Chapter 4. Chapter 4: Completing a Four-Point Testcross

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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:

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.

• Part A (steps 1-3): Look carefully at the progeny data in the table. Look carefully at the specific genotypes and the number of each genotype among the 1000 progeny.
• Part B (steps 4 and 5): Determine which genes are linked by understanding how the different progeny genotypes came to be and why pairs of genotypes appear in similar numbers among the progeny.
• Part C (steps 6-9):Calculate recombination frequencies to determine map distances and the relative gene order.
• Part D (steps 10 and 11): Calculate interference between adjacent crossovers.

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.