A Drosophila experiment examining potential genetic linkage of X-...

Question

A Drosophila experiment examining potential genetic linkage of X-linked genes studies a recessive eye mutant (echinus), a recessive wing-vein mutation (crossveinless), and a recessive bristle mutation (scute). The wild-type phenotypes are dominant. Trihybrid wild-type females (all have the same genotype) are crossed to hemizygous males displaying the three recessive phenotypes. Among the 20,765 progeny produced from these crosses are the phenotypes and numbers listed in the table. Any phenotype not given is wild type.

Phenotype Number
1. Echinus     8576
2. Scute       977
3. Crossveinless        716
4. Echinus, scute       681
5. Scute, crossveinless     8808
6. Scute, crossveinless, echinus         4
7. Echinus, crossveinless     1002
8. Wild type         1
20,765

Use chi-square analysis to demonstrate that the data in this experiment are not the result of independent assortment.

Accepted Answer

Hello, everyone and welcome to today's video. So A P plan experiment is conducted to study the genetic linkage of two genes, seed color represented by Y and seed shape, which is represented by R A di hybrid heterozygous is crossed with another di hybrid heteros agus plant. They produce the following progeny. So we have yellow and round 1 20 yellow and wrinkled, 43 green and brown, 39 green and wrinkled eight for a total of 210 progeny. So what is the chi square value as answer choice? A we have 4.25 B is 1. C is 2.20 3d is 3.62. Well, in order to calculate the K square value, remember from previous videos that the formula is going to be the following that we're going to have on the screen. It's going to be this X square or this K square is going to be equal to the sum of the observed minus the expected squared divided by the expected value. Now, we are given in the problem, the observed value in the progeny which we have here. However, we don't know the expected value we need to calculate and get this somehow. Now remember from previous videos as well that the typic ratio for these di hybrid heterozygous process is going to be nine three three one. This means that it's going to be of 16 progeny. This is this is the ratio that we're going to be obtaining 9331 for these phenotypes. Now nine out of 16, we're going to put it for this first phenotype three out of 16 is going to correspond to the second phenotype three out of 16 for the third and then 1/16 for this last one, which is going to be the recessive one. Now these, we multiply by the total number of progeny and this is how we obtain the expected value according to statistics. So we multiply by in each case. And we obtain for the 1st 1, 118.1 25. For the second one, we obtain 41.25. For the third one, we obtain 41.25 as well. And then for a third one, we obtained 13 point 125. So these are the four expected value that we obtain for each phenotype. Now, all we have to do is create these K square, this calculation for each phenotype just adding them up so that we can obtain the high square value. So let's do just that. So the first one is going to be these 18, 118.1 25. The way that we're going to set this up is that we're going to see that the observed value is 1 20 per, what the problem is telling us minus 1 square over 1 18 because that's the expected value. That's what we calculated using the phenotypic ratio. Now, we're going to add this for the second ratio, which is this 3/16 and this is 41.25. So the X the observed is going to be 43 minus 41.25 which is the observed which is going to be dividing 41.25 the stop is going to be squared. And then we're going to make the third ratio, which is going to be exactly the same as the second one is going to be 41.25. Now, the observed value however, is 39 it's going to be 39 minus 41 0.25 squid over 41.25. And then finally, we're going to do the last ratio, which is this ring and wrinkled, which has eight absorbed phenotypes. And this is going to be eight minus 13.25 square over 13.25. So this is going to be a very tedious problem in the sense that we have to write a lot and we have to do a lot of things in order to get our answer. But once we compute this into the calculator, the answer for this K square value is going to be two point 23. And that is going to be represented by answer choice C 2.23. I really hope this video helped you and I hope to see you on the next one.

Key Concepts

Genetic Linkage

Chi-Square Analysis

Phenotypic Ratios

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