Multiple Cross Overs and Interference - Online Tutor, Practice Problems & Exam Prep

Multiple crossovers complicate genetic mapping due to their rarity and difficulty in detection. The product law helps calculate the likelihood of double crossovers, where the recombination frequency is determined by multiplying individual crossover probabilities. Interference occurs when a crossover in one region reduces the chance of another in an adjacent region, calculable via the coefficient of coincidence (COC). The formula for interference is $I=1-\frac{\mathrm{observed}}{\mathrm{expected}}$, highlighting the impact of crossovers on genetic outcomes.

Multiple Cross Overs and Interference

So, as you probably just learned, tracking multiple crossovers is more difficult because there could be double crossovers, and they're hard to find unless you know the order of the genes and you've done a lot of work, but we still need to know about them. Multiple crossovers occur when more than one crossover causes two or more changes in the gamete genotype. A single crossover event is this; a multiple crossover event is this, where you have something like this happening twice. And these are much harder to detect, but they also happen less frequently. And so, you can calculate information on how likely it is a double crossover will happen using the product law, which we've talked about before. So, you can double count. I'll show you the product law example in the first. But, the example that we've gone through before in the previous video, this was in the Trihybrid Cross video. Now, if you don't understand what I'm about to say and you're confused, I highly suggest you go back and look at this because it explains it very well at the end. But you have to double count the double crossovers when calculating the recombination frequency, and this helps you accurately map the genes. Like I said, if you don't know what I'm talking about about double counting the double crossovers, then go back and watch this video now. But if you know what I'm talking about and you're ready to learn something else, what we want to talk about next is interference. Interference is when crossovers in one region affect the chance of a crossover in a second region. So it would be like if you have genes a, b, and c, and a, b, and c on two chromosomes. Say that if this crossover event happens, then it's much less likely that a second crossover event here will happen. And so, independent crossovers happen a lot of the time, and this means there's no interference. And you can calculate this using a double recombination frequency, and this is the product law thing I was just talking about. So the question would look like this: If the frequency of crossing over is 20% for genes a and b, so we'll just say this is exactly like this, and 30% for genes b and c. So the chance of this happening is 20%, and the chance of this happening is 30%. Then what is the frequency of a double crossover? So the frequency of both happening. Well, you just use the product law. So you take 20% and you turn it into a decimal. And do the same thing for 30, and you multiply them together and multiply by 100. So, your answer here is 6%. So the frequency of a double crossover happening here and here is 6%. And this is a good formula to know, because you definitely may see a question like this. Now, this is the predictive. Right? This is the ideal situation, no interference occurring. And so, if you want if if you're kind of doing it if you're setting up an experiment and you want to know, you know, what percentage of double crossover should I expect, you can use this formula. But sometimes, if you have interference, this means that the calculated value, so the 6% we just calculated, isn't actually what you observed in your experiment, and so this means that interference is affecting the data. So what you do is you use a different formula. This formula uses the coefficient of coincidence, and that's here. This is the COC, what I just highlighted in this paragraph. So the formula for interference is I equals 1 minus the COC. Now the COC is going to be the observed double recombinant, over the expected. So in this formula, this is going to be what you see, and this is going to be what you calculated, which is 6%. And you do 1 minus. So if we're doing this, calculating the interference, say we observed 4, but we expected 6, say this is this exact same experiment that we talked about earlier, where the chance was 20% here and 30% here, so you did this, times 100, and that equals 6%, so that's going to be your expected. But you did the experiment, and what you got was you got 4 offspring instead of 6. And so, then you use this interference formula, and so that's going to be 1 minus 4 over 6, which equals 0.33. You multiply that by 100 to get a percentage, which is 33%. So you say, okay, the interference is 33%. But what does that actually mean in a real sentence? Well, that means there were 33% fewer double crossovers occurred than expected, meaning that crossing over at one location, either the a or the b, decreased the chance of crossing over at an adjacent location, either the b or the c. Now, I don't know which one. I don't know which one was more likely or which, you know, I can't tell you that. But I can tell you that crossing over at one of these locations decreased it by 33%, or decreased it at an adjacent location, meaning that we got 33% fewer double crossovers than we expected. So, that's genetic interference, and you really need to know how to calculate it. Genetics is a lot of formulas; there's a lot more math than you were probably expecting, but these formulas are important, and you do need to know how to calculate these. So, with that, let's now move on.

A female with the following genotype can produce a number of different gametes. Choose the gamete produced if no crossovers have occurred. Genotype = a b + / + + c

• A. a + c
• B. + b c
• C. a b +
• D. + b +

A female with the following genotype can produce a number of different gametes. Choose the gamete produced if a single crossover has occurred. Genotype = a b + / + + c

• A. + + c
• B. + b c
• C. a b +
• D. a b c

Here’s what students ask on this topic:

What is the product law in genetics and how is it used to calculate the frequency of double crossovers?

The product law in genetics is used to calculate the likelihood of multiple independent events occurring together. To find the frequency of double crossovers, you multiply the individual crossover probabilities. For example, if the crossover frequency between genes A and B is 20% (0.2) and between genes B and C is 30% (0.3), the frequency of a double crossover is calculated as follows:

$0.2\times 0.3\times 100=6\%$

This means the expected frequency of a double crossover is 6%.

What is genetic interference and how is it calculated?

Genetic interference occurs when a crossover in one region of a chromosome affects the likelihood of a crossover in an adjacent region. It is calculated using the coefficient of coincidence (COC), which is the ratio of observed double recombinants to expected double recombinants. The formula for interference (I) is:

$I=1-\frac{\mathrm{Observed}}{\mathrm{Expected}}$

For example, if you observe 4 double recombinants but expect 6, the interference is:

$1-\frac{4}{6}=0.33\times 100=33\%$

This means there were 33% fewer double crossovers than expected.

Why are multiple crossovers more difficult to detect in genetic mapping?

Multiple crossovers are more difficult to detect in genetic mapping because they occur less frequently and can mask each other. A single crossover changes the genotype of a gamete, but multiple crossovers can reverse these changes, making it hard to identify the events without knowing the gene order. Accurate detection often requires extensive data and careful analysis. Additionally, double crossovers can be mistaken for no crossover events if not properly accounted for, complicating the calculation of recombination frequencies and gene mapping.

How does the coefficient of coincidence (COC) relate to genetic interference?

The coefficient of coincidence (COC) measures the actual occurrence of double crossovers compared to the expected occurrence. It is calculated as:

$\frac{\mathrm{Observed}}{\mathrm{Expected}}$

Genetic interference (I) is then determined using the formula:

$I=1-\frac{\mathrm{Observed}}{\mathrm{Expected}}$

If the observed number of double crossovers is less than expected, interference is positive, indicating that a crossover in one region reduces the likelihood of another nearby. Conversely, if the observed number is higher, interference is negative.

What is the significance of calculating recombination frequency in genetic mapping?

Calculating recombination frequency is crucial in genetic mapping as it helps determine the distance between genes on a chromosome. Recombination frequency is the proportion of recombinant offspring produced in a cross, reflecting the likelihood of crossover events between genes. A higher recombination frequency indicates genes are further apart, while a lower frequency suggests they are closer. This information is used to create genetic maps, which are essential for understanding gene linkage, inheritance patterns, and for identifying the location of genes associated with specific traits or diseases.