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.
What is a reciprocal translocation?
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What is an acentric chromosome?
What are homologous chromosomes?
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.
The F1 are semisterile and phenotypically Pr. This means that the F1 are:
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Which stem color is dominant?
In the F1 does each cell have a normal set of chromosomes?
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.
The backcross was performed with the parent having standard chromosomes. Why was the backcross not performed with the other parent?
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What is another name for the parent that was used in the backcross?
Why is it valuable to use a tester when performing a backcross in order to map the distance between two locations on a chromosome?
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.
This is a mapping problem. What is map distance equivalent to?
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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?
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.
Based on Figure 4-8 and the number of different types of progeny in the backcross, which progeny should be classified as parental type?
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The parental type progeny are found
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.
How many recombinants are there among the backcross progeny?
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There are more recombinants than parental type progeny in the backcross results. Is this statement true or false?
Given the results, would you expect the locus for the Pr gene to be located on chromosome 2 or 5?
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.
What is the map distance between the Pr locus and the translocation point?
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The map distance is equal to the recombinant frequency. Is this statement true or false?
The recombinant frequency is calculated as the
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.