Interference & Coefficient of Coincidence - Genetics

1. In some organisms, such as certain fungi, a crossover event can sometimes increase the probability of a second crossover event nearby. If this phenomenon, known as negative interference, is occurring, what would be the expected relationship between the observed and expected double crossover (DCO) frequencies?
   Answer: Observed DCO frequency would be greater than the expected DCO frequency, resulting in I < 0. Negative interference describes the situation where one crossover enhances the chance of a second crossover. This means more double crossovers (DCOs) will be observed than expected if the events were independent. Therefore, the observed DCO frequency is greater than the expected DCO frequency. This leads to a coefficient of coincidence (C = Obs/Exp) greater than 1, and an interference (I = 1 - C) that is negative (less than 0).
2. A geneticist calculates a coefficient of coincidence of 0.80 and an interference of 0.20. Which value is a more direct measure of the reduction in expected double crossovers, and which value reflects the proportion of expected double crossovers that actually occur?
   Answer: Reduction: 0.20; Proportion: 0.80 This question tests the precise definitions of interference and coefficient of coincidence. The coefficient of coincidence (C = Obs/Exp) directly represents the fraction or proportion of expected double crossovers that are observed (in this case, 0.80 or 80%). Interference (I = 1 - C) represents the degree to which expected double crossovers are not observed, i.e., the reduction or inhibition (in this case, 0.20 or 20%).
3. Two genes, A and B, are 10 map units apart, and two other genes, C and D, are also 10 map units apart, but they are in different regions of the genome. A cross involving A and B shows an interference of 0.8, while a cross involving C and D shows an interference of 0.3 when measured with a third gene. What is a valid conclusion from this comparison?
   Answer: Fewer double crossovers are observed than expected in the A-B region compared to the C-D region. Interference (I) reflects the reduction in observed double crossovers (DCOs) compared to expected. A higher interference value means a greater reduction. An interference of 0.8 in the A-B region means the observed DCO frequency is only 1 - 0.8 = 0.2 (20%) of the expected frequency. An interference of 0.3 in the C-D region means the observed DCO frequency is 1 - 0.3 = 0.7 (70%) of the expected frequency. Therefore, there is a much stronger suppression of double crossovers in the A-B region than in the C-D region.
4. A plant breeder is performing a trihybrid test cross. The three genes are linked in the order R-S-T. The map distance between R and S is 20 cM and between S and T is 20 cM. Interference in this region is 0.5. What is the expected frequency of parental gametes (R S T and r s t) from the trihybrid parent?
   Answer: 62% When you encounter linked gene problems with interference, you need to account for how crossovers in one region affect crossovers in adjacent regions. This requires calculating both single and double crossover frequencies. Start by finding the recombination frequencies: R-S distance is 20 cM (20% recombination) and S-T distance is 20 cM (20% recombination). If genes were unlinked, you'd expect 4% double crossovers (0.20 × 0.20 = 0.04). However, interference = 0.5 means only half the expected double crossovers actually occur, so the actual double crossover frequency is 2%. Now calculate the gamete frequencies: - Single crossover between R-S: 20% - 2% = 18% - Single crossover between S-T: 20% - 2% = 18% - Double crossovers: 2% - Parental types: 100% - 18% - 18% - 2% = 62% Therefore, parental gametes (RST and rst) appear at 62% frequency. Answer choice A (40%) ignores interference entirely and miscalculates recombinant frequencies. Answer choice B (34%) appears to incorrectly subtract total recombination from 100% without properly accounting for double crossovers. Answer choice C (68%) likely represents a calculation error where someone added instead of subtracted recombination frequencies. Study tip: Always remember that interference reduces double crossover frequency below the expected value (distance₁ × distance₂). The coefficient of coincidence equals (1 - interference), so multiply expected double crossovers by this value to get the actual frequency.
5. The map distances for three linked genes are A-15cM-B and B-25cM-C. If the coefficient of coincidence is 0.8 in this region, what is the expected frequency of double-crossover gametes produced by an individual with genotype ABC/abc?
   Answer: 3.00% The frequency of double-crossover gametes is the observed double crossover frequency. 1. Calculate the expected double crossover (DCO) frequency by multiplying the recombination frequencies of the two intervals: Expected DCO = 0.15 × 0.25 = 0.0375 (or 3.75%). 2. Use the coefficient of coincidence (C) to find the observed DCO frequency: Observed DCO = C × Expected DCO = 0.8 × 0.0375 = 0.030 (or 3.00%).
6. In a three-point cross, the coefficient of coincidence is calculated as 0. This implies that:
   Answer: the interference is 1, and no double crossovers were observed. The coefficient of coincidence (C) is the ratio of observed to expected double crossovers. If C=0, it means the observed number of double crossovers was zero. Interference (I) is calculated as I = 1 - C. Therefore, if C=0, then I = 1 - 0 = 1. This situation is known as complete or total interference, where a crossover in one region completely prevents a crossover in the adjacent region, leading to an absence of double-crossover progeny.
7. In a three-point test cross analyzing genes P, Q, and R, the recombination frequency between P and Q is 12%, and between Q and R is 18%. The observed frequency of double crossovers is 1.512%. What is the value of the interference?
   Answer: 0.30 Interference (I) is calculated as 1 - Coefficient of Coincidence (C). The Coefficient of Coincidence is the ratio of observed double crossover (DCO) frequency to expected DCO frequency. 1. Calculate the expected DCO frequency: Expected DCO = RF(P-Q) × RF(Q-R) = 0.12 × 0.18 = 0.0216. 2. The observed DCO frequency is given as 1.512%, or 0.01512. 3. Calculate the Coefficient of Coincidence (C): C = Observed DCO / Expected DCO = 0.01512 / 0.0216 = 0.70. 4. Calculate Interference (I): I = 1 - C = 1 - 0.70 = 0.30.
8. In a three-point mapping experiment, the calculated coefficient of coincidence is 1.2. What type of interference does this indicate, and what does it imply about the crossover events?
   Answer: Negative interference; one crossover enhances the likelihood of another. When you encounter coefficient of coincidence values in genetic mapping problems, you're dealing with the relationship between observed and expected double crossovers. The coefficient of coincidence (COC) equals observed double crossovers divided by expected double crossovers. A COC of 1.2 means you observed 20% more double crossovers than expected if crossovers occurred independently. This indicates negative interference – the occurrence of one crossover actually increases the probability of another crossover nearby. Answer A correctly identifies this relationship: when COC > 1.0, one crossover enhances the likelihood of another. Let's examine why the other options are incorrect. Answer B describes positive interference, which occurs when COC < 1.0, meaning one crossover reduces the chance of another. Answer C represents complete interference (COC = 0), where double crossovers cannot occur at all. Answer D describes no interference (COC = 1.0), where crossovers happen independently with no influence on each other. The key insight is that negative interference, while less common than positive interference in most organisms, does occur in certain situations – particularly in some fungi and when crossovers are far apart on chromosomes. Study tip: Remember the COC scale: COC > 1.0 = negative interference (crossovers promote more crossovers), COC = 1.0 = no interference (independent events), COC < 1.0 = positive interference (crossovers inhibit more crossovers). Most organisms show positive interference, but negative interference questions test your understanding of the complete spectrum.
9. Positive interference is thought to be a consequence of the physical nature of chromosomes and crossing over. Which molecular mechanism is the most accepted explanation for this phenomenon?
   Answer: Mechanical rigidity of the chromosome and synaptonemal complex, which makes it difficult to form two chiasmata close together. When you encounter questions about positive interference in genetics, focus on the physical constraints that limit where crossovers can occur along chromosomes. Positive interference refers to the observation that when one crossover forms, it reduces the probability of another crossover forming nearby. The most widely accepted explanation involves the mechanical properties of chromosomes during meiosis. Answer A correctly identifies that the physical rigidity of the synaptonemal complex and chromosome structure creates spatial constraints that prevent chiasmata from forming too close together. Think of it like trying to tie two knots very close together on a stiff rope—the physical properties of the material make this mechanically difficult. Answer B incorrectly focuses on Spo11 depletion. While Spo11 does initiate double-strand breaks, its localized depletion isn't the primary mechanism explaining positive interference. Multiple breaks can still occur nearby even with varying Spo11 concentrations. Answer C misrepresents the timing of meiotic progression. The first chiasma doesn't trigger immediate entry into anaphase I—cells remain in prophase I for extended periods, giving ample time for additional crossovers to form if interference weren't operating. Answer D incorrectly attributes interference to limited hotspots. While recombination hotspots exist, positive interference occurs even in regions with abundant potential crossover sites. The phenomenon isn't about hotspot availability but about physical spacing constraints. Remember: positive interference questions typically test your understanding of the physical, mechanical aspects of chromosome behavior during meiosis, not just the biochemical processes involved in recombination.
10. If a geneticist observes complete (or total) positive interference for a set of three very closely linked genes, what is the expected coefficient of coincidence (C), and which progeny class from a test cross would be entirely absent?
    Answer: C = 0; the double-crossover class would be absent. Complete positive interference means that the occurrence of one crossover completely prevents the occurrence of another crossover nearby. This is represented by an interference value (I) of 1. The coefficient of coincidence (C) is calculated as C = 1 - I. Therefore, C = 1 - 1 = 0. A C value of 0 means that the observed number of double crossovers is zero, so this progeny class would be entirely absent from the results of a cross.
11. The genetic map for three genes in tomato is d - 20 cM - m - 10 cM - p. In a test cross of a trihybrid plant (D M P / d m p), interference is 0.4. Out of 5,000 progeny scored, what is the expected number of individuals with the genotype d M p or D m P?
    Answer: 60 The genotypes d M p and D m P represent the double crossover (DCO) progeny. The number of DCO progeny depends on the expected DCO frequency and the interference. 1. Calculate the expected DCO frequency: RF(d-m) × RF(m-p) = 0.20 × 0.10 = 0.02 (or 2%). 2. Calculate the coefficient of coincidence (C) from interference (I): C = 1 - I = 1 - 0.4 = 0.6. 3. Calculate the observed DCO frequency: Observed DCO = C × Expected DCO = 0.6 × 0.02 = 0.012 (or 1.2%). 4. Calculate the number of DCO progeny: Number = Observed DCO frequency × Total progeny = 0.012 × 5,000 = 60.
12. A student analyzes data from a three-point cross. They calculate the recombination frequency for region I as 20% and for region II as 30%. They correctly calculate the expected double crossover (DCO) frequency as 6%. However, upon observing only 3% DCOs in the progeny, they conclude that I = 1 - (0.06 / 0.03) = -1.0. What was the student's primary mistake?
    Answer: They inverted the ratio for the coefficient of coincidence calculation. The student's calculation for interference (I) was I = 1 - C. Their calculation for C was Expected DCO / Observed DCO (0.06 / 0.03 = 2.0), which is incorrect. The correct formula for the coefficient of coincidence (C) is Observed DCO / Expected DCO. The correct calculation would be C = 0.03 / 0.06 = 0.5, leading to an interference of I = 1 - 0.5 = 0.5. The student inverted the ratio in the C calculation.
13. As the distance between two adjacent linked genes increases, how does the value of interference typically change, and what does a coefficient of coincidence approaching 1.0 imply?
    Answer: Interference decreases; crossovers occur more independently. Interference is strongest over short distances and diminishes as the distance between genes increases. As genes get farther apart, the physical constraints that cause one crossover to inhibit another are reduced. Consequently, the coefficient of coincidence (C) approaches 1.0, meaning the observed double crossovers become equal to the expected number. This signifies that the two crossover events are occurring independently of one another.
14. Given three linked genes, A, B, and C, with recombination frequencies of 10% between A and B, and 15% between B and C. If there is no interference, how many double-crossover individuals would be expected among 2000 progeny of a test cross?
    Answer: 30 When you encounter questions about double crossovers in linked genes, you need to understand that these events involve simultaneous crossing over in two different regions. The key principle is that when there's no interference, the probability of independent events occurring together equals the product of their individual probabilities. To find the expected double-crossover frequency, multiply the recombination frequencies of the two intervals: A-B has 10% recombination and B-C has 15% recombination. Since these are independent events with no interference, the double-crossover frequency is $0.10 \times 0.15 = 0.015$ or 1.5%. Among 2000 progeny, you'd expect $2000 \times 0.015 = 30$ double-crossover individuals, making D correct. Let's examine why the other answers miss the mark. Choice A (15) represents only 0.75% of the progeny, which would result from incorrectly calculating $0.05 \times 0.15$ - perhaps halving one of the recombination frequencies. Choice B (50) equals 2.5% of progeny, which might come from adding the percentages incorrectly ($0.10 + 0.15 = 0.25$, then halving). Choice C (25) represents 1.25% of progeny and doesn't correspond to any logical calculation using the given recombination frequencies. Remember this formula: double-crossover frequency = (recombination frequency of region 1) × (recombination frequency of region 2). This multiplicative relationship is crucial because double crossovers are much rarer than single crossovers, reflecting the low probability of two independent crossing-over events occurring simultaneously in the same meiosis.
15. A coefficient of coincidence of 0.65 is calculated for three linked genes in Neurospora. Which of the following statements provides the most accurate interpretation of this value?
    Answer: The number of observed double crossovers was 65% of the number expected based on the individual crossover frequencies. The coefficient of coincidence (C) is the ratio of the observed frequency of double crossovers (DCOs) to the expected frequency of DCOs. A value of 0.65 means that the actual number of DCOs observed is 65% of the number that would be expected if the two crossover events occurred independently of each other. Choice B describes interference (I), which would be 1 - 0.65 = 0.35 or 35%. Choice C makes an irrelevant comparison to single crossovers. Choice D incorrectly equates the coefficient of coincidence with genetic distance.
16. A three-point test cross yields an interference value of 1.0. Which statement is the least likely to be true about the genes being studied?
    Answer: The genes are on different chromosomes. Interference is a phenomenon that applies to linked genes on the same chromosome. If the genes were on different chromosomes, they would assort independently, and the concepts of linkage, recombination frequency between them, and interference would not be applicable. An interference of 1.0 (complete interference) means C=0 and no double crossovers are observed, a situation that occurs when genes are very tightly linked.
17. In a test cross for three linked genes, y, f, and m, the following recombination frequencies were determined: RF(y-f) = 22 cM and RF(f-m) = 14 cM. If 56 double crossovers were observed in a total of 20,000 progeny, what was the interference?
    Answer: 0.91 When you encounter questions about interference in genetics, you're dealing with how crossover events affect each other's probability. Interference measures how much the occurrence of one crossover reduces the likelihood of a second crossover nearby. To solve this, you need to compare expected versus observed double crossovers. The expected frequency of double crossovers equals the product of individual recombination frequencies: $0.22 \times 0.14 = 0.0308$. In 20,000 progeny, this predicts $20,000 \times 0.0308 = 616$ double crossovers. However, only 56 double crossovers were actually observed. The coefficient of coincidence (COC) is the ratio of observed to expected: $\frac{56}{616} = 0.091$. Interference is calculated as $1 - COC = 1 - 0.091 = 0.909$, which rounds to 0.91. Answer A (0.09) represents the coefficient of coincidence, not interference—a common mix-up since students sometimes forget to subtract from 1. Answer B (0.82) might result from incorrectly calculating the expected double crossovers or making arithmetic errors in the COC calculation. Answer C (0.18) could come from using the wrong formula entirely, perhaps confusing recombination frequency calculations with interference. Remember this key relationship: interference = 1 - (observed double crossovers ÷ expected double crossovers). High interference (close to 1) means crossovers strongly inhibit nearby crossovers, while low interference (close to 0) means crossovers occur independently. Always calculate the coefficient of coincidence first, then subtract from 1.

Start this quiz