• Step 1
  • Step 2
  • Step 3
  • Step 4
  • Step 5
  • Step 6
  • Step 7
  • Step 8
  • Step 9
  • Step 10
  • Step 11

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

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
1

What is the expected number of possible gamete genotypes for a four-point testcross?

A.
B.
C.
D.

1

The tester genotype in this testcross is

A/a·B/b·C/c·D/d
A·B·C·D
a·b·c·d
a/a·b/b·c/c·d/d

Why isn’t the tester’s gamete contribution listed in the table of 1000 progeny?

The tester does not contribute any genes to the progeny.
The tester contributes gametes carrying only recessive alleles, so the progeny phenotypes directly reveal the alleles contributed by the gametes of the heterozygous parent.
The gametes contributed by the tester gametes carry only dominant alleles, so the progeny phenotypes directly reveal the alleles contributed by the gametes of the heterozygous parent.
The gametes contributed by the tester are lost during development of the progeny, so the progeny phenotypes directly reveal the alleles contributed by the gametes of the heterozygous parent.

What is the expected number of possible gamete genotypes for a dihybrid testcross?

2
4
6

What is the expected number of possible gamete genotypes for a three-point testcross?

8
16
24

Another way to approach this problem is to write out every genotype: A·B·C·D, A·B·C·d, and so on.

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
2

Are any gamete genotypes missing from this four-point testcross? If so, how many?

A.
B.
C.
D.

1
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

less than the number of possible gamete genotypes in a three-point testcross.
equal to the number of possible gamete genotypes in a three-point testcross.
greater than the number of possible gamete genotypes in a three-point testcross.

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.

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
3

Which pairs of alleles never appear together among the gamete genotypes?

A.
B.
C.
D.
E.

1
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.

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
4

Describe a test that is diagnostic for linkage by filling in the blanks in the sentence below.
Analyze the at a time. They are linked when the is .
Correct. When the progeny in a testcross reveal a recombinant frequency of less than 50%, then the pair of genes being analyzed must be linked.
Incorrect. Two genes that are close together on the same chromosome pair do not assort independently, so they produce a recombinant frequency of less than 50%. Thus, when the progeny in a testcross reveal a recombinant frequency of less than 50%, then the pair of genes being analyzed must be linked.
1
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?

True
False

For a dihybrid with the genes on separate chromosomes, the four possible gamete genotypes should be in which of the following
ratios?

9:3:3:1
4:3:2:1
3:3:1:1
1:1:1:1

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?

True
False
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
5

Which genes are linked? Fill in the table by choosing the correct answer from each pull-down menu.
A and B
A and C
A and D
B and C
B and D
C and D
1
Correct. Every pair of genes has a recombination frequency of less than 50%, so all of the genes are linked.
Incorrect. Every pair of genes has a recombination frequency of less than 50%, so all of the genes are linked. Review how you carried out the recombination frequency calculations.
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?

They are the result of multiple crossover events during meiosis.
They reflect the parental genotypes, so the genes were inherited as a package.
They are the result of a dihybrid crossover event.
They reflect the result of a single crossover between chromatids during meiosis.

When we say that genes are linked, we mean

they code for the same phenotype.
they are located on the same chromosome.
they never undergo recombination during meiosis.
they frequently undergo recombination during meiosis.

Recall that Step 4 explained the connection between recombination frequency and linkage. Genes are linked when the recombination frequency is

less than 50%.
equal to 50%.
more than 50%.

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.)

a·B·C·D, A·b·c·d, A·B·C·d, and a·b·c·D
a·B·C·D, A·b·C·d, A·B·c·d, and a·b·C·D
a·B·C·D, A·b·C·d, a·B·c·D, and A·B·c·d
a·B·C·D, A·b·c·d, a·B·c·D, and A·b·C·d

How many recombinants for loci A and B are present among the 1000 progeny?

100
372
300
630
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
6

Do any pair(s) of genes fail to exhibit recombination?

A.
B.
C.
D.
E.

1
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

100%.
greater than 50% and less than 100%.
greater than 0% and less than 1%.
0%.
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
7

If a pair of genes does not undergo recombination, then what can you conclude?

A.
B.
C.
D.

1
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

less than 50%.
equal to 50%.
more than 50%.

What is the relationship between recombination frequency (RF) and distance between two loci?

The larger the RF, the greater the distance.
The smaller the RF, the greater the distance.
The larger the RF, the smaller the distance.

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.

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
8

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.

1
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?

Yes
No

Which genotypes are the most abundant ones found among the 1000 progeny? (Consult the progeny table by clicking the SHOW PROGENY TABLE button above.)

The parental genotypes
The nonrecombinant genotypes
The recombinant genotypes
The first and second answers are both correct
The first and third answers are both correct

Is the order of the genes as presented in the progeny table always the order of the genes as found on the chromosome?

Yes
No
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
9

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.

1
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

XYZ or XZY
XZY or YXZ
YXZ or XYZ

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

XYZ
YXZ
XZY

What is the relationship between recombination frequency (RF) and the distance (in map units) between two loci?

RF is always greater than map units.
RF is always equivalent to map units.
RF is always less than map units.
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
10

Is there evidence of interference in this testcross? Why or why not?

A.
B.
C.
D.

1

Interference occurs when

two adjacent crossovers intertwine and alter the outcome of the chromatid exchange.
a crossover in one chromosome affects the likelihood that a crossover will occur in another chromosome.
a crossover in one region of a chromosome affects the likelihood that a crossover will occur in an adjacent region.

If crossovers occur independently, then

the product rule can be used to calculate the expected frequency of a double crossover.
the sum rule can be used to calculate the expected frequency of a double crossover.
the associative rule can be used to calculate the expected frequency of a double crossover.
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

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.

  • 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.
11

Calculate I, the interference, if appropriate.

A.
B.
C.
D.
E.

1

What is the coefficient of coincidence?

The ratio of expected double recombinants to observed double recombinants
The ratio of observed double recombinants to expected double recombinants
1- (the ratio of expected double recombinants to observed double recombinants)
1- (the ratio of observed double recombinants to expected double recombinants)

What is the formula for interference?

1 + (the ratio of expected double recombinants to observed double recombinants)
1 + (the ratio of observed double recombinants to expected double recombinants)
1 - (the ratio of expected double recombinants to observed double recombinants)
1 - (the ratio of observed double recombinants to expected double recombinants)

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.