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Chapter 17. Chapter 17: Mapping Chromosomal Translocations

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

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 are semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

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): Define the terms used in the problem.
  • Part B (steps 4-6): Set up the mapping problem.
  • Part C (step 7): Solve the mapping problem.
1

What is a reciprocal translocation?

A.
B.
C.
D.

1

What is an acentric chromosome?

A chromosome that lacks a properly defined center
A chromosome that is missing proper telomeres
A chromosome that has been irradiated
A chromosome that is missing a centromere

What are homologous chromosomes?

Chromosomes that have similar ancestry
Chromosomes that have the same level of gene expression
Chromosomes that pair with each other at meiosis
Chromosomes that have identical gene patterns

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 are semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

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): Define the terms used in the problem.
  • Part B (steps 4-6): Set up the mapping problem.
  • Part C (step 7): Solve the mapping problem.
2

The F1 are semisterile and phenotypically Pr. This means that the F1 are:

A.
B.
C.
D.

1

Which stem color is dominant?

Green stems
Purple stems
They are co-dominant.
It depends on the effects of other genes.

In the F1 does each cell have a normal set of chromosomes?

Yes, because the pr/pr parent contributed only normal chromosomes.
No, chromosomes 2 and 5 will be heterozygous for the reciprocal translocation.

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 are semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

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): Define the terms used in the problem.
  • Part B (steps 4-6): Set up the mapping problem.
  • Part C (step 7): Solve the mapping problem.
3

The backcross was performed with the parent having standard chromosomes. Why was the backcross not performed with the other parent?

A.
B.
C.

1

What is another name for the parent that was used in the backcross?

Recombinant
Tester
Dihybrid
Normal

Why is it valuable to use a tester when performing a backcross in order to map the distance between two locations on a chromosome?

The tester contributes gametes that carry only dominant alleles, so the offspring phenotypes reveal the meiotic products of the F1.
The tester contributes gametes that carry only recessive alleles, so the offspring phenotypes reveal the meiotic products of the F1.
The tester does not undergoing crossing over during meiosis, so all recombinants must come from gametes of the F1.

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 are semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

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): Define the terms used in the problem.
  • Part B (steps 4-6): Set up the mapping problem.
  • Part C (step 7): Solve the mapping problem.
4

This is a mapping problem. What is map distance equivalent to?

A.
B.
C.
D.
E.
F.

1

The distance between two locations on a chromosome has no effect on the number of crossover events that could occur within that space. Is this statement true or false?

True
False

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 are semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

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): Define the terms used in the problem.
  • Part B (steps 4-6): Set up the mapping problem.
  • Part C (step 7): Solve the mapping problem.
5

Based on Figure 4-8 and the number of different types of progeny in the backcross, which progeny should be classified as parental type?

A.
B.
C.
D.
E.
F.

1

The parental type progeny are found

in the greatest numbers among the progeny.
in the fewest numbers among the progeny.
in the largest single group of progeny (in this problem that is semisterile Pr).

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 are semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

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): Define the terms used in the problem.
  • Part B (steps 4-6): Set up the mapping problem.
  • Part C (step 7): Solve the mapping problem.
6

How many recombinants are there among the backcross progeny?

A.
B.
C.
D.
E.
F.

1

There are more recombinants than parental type progeny in the backcross results. Is this statement true or false?

True
False

Given the results, would you expect the locus for the Pr gene to be located on chromosome 2 or 5?

Yes, because the two phenotypes do not appear to assort independently.
No, you cannot tell from these results.

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 are semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

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): Define the terms used in the problem.
  • Part B (steps 4-6): Set up the mapping problem.
  • Part C (step 7): Solve the mapping problem.
7

What is the map distance between the Pr locus and the translocation point?

A.
B.
C.
D.
E.

1

The map distance is equal to the recombinant frequency. Is this statement true or false?

True
False

The recombinant frequency is calculated as the

sum of all the recombinant types.
(total number of recombinants)/(total number of parentals).
(total number of recombinants)/(total number of progeny).
(parentals - recombinants)/(total number of progeny).

Conclusion

This is essentially a dihybrid mapping problem in which one of the gene loci has been replaced with the translocation break point. Recognizing this and that both green stems and semisterility are dominant in the heterozygote makes it possible to choose the proper tester to perform the backcross. The tester in this case is the parent that is purple stemmed and has normal chromosomes (since these are recessive in the F1). The progeny from the backcross are not in a 1:1:1:1 ratio and instead are grouped into a large number of parental type offspring and a much smaller group of recombinants. The latter group results from crossovers between the Pr locus and the translocation breakpoint on either chromosome 2 or 5 (you cannot tell which from the data provided). The number of recombinants is the sum of the two smaller classes. The frequency of recombinants is that number divided by the total progeny. In other words, (145 + 186)/(145 + 186 + 727 + 764) = 331/1822 = 0.1817. As a percentage, this is 18.17% or 18.17 map units.