An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
What is the expected number of possible gamete genotypes for a four-point testcross?
A. |
B. |
C. |
D. |
The tester genotype in this testcross is
Why isn’t the tester’s gamete contribution listed in the table of 1000 progeny?
What is the expected number of possible gamete genotypes for a dihybrid testcross?
What is the expected number of possible gamete genotypes for a three-point testcross?
Another way to approach this problem is to write out every genotype: A·B·C·D, A·B·C·d, and so on.
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
Are any gamete genotypes missing from this four-point testcross? If so, how many?
A. |
B. |
C. |
D. |
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
The number of possible gamete genotypes in a four-point testcross is
Having solved the problem in the previous step to determine the number of possible gamete genotypes, compare that number with the number of genotypes observed among the 1000 progeny.
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
Which pairs of alleles never appear together among the gamete genotypes?
A. |
B. |
C. |
D. |
E. |
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Review the progeny table (click SHOW PROGENY TABLE above) and organize the genotypes so that you can see which pairs of alleles are present. For example, all 4 alleles for genes A and B are observed in all 4 possible combinations: a with B, a with b, A with b, and A with B. Compare that with the 16 possible genotypes and deduce which pairs of alleles are missing among the gamete genotypes contributed by the heterozygous parent.
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
If genes under study are on separate chromosomes, then the genotypes in the testcross results should be in roughly equal amounts because they assort independently. Is this statement true or false?
For a dihybrid with the genes on separate chromosomes, the four possible gamete genotypes should be in which of the following
ratios?
When genes under study are linked on the same chromosome, then the genotypes in the testcross results should be in roughly equal amounts. Is this statement true or false?
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
A and B | |
A and C | |
A and D | |
B and C | |
B and D | |
C and D |
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Why are the genotypes a·b·C·D and A·B·c·d the most prevalent among the 1000 progeny?
When we say that genes are linked, we mean
Recall that Step 4 explained the connection between recombination frequency and linkage. Genes are linked when the recombination frequency is
When calculating the recombination frequency for the A and B gene pair based on the observed gamete genotypes in the table, which set of genotypes contains only the recombinants for loci A and B? (To consult the progeny table, click SHOW PROGENY TABLE above.)
How many recombinants for loci A and B are present among the 1000 progeny?
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
Do any pair(s) of genes fail to exhibit recombination?
A. |
B. |
C. |
D. |
E. |
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
If no crossing over occurs between two loci during meiosis, then the recombination frequency is
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
If a pair of genes does not undergo recombination, then what can you conclude?
A. |
B. |
C. |
D. |
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Two genes are linked when their recombination frequency is
What is the relationship between recombination frequency (RF) and distance between two loci?
Recall that recombinants are formed by crossing over, and there is a relationship between crossover frequency between two genes and the physical distance between those genes on a chromosome.
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? (The pair of alleles in parentheses are so close together that their order cannot be determined.)
A. |
B. |
C. |
D. |
E. |
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Can pure-breeding lines be heterozygous at any of the genes under study?
Which genotypes are the most abundant ones found among the 1000 progeny? (Consult the progeny table by clicking the SHOW PROGENY TABLE button above.)
Is the order of the genes as presented in the progeny table always the order of the genes as found on the chromosome?
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
Identify the gene order and the map distances. (Note that two of the genes are so close together that their order cannot be determined. Write them as follows: (X,Y), where X and Y represent two of the genes in the problem, A, B, C, and D.)
A. |
B. |
C. |
D. |
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
If the recombination frequency (RF) between genes X and Y is 17% and the RF between Y and Z is 9%, then the possible order of these three genes is limited to
If the recombination frequency (RF) between genes X and Y is 17%, the RF between Y and Z is 9%, and the RF between X and Z is 8%, then the gene order must be
What is the relationship between recombination frequency (RF) and the distance (in map units) between two loci?
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
Is there evidence of interference in this testcross? Why or why not?
A. |
B. |
C. |
D. |
Interference occurs when
If crossovers occur independently, then
An individual heterozygous for four genes, A/a·B/b·C/c·D/d, is testcrossed with a/a·b/b·c/c·d/d, and 1000 progeny are classified by the gamete contribution of the heterozygous parent as follows:
a·B·C·D | 42 |
A·b·c·d | 43 |
A·B·C·d | 140 |
a·b·c·D | 145 |
a·B·c·D | 6 |
A·b·C·d | 9 |
A·B·c·d | 305 |
a·b·C·D | 310 |
Which genes are linked? If two pure-breeding lines had been crossed to produce the heterozygous individual, what would their genotypes have been? Draw a linkage map of the linked genes, showing the order and the distances in map units. Calculate an interference value, if appropriate.
Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.
Calculate I, the interference, if appropriate.
A. |
B. |
C. |
D. |
E. |
What is the coefficient of coincidence?
What is the formula for interference?
Conclusion
This four-point testcross reduces to the familiar three-point testcross because genes A and D are so close together on the chromosome that they do not exhibit any crossing over and hence no recombination. A clue that this four-point testcross had an anomaly was the presence of only 8 gamete genotypes instead of the expected 16. The recombination frequencies can be calculated from the table provided of gamete genotypes found in the 1000 progeny. These values are then used to determine the gene order and the map distances between the genes. The most frequent gamete genotypes among the progeny are the parentals and reveal the alleles that are grouped together on each individual chromosome.