color in these plants is controlled by two genes, A and B. Gene A codes for the enzyme that synthesizes the red pigment from a colorless precursor, while gene B codes for the enzyme that converts the red pigment into a purple pigment. The red pigment is the result of the action of gene A, and the purple pigment is the result of the combined action of gene A and gene B. B. Genotypes and ratios for each F2 phenotypic class: - Purple plants: AA or Aa (genotypes) - Red plants: AAbb or Aabb (genotypes) - White plants: aabb (genotype) C. Phenotypic ratio expected in the progeny of a testcross using the F1 generation: If the F1 generation is used in a testcross, the expected phenotypic ratio in the progeny would be 1:1:1:1, meaning equal proportions of purple, red, and white plants.

Transcript

00:01 In order to answer this question let's talk about inheritance.

00:02 It says in some plants a great pigment is synthesized from a colorless precursor by a single enzyme coded for by gene a.

00:11 So you have here a colorless precursor and you need the dominant a enzyme to produce great.

00:22 And it says the great pigment can be changed into purple pigment by another enzyme coded for like this.

00:32 It is a cross between pure breeding purple plants and white plants.

00:37 Remember that true breeding means homozygous.

00:39 So in this case if you have a true breeding or pure breeding it means homozygous purple plants.

00:44 It means that you have homozygous dominant a and homozygous dominant b.

00:48 And you're crossing it with white plants.

00:51 It means colorless.

00:52 Okay it means homozygous is for a.

00:58 And well also homozygous is for b.

01:01 So in this case if you perform this cross this plant can only transmit dominant a and this plant can only transmit recessive a.

01:08 So you have that all of the offspring are going to be heterozygous for a and also heterozygous for b.

01:13 So if you want to produce your f2 you have to perform this cross.

01:17 And this is a dihybrid cross.

01:19 And as you may know in a normal dihybrid cross you have 9 3 3 1 phenotypic ratio where 9 are going to be dominant a dominant b.

01:28 3 are going to be dominant a recessive b.

01:30 3 are going to be recessive a dominant b.

01:32 And 1 is going to be recessive a and recessive b.

01:35 But in this case according to the information given it says that you're getting 94 purple.

01:44 You're getting 31 red and 43 white.

01:51 So let's find the ratios for this.

01:53 Okay if you add these values here you're going to get 8 and here you have 168.

02:00 So if you have to find the ratios you just have to divide each of these numbers by the total that is 168.

02:07 So you have 94 divided by 168 and this is going to be equal to 0 .56 approximately.

02:19 If you do the same with the red you get 31 divided by 168 and this is going to be equal to 0 .19 or well 18 approximately.

02:34 And if you do the same here 43 divided by 168 you're going to have 0 .26 approximately.

02:46 Okay so in this case this is for a dihybrid cross remember when you have a complete dominance.

02:52 But in this case this is different because when you have the dominant a and the dominant b you're going to have purple.

02:57 So this is for purple.

02:59 This is going to be dominant a and recessive b.

03:01 So you have the dominant a and the recessive b you're going to have red because the recessive b is not going to allow you to convert red to purple.

03:09 So you're going to have that this is for red.

03:12 This one here has the recessive a and in this case whenever you have the recessive a it doesn't matter what you have here because you're going to have directly white.

03:21 So in this case both of them are going to call for white.

03:25 So in this case you have a case of recessive epistasis because the homozygous recessive for a is going to mask the effects of this gene here.

03:33 So in this case you actually have a 9, 3, and 4 phenotypic ratio out of 16.

03:40 So in this case 9 divided by 16 is equal to 9 divided by 16 is equal to 0 .56.

03:50 3 divided by 16 is equal to 0 .19 approximately and 4 16s is equal to 0 .25.

04:07 So as you can see these frequencies are very close to this one here.

04:11 So practically this is happening.

04:12 Okay this is a case of recessive epistasis...

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