In a diploid plant species, an F₁ with the genotype Gg Ll Tt is t...

Question

In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows: Genotype Number Gg Ll Tt 621 Gg Ll tt 3 Gg ll Tt 64 Gg ll tt 109 gg Ll Tt 103 gg Ll tt 67 gg ll Tt 7 gg ll tt 626 1600 What is the order of these three linked genes?

Table showing offspring genotypes and their counts from a trihybrid cross in genetics.

Accepted Answer

Welcome back. Let's look at our next problem. Just a quick note before I read it that in this problem, the plus sign is used to symbolize the wild type of an allele. So when I read the word plus it is that symbol for the wild type allele. So our problem says inro Sophal CPA I S lowercase S E curled wings, lowercase C U and ebony body, lowercase E are encoded by recessive genes found on chromosome three. A researcher crosses S E C U E slash plus plus plus females with S E C U E slash S E C U E males and obtains the following progeny data. And then we have a table which on the left side has the genotype of offspring, all the different possible genotypes from this cross and number of offspring for each type. And I'll come back to this table and describe it. Uh After we look at the rest of the question a little bit, the question then says, determine the recombination frequency between S E and E. And our answer choices are choice, a 19.4% choice B 22% choice C 33.4% and choice d 36%. So, before we look at this chart, let's just very briefly look at the cross that we're doing to think about all the different recombination possibilities that will lead to the genotypes on this chart. And our female who is heterozygous for the mutant all and the wild type, we know all three genes are on the same chromosome. So we'll write them on top of a line, S E C U and E line underneath and then plus plus plus for the wild type alls that's crossed with a male whose homozygous recessive. So he has S E C U E both top and bottom. And now we'll think about recombination, recombination. Of course, doesn't matter with the male because all the possibilities will result in the same genotype. He will always give the recessive, all of all the genes is a typical test cross. So that then the offspring you can tell whether they've inherited the recessive gene or the wild type from the heterozygous parent. So now let's look at our table and what each genotype comes from. So we'll start with the first two rows, which would be the parental genotypes. That's S E C U E slash S E C U E, Which is 340 offspring. So, rather than read them each time I'm just gonna read the top row um from the heterozygous female. So in this case, inherited all three all and no crossing over, Then the second row is plus plus plus, which is 300 offspring. So those are the two parental genotypes. I'm going to mark them with A P next to them. So remember that no crossing over. Well, now we get down to the genotypes that involve a crossover. So here's the next row, it says S E C U plus. So in that case, that involves a crossover between the C U and E genes That results in that 3rd alley being the wild type. So I've indicated that crossover on my, where I've written the cross with a red X between C U and E there. And I'm going to put a red dot on that line of the table. So that SECU plus 60 offspring, the next row says S E plus plus And that has 110 offspring and that would be a crossover between S E and C U. So I'm going to indicate that with a blue X between those two and a blue dot Next to this line of the table, the next one is plus C U E. So that would be another crossover between S E and C U. So blue dot there, the next row says, oh, I'm sorry, let's look back at that previous one plus C U E that was 84 offsprings the next row plus plus E and that will be crossover between C U and E. So a red dot And that's 80 offspring. Now, the final two rows involve crossing over in both places. So if there's crossover between S E and C U and C U and E, we get S E plus E with 10 offspring and plus C U plus with 16 offspring. So these, I will give both a red and blue dot To remember that they're crossing over in both places. So now we've looked at our entire table and the different crossovers that occur and we are looking for a recombination frequency between S E and E. So those are our two outer genes. So how often does recombination occur somewhere between S E on the one side and E on the other? Well, we need to look at for recombination frequency, we would calculate that with the number of recombinant offspring divided by the total number of offspring And then expressed as one. So times 100% or multiplied by 100%. So I need to calculate how many recombinant offspring we will have with the recombination occurring between S E N E. So I will need to add all of the offspring that have recombination between S E and C U. So my two blue. here. So that's 110 and Which is going to equal 194 recommence between S E and C U. And then I will also need to add up all of the ones that had a recombination between C U and E. So those I put a red dot by, so that's gonna be 60 and Which will add up to 140. Now, the tricky question, do I need to add in the ones that recombined twice that recombined in both places? And you would think the answer is yes. However, when I look at the results, we'll have to look at the number of recombinant offspring. So looking at the offsprings genotype, if two recombination occur, we're looking for recombination between S E and E. Well, when we look at our offspring genotype, we see that although two crossings over occurred, the outer genes in the one case are still S E and E just the same as the parental genotype, S E and E together the values in the middle. But we're just talking about between S E N E, same with the other genotype for two crossovers which would be plus C U plus. But again, two wild type alls on either side, just the same as that parental genotype plus and plus. So even though recombination has occurred, the offspring are not recombinant because they have this, they match the parental genotype at the S E and the E um gene loci. So that's why we will not include those double recombinant in our calculation of recombination frequency. So we have our numbers here for recommit and offspring. We have with the crossover between S E N C U. So our number of recombinant offspring will be 194. And we're going to add that to the 140 recommit offspring from a crossover between CU and E so plus sign And 140. So that would give us our total number of recommit and offspring and that will be divided by the total number of offspring. So we need to add together all our numbers from the table to get that total number. And when we add all those numbers together, we get 1000 as our total number of offspring. So we'll be dividing our sum of 194 and 140 x 1000. And that is going to equal 334 over which will equal 0.334. And now we just need to multiply by 100% to get it expressed as a percent. So that's going to be 33.4% as our recombination frequency between S E and E. And when we go over to our answer choices, that will be choice C Which is that 33.4%. See you in the next video.

Key Concepts

Linkage and Gene Mapping

Test Cross

Recombination Frequency

Watch next