In Drosophila melanogaster, cut wings (ct) is recessive to normal wings (ct+), sable body (s) is recessive to gray body (s+), and vermilion eyes (v) is recessive to red eyes (v+). All three recessive mutations are X- linked. A female fly with cut wings, sable body, and vermilion eyes is crossed to a male with normal wings, gray body, and red eyes. The F1 females produced by this cross were mated with cut, sable, vermilion males in a testcross. The following are the progeny resulting from the testcross.
v ct s 510
v+cts 1
v+ ct+ s 14
v+ ct+ s+ 500
v+cts+ 73
v ct s+ 20
v ct+ s 81
v ct+ s+ 1
Total 1200
Determine the order of these genes on the chromosome.
Calculate the map distances between the genes.
Determine the coefficient of coincidence and the interference among these genes.
s 3 v 13 ct
To determine the order of the genes on the chromosome, we can start by looking at the progeny resulting from the testcross. We'll focus on the progeny with the double mutant phenotype (v ct s).
From the testcross, we obtain the following progeny:
- v ct s: 510
- v+ ct+ s: 14
- v ct+ s: 81
- v ct s+: 20
Now, let's compare the numbers of each phenotype to determine the order of the genes:
- v ct s: This progeny has the double mutant phenotype, so it must have received the recessive alleles for all three genes from both parents. Therefore, we can conclude that the genotype of the female parent is v ct s.
- v+ ct+ s and v ct+ s: Both these progeny have the phenotype of the female parent, but they also have at least one of the dominant alleles from the male parent. This suggests that the genes are in a different order than the female parent. However, we don't know the individual order of ct and s from these progeny alone.
- v ct s+: This progeny has the same phenotype as the female parent, but it carries one copy of the dominant allele for vermilion eyes from the male parent. This suggests that the gene for vermilion eyes (v) is located between the genes for cut wings (ct) and sable body (s).
- v+ ct+ s+: This progeny has the phenotype of the male parent, indicating that it has at least one copy of each dominant allele from the male parent. This suggests that the gene for gray body (s) is located between the genes for cut wings (ct) and vermilion eyes (v).
Based on these observations, we can conclude that the order of the genes on the chromosome is: ct - v - s.
Next, let's calculate the map distances between the genes.
Map distance between ct and v:
To calculate the map distance, we divide the number of recombinant progeny (v ct+ s and v+ ct s) by the total number of progeny. In this case, the number of recombinant progeny is 14 + 81 = 95, and the total number of progeny is 1200. Therefore, the map distance between ct and v is 95/1200 = 0.0792 or 7.92 cM.
Map distance between v and s:
Similarly, we divide the number of recombinant progeny (v ct s+ and v+ ct s+) by the total number of progeny. The number of recombinant progeny is 20 + 1 = 21, and the total number of progeny is 1200. Therefore, the map distance between v and s is 21/1200 = 0.0175 or 1.75 cM.
Finally, let's determine the coefficient of coincidence and interference.
Coefficient of coincidence (C.O.C.):
The coefficient of coincidence is calculated by dividing the observed double recombinant progeny (v ct+ s+ and v+ ct s) by the expected double recombinant progeny. In this case, the observed double recombinant progeny is 1 + 14 = 15, and the expected double recombinant progeny can be calculated by multiplying the individual recombinant frequencies: (95/1200) * (21/1200) = 0.0197. Therefore, the coefficient of coincidence is 15/0.0197 = 761.42.
Interference:
The interference can be calculated by subtracting the coefficient of coincidence from 1. In this case, 1 - 761.42 = -760.42.
Therefore, the coefficient of coincidence is 761.42 and the interference is -760.42.
To summarize:
- The order of the genes on the chromosome is ct - v - s.
- The map distance between ct and v is 7.92 cM.
- The map distance between v and s is 1.75 cM.
- The coefficient of coincidence is 761.42.
- The interference is -760.42.
To determine the order of these genes on the chromosome, we can use the information provided by the progeny resulting from the testcross.
First, let's analyze the F1 females produced by crossing the female with cut wings, sable body, and vermilion eyes with the male with normal wings, gray body, and red eyes.
From the testcross progeny data, we can determine the genotypes of the F1 females:
v ct s 510
v+cts 1
v+ ct+ s 14
v+ ct+ s+ 500
v+cts+ 73
v ct s+ 20
v ct+ s 81
v ct+ s+ 1
Based on the data, we can conclude that the F1 females produced four different genotypes:
1. v+ ct+ s+
2. v+ct+ s
3. v ct+ s
4. v ct+ s+
Next, let's analyze the map distances between the genes. To do this, we need to calculate the recombination frequencies between each pair of genes.
1. v+ ct+ s+ and v+ct+ s:
Recombinant progeny (v+ ct+ s): 14
Total progeny: 1200
Recombination frequency = Recombinant progeny / Total progeny = 14 / 1200 = 0.0117
2. v+ ct+ s+ and v ct+ s:
Recombinant progeny (v ct+ s+): 1
Total progeny: 1200
Recombination frequency = Recombinant progeny / Total progeny = 1 / 1200 = 0.0008
3. v+ ct+ s+ and v ct+ s+:
Recombinant progeny (v ct+ s): 81
Total progeny: 1200
Recombination frequency = Recombinant progeny / Total progeny = 81 / 1200 = 0.0675
4. v+ct+ s and v ct+ s+:
Recombinant progeny (v ct+ s): 73
Total progeny: 1200
Recombination frequency = Recombinant progeny / Total progeny = 73 / 1200 = 0.0608
Based on these recombination frequencies, we can determine the gene order:
v ct+ s - v+ct+ s - v ct+ s+ - v+ ct+ s+
Now, let's determine the coefficient of coincidence and interference. The coefficient of coincidence (C.O.C.) is the ratio of the observed double crossovers to the expected double crossovers. The interference is 1 minus the coefficient of coincidence.
From the data, we can calculate the observed double crossovers:
Observed double crossovers (v+cts): 1
Now we need to calculate the expected double crossovers based on the recombination frequencies:
Expected double crossovers = Recombination frequency between v+ ct+ s+ and v ct+ s * Recombination frequency between v ct+ s and v+ ct+ s+ * Total progeny
Expected double crossovers = 0.0008 * 0.0608 * 1200 = 0.0584
Now we can calculate the coefficient of coincidence:
C.O.C. = Observed double crossovers / Expected double crossovers = 1 / 0.0584 = 17.12
The interference is 1 - C.O.C.:
Interference = 1 - 17.12 = -16.12
Note: The negative value for the interference indicates a greater number of double crossovers observed than expected, suggesting interference is occurring.
Therefore, the order of the genes on the chromosome is v ct+ s - v+ct+ s - v ct+ s+ - v+ ct+ s+, the map distances between the genes are 0.0117, 0.0008, and 0.0675, and the coefficient of coincidence is 17.12 with an interference of -16.12.