Genetics Quiz: Three Point Gene Maps

What this quiz covers

This quiz focuses on Three Point Gene Maps, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Gametes A B C and a b c
2. Gametes A b c and a B C
3. Gametes A B c and a b C
4. Gametes A b C and a B c (correct answer)

Explanation: The frequency of different gamete types depends on the crossover events that produce them. Parental gametes (A B C and a b c) are produced with no crossovers and are the most frequent. Single-crossover gametes result from one crossover in either the A-B interval (A b c, a B C) or the B-C interval (A B c, a b C). Double-crossover gametes (A b C, a B c) result from simultaneous crossovers in both intervals. The probability of a double crossover is the product of the probabilities of single crossovers in each interval, making it the rarest event. Therefore, the gametes resulting from a double crossover will be the least frequent.

Question 2

1. P - R - S (correct answer)
2. R - S - P
3. S - P - R
4. The order cannot be determined from this information.

Explanation: To determine the gene order, we compare the allele combination on a parental chromosome with the combination on a double-crossover (DCO) chromosome. The parental chromosomes are P r S and p R s. The DCO chromosomes are p r s and P R S. Let's compare a parental, P r S, with a DCO, P R S. The alleles for P and S are identical between the two, but the allele for R is different (r vs. R). The gene that is exchanged between the parental and DCO classes is the middle gene. Therefore, R is the middle gene, and the order is P-R-S.

Question 3

1. Because double-crossover events result in non-viable gametes, reducing their observed frequency.
2. Because the probability of two independent crossover events occurring between three linked genes is the product of their individual probabilities. (correct answer)
3. Because double-crossover events can only occur in repulsion phase heterozygotes, which are less common.
4. Because positive interference always enhances the rate of double crossovers above the expected frequency.

Explanation: Crossover events in adjacent regions of a chromosome are largely independent. The probability of a double crossover is the probability of a crossover happening in the first interval multiplied by the probability of a crossover happening in the second interval. Since these individual probabilities (recombination frequencies) are always less than 0.5, their product will be a much smaller number than either individual probability. For example, if P(crossover 1) = 0.2 and P(crossover 2) = 0.1, then P(double crossover) = 0.2 * 0.1 = 0.02. This low joint probability makes the double-crossover class the rarest among the progeny.

Question 4

1. Because double-crossover events result in non-viable gametes, reducing their observed frequency.
2. Because the probability of two independent crossover events occurring between three linked genes is the product of their individual probabilities. (correct answer)
3. Because double-crossover events can only occur in repulsion phase heterozygotes, which are less common.
4. Because positive interference always enhances the rate of double crossovers above the expected frequency.

Explanation: Crossover events in adjacent regions of a chromosome are largely independent. The probability of a double crossover is the probability of a crossover happening in the first interval multiplied by the probability of a crossover happening in the second interval. Since these individual probabilities (recombination frequencies) are always less than 0.5, their product will be a much smaller number than either individual probability. For example, if P(crossover 1) = 0.2 and P(crossover 2) = 0.1, then P(double crossover) = 0.2 * 0.1 = 0.02. This low joint probability makes the double-crossover class the rarest among the progeny.

Question 5

1. 618 (correct answer)
2. 700
3. 1236
4. 1400

Explanation: First, calculate the frequency of all recombinant gametes. This is the sum of all single crossover (SCO) and double crossover (DCO) gamete frequencies. Expected DCO freq = RF(P-S) × RF(S-T) = 0.25 × 0.16 = 0.04. Interference (I) = 0.3, so C = 1 - I = 0.7. Observed DCO freq = 0.04 × 0.7 = 0.028. Freq(SCO P-S) = RF(P-S) - Obs DCO = 0.25 - 0.028 = 0.222. Freq(SCO S-T) = RF(S-T) - Obs DCO = 0.16 - 0.028 = 0.132. Total frequency of recombinant gametes = Freq(SCO P-S) + Freq(SCO S-T) + Freq(DCO) = 0.222 + 0.132 + 0.028 = 0.382. The frequency of parental gametes is 1 - (total recombinant frequency) = 1 - 0.382 = 0.618. There are two parental gametes (PST and pst), so they share this frequency. Freq(PST) = 0.618 / 2 = 0.309. Expected number of P S T progeny = 0.309 × 2000 = 618.

Question 6

1. 50
2. 100
3. 450 (correct answer)
4. 500

Explanation: The phenotype r S w corresponds to the gamete rSw. This gamete is formed by a single crossover (SCO) between genes R and S. The frequency of recombination (RF) in this interval is given as 20 cM or 0.20. The RF is the sum of SCO and double crossover (DCO) frequencies in that interval. Assuming no interference (I=0), the expected DCO frequency is the product of the two interval RFs: Freq(DCO) = RF(R-S) × RF(S-W) = 0.20 × 0.10 = 0.02. The frequency of SCO events in the R-S interval is Freq(SCO R-S) = RF(R-S) - Freq(DCO) = 0.20 - 0.02 = 0.18. There are two reciprocal SCO gametes (rSw and RsW), so the frequency of the rSw gamete is half of the SCO frequency: 0.18 / 2 = 0.09. The expected number of r S w progeny is this frequency multiplied by the total number of progeny: 0.09 × 5000 = 450.

Question 7

1. 0.139 (correct answer)
2. 0.861
3. 1.44
4. 7.20

Explanation: The coefficient of coincidence (C) is the ratio of the observed double crossover (DCO) frequency to the expected DCO frequency. First, calculate the expected DCO frequency. This is the product of the recombination frequencies of the two adjacent intervals: Expected DCO freq = RF(A-B) × RF(B-C) = 0.18 × 0.24 = 0.0432. Next, calculate the number of expected DCO progeny in the sample: Expected DCO count = 0.0432 × 5000 = 216. The observed DCO count is given as 30. Finally, calculate the coefficient of coincidence: C = Observed DCO / Expected DCO = 30 / 216 ≈ 0.139.

Question 8

1. The gene order is b-a-c, and the parental chromosome configuration was b+ a+ c / b a c+.
2. The gene order is a-b-c, and the parental chromosome configuration was a+ b+ c / a b c+.
3. The gene order is a-c-b, and the parental chromosome configuration was a+ c b+ / a c+ b. (correct answer)
4. The gene order is c-b-a, and the parental chromosome configuration was c b+ a+ / c+ b a.

Explanation: When you encounter a three-factor cross like this, you're dealing with gene mapping through recombination analysis. The key insight is that crossing over frequency reveals both gene order and which chromosomes were present in the original parents. The most frequent progeny classes represent the parental types (no crossover), while the least frequent represent double crossover types. Here, the parentals are a+ b+ c and a b c+, and the double crossovers are a+ b+ c+ and a b c. To find gene order, compare the parental and double crossover classes. In double crossovers, the middle gene gets "flipped" relative to the outer genes. Looking at the first chromosome: parental a+ b+ c becomes a+ b+ c+ in the double crossover. The a+ stayed the same, b+ stayed the same, but c changed to c+. This means b is in the middle, giving us the gene order a-c-b. Now you can determine the parental configuration. If the order is a-c-b, then a+ b+ c becomes a+ c b+, and a b c+ becomes a c+ b. So the female parent was a+ c b+ / a c+ b. Answer A is wrong because it places b first, not in the middle. Answer B incorrectly keeps the original gene order without recognizing the crossing over pattern. Answer D reverses the entire order incorrectly. Study tip: Always identify parental vs. double crossover classes first, then use the "middle gene flips" rule to determine gene order before writing the final chromosome configuration.

Question 9

1. Order a-b-c; expected DCO = 0.96% (correct answer)
2. Order a-c-b; expected DCO = 1.44%
3. Order a-b-c; expected DCO = 2.40%
4. Order c-a-b; expected DCO = 0.96%

Explanation: The gene order is determined by comparing the recombination frequencies. The two smaller distances should sum to the largest distance. Here, RF(a-b) + RF(b-c) = 8% + 12% = 20%, which equals RF(a-c). This indicates that gene b is located between genes a and c. The correct gene order is a-b-c. The expected frequency of double crossovers (DCO) is the product of the recombination frequencies of the two adjacent intervals. Expected DCO freq = RF(a-b) × RF(b-c) = 0.08 × 0.12 = 0.0096, or 0.96%.

Question 10

1. The observed number of double crossovers was 25% lower than the expected number, indicating positive interference.
2. The observed number of double crossovers was 25% higher than the expected number, indicating negative interference. (correct answer)
3. A calculation error must have occurred, as interference cannot be a negative value.
4. The genes are assorting independently, and the result is due to sampling error.

Explanation: Interference (I) is calculated as I = 1 - C, where C is the coefficient of coincidence (Observed DCO / Expected DCO). If I = -0.25, then 1 - C = -0.25, which means C = 1.25. A coefficient of coincidence of 1.25 means that the observed number of double crossovers was 25% higher than the number expected based on single crossover frequencies. This phenomenon, where one crossover event increases the probability of a second nearby crossover, is called negative interference.

Question 11

1. 18
2. 36
3. 64
4. 82 (correct answer)

Explanation: The phenotype a B c corresponds to a gamete aBc. This gamete is produced by a single crossover (SCO) event between genes A and B. To find the number of these progeny, we must first calculate the frequency of such SCO events. The recombination frequency (RF) for an interval includes both SCOs and double crossovers (DCOs). Therefore, Freq(SCO) = RF - Freq(DCO). First, find the observed DCO frequency: Expected DCO freq = RF(A-B) × RF(B-C) = 0.20 × 0.30 = 0.06. The coefficient of coincidence (C) is Observed DCO / Expected DCO. So, Observed DCO freq = Expected DCO freq × C = 0.06 × 0.6 = 0.036. Now, find the SCO frequency for the A-B interval: Freq(SCO A-B) = RF(A-B) - Freq(DCO) = 0.20 - 0.036 = 0.164. There are two types of SCO gametes for this interval (aBc and AbC), so the frequency of the aBc gamete is half of this: 0.164 / 2 = 0.082. The expected number of a B c progeny is 0.082 × 1000 = 82.

Question 12

1. 1.64
2. 0.61
3. 0.39 (correct answer)
4. -0.64

Explanation: When you encounter genetics problems involving map distances and crossovers, you're dealing with chromosome mapping and interference. Interference measures how one crossover event affects the probability of another crossover occurring nearby. To solve this, you need to compare expected versus observed double crossovers. The expected number of double crossovers equals the product of individual crossover frequencies times total progeny: $0.13 \times 0.063 \times 2000 = 16.38$ expected double crossovers. The coefficient of coincidence (COC) is: $\frac{\text{observed}}{\text{expected}} = \frac{10}{16.38} = 0.61$ Interference is calculated as: $\text{Interference} = 1 - \text{COC} = 1 - 0.61 = 0.39$ This means 39% interference occurred, so answer C is correct. Let's examine why the other answers are wrong: Answer A (1.64) likely results from incorrectly calculating interference as expected/observed instead of 1 - (observed/expected). Answer B (0.61) is actually the coefficient of coincidence, not interference - this is a common trap where students confuse these related but distinct concepts. Answer D (-0.64) might come from a sign error or misunderstanding the interference formula. Remember that interference always ranges from 0 to 1, representing the degree to which one crossover reduces the probability of another nearby crossover. When you see mapping problems, always distinguish between coefficient of coincidence (observed/expected) and interference (1 - COC).

Question 13

1. 12.0 cM
2. 13.0 cM (correct answer)
3. 14.0 cM
4. 15.0 cM

Explanation: The map distance between two genes is calculated as the percentage of total recombination events occurring between them. This includes both single crossovers (SCOs) in that interval and all double crossovers (DCOs), as a DCO event involves a crossover in that interval. The formula is: Map Distance = [(Number of SCOs in interval) + (Number of DCOs)] / (Total Progeny) × 100. For the interval between L and M: Map Distance (L-M) = (96 + 8) / 800 × 100 = 104 / 800 × 100 = 0.13 × 100 = 13.0 cM.

Question 14

1. 26.0% (correct answer)
2. 28.0%
3. 30.0%
4. 32.0%

Explanation: The recombination frequency between the two outer genes is the proportion of progeny that have a new combination of alleles for those genes compared to the parental generation. Single crossovers (in either interval) create new combinations for the outer genes. However, double crossovers restore the parental combination of alleles for the two outer genes. Therefore, to calculate the recombination frequency between the outer genes, we sum the single crossovers and divide by the total progeny. RF(outer genes) = (Number of SCO(1) + Number of SCO(2)) / Total Progeny = (160 + 100) / 1000 = 260 / 1000 = 0.26 or 26.0%. Note this is different from the map distance, which would be the sum of the two interval distances: [(160+20)/1000 + (100+20)/1000]*100 = 18 + 12 = 30 cM.

Question 15

1. 38%
2. 44% (correct answer)
3. 50%
4. 47%

Explanation: Recombination frequency (RF) between outer markers (X and Z) is the sum of single crossover frequencies in each interval. First, calculate the observed double crossover (DCO) frequency. Expected DCO = RF(X-Y) * RF(Y-Z) = 0.30 * 0.20 = 0.06. Interference (I) = 1 - Coefficient of Coincidence (C). So, C = 1 - I = 1 - 0.5 = 0.5. Observed DCO = Expected DCO * C = 0.06 * 0.5 = 0.03. The frequency of single crossovers in the first interval is Freq(SCO X-Y) = RF(X-Y) - Obs DCO = 0.30 - 0.03 = 0.27. The frequency of single crossovers in the second interval is Freq(SCO Y-Z) = RF(Y-Z) - Obs DCO = 0.20 - 0.03 = 0.17. The total recombination frequency between X and Z is the sum of the single crossover frequencies: RF(X-Z) = Freq(SCO X-Y) + Freq(SCO Y-Z) = 0.27 + 0.17 = 0.44, or 44%.

Question 16

1. The observed number of double crossovers was 25% lower than the expected number, indicating positive interference.
2. The observed number of double crossovers was 25% higher than the expected number, indicating negative interference. (correct answer)
3. A calculation error must have occurred, as interference cannot be a negative value.
4. The genes are assorting independently, and the result is due to sampling error.

Explanation: Interference (I) is calculated as I = 1 - C, where C is the coefficient of coincidence (Observed DCO / Expected DCO). If I = -0.25, then 1 - C = -0.25, which means C = 1.25. A coefficient of coincidence of 1.25 means that the observed number of double crossovers was 25% higher than the number expected based on single crossover frequencies. This phenomenon, where one crossover event increases the probability of a second nearby crossover, is called negative interference.

Question 17

1. 0.139 (correct answer)
2. 0.861
3. 1.44
4. 7.20

Explanation: The coefficient of coincidence (C) is the ratio of the observed double crossover (DCO) frequency to the expected DCO frequency. First, calculate the expected DCO frequency. This is the product of the recombination frequencies of the two adjacent intervals: Expected DCO freq = RF(A-B) × RF(B-C) = 0.18 × 0.24 = 0.0432. Next, calculate the number of expected DCO progeny in the sample: Expected DCO count = 0.0432 × 5000 = 216. The observed DCO count is given as 30. Finally, calculate the coefficient of coincidence: C = Observed DCO / Expected DCO = 30 / 216 ≈ 0.139.

Question 18

1. 18
2. 36
3. 64
4. 82 (correct answer)

Explanation: The phenotype a B c corresponds to a gamete aBc. This gamete is produced by a single crossover (SCO) event between genes A and B. To find the number of these progeny, we must first calculate the frequency of such SCO events. The recombination frequency (RF) for an interval includes both SCOs and double crossovers (DCOs). Therefore, Freq(SCO) = RF - Freq(DCO). First, find the observed DCO frequency: Expected DCO freq = RF(A-B) × RF(B-C) = 0.20 × 0.30 = 0.06. The coefficient of coincidence (C) is Observed DCO / Expected DCO. So, Observed DCO freq = Expected DCO freq × C = 0.06 × 0.6 = 0.036. Now, find the SCO frequency for the A-B interval: Freq(SCO A-B) = RF(A-B) - Freq(DCO) = 0.20 - 0.036 = 0.164. There are two types of SCO gametes for this interval (aBc and AbC), so the frequency of the aBc gamete is half of this: 0.164 / 2 = 0.082. The expected number of a B c progeny is 0.082 × 1000 = 82.

Question 19

1. 50
2. 100
3. 450 (correct answer)
4. 500

Explanation: The phenotype r S w corresponds to the gamete rSw. This gamete is formed by a single crossover (SCO) between genes R and S. The frequency of recombination (RF) in this interval is given as 20 cM or 0.20. The RF is the sum of SCO and double crossover (DCO) frequencies in that interval. Assuming no interference (I=0), the expected DCO frequency is the product of the two interval RFs: Freq(DCO) = RF(R-S) × RF(S-W) = 0.20 × 0.10 = 0.02. The frequency of SCO events in the R-S interval is Freq(SCO R-S) = RF(R-S) - Freq(DCO) = 0.20 - 0.02 = 0.18. There are two reciprocal SCO gametes (rSw and RsW), so the frequency of the rSw gamete is half of the SCO frequency: 0.18 / 2 = 0.09. The expected number of r S w progeny is this frequency multiplied by the total number of progeny: 0.09 × 5000 = 450.

Question 20

1. Order a-b-c; expected DCO = 0.96% (correct answer)
2. Order a-c-b; expected DCO = 1.44%
3. Order a-b-c; expected DCO = 2.40%
4. Order c-a-b; expected DCO = 0.96%

Explanation: The gene order is determined by comparing the recombination frequencies. The two smaller distances should sum to the largest distance. Here, RF(a-b) + RF(b-c) = 8% + 12% = 20%, which equals RF(a-c). This indicates that gene b is located between genes a and c. The correct gene order is a-b-c. The expected frequency of double crossovers (DCO) is the product of the recombination frequencies of the two adjacent intervals. Expected DCO freq = RF(a-b) × RF(b-c) = 0.08 × 0.12 = 0.0096, or 0.96%.