Genetic maps are based on recombination frequencies. Because both odd and even numbers of crossovers can occur between any two gene loci, as the physical distance between two loci increases, the maximum recombination frequency levels off at 50%. However, suppose you discovered a species where only an even number of crossovers can occur between any two gene loci. In this case, as the physical distance between two loci increases, you would expect the maximum recombination frequency to
A. remain at zero.
B. increase with no limit.
C. level off at 25%.
D. level off at 75%.
E. level off at 100%.
Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain?
Gather Content
· What do you already know about recombination frequency?
Consider Possibilities
· What other information is related to the question? Which information is most useful?
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
A. remain at zero.
Clarify Question
· What is the key concept addressed by the question?
o This question addresses recombination frequency.
· What type of thinking is required?
o You need to Analyze the information given, using logic, to dissect the problem and determine the answer.
· What key words does the question contain?
o “Gene loci” is just another way of saying genes. The loci are the locations on the chromosome. Recombination frequency is the percent of progeny with a recombined chromosome – that is, a set of alleles that differs from the original parental chromosomes.
Gather Content
· What do you already know about recombination frequency?
o Recombination frequency is the percent of progeny with a recombined chromosome – that is, a set of alleles that differs from the original parental chromosomes.
o Normally, the frequency increases with distance because there is more physical room for the crossover of DNA strands to occur.
Consider Possibilities
· What other information is related to the question? Which information is most useful?
o Drawing a diagram is a useful way to attack this problem.
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
o Draw a chromosome pair with two genes, A and B on the top chromosome, and a and b on the bottom chromosome.
o If one crossover event occurs (draw an X between the chromosomes), you can now see that A and b are connected on one chromosome, and a and B are connected on the other chromosome.
o But if a second crossover event occurs (draw another X next to the first), the A allele is NOW connected to the B allele – just like the parental chromosome.
o So normally, as distance increases, the even-numbered crossover events re-connect the parental combination. This counteracts the effect of the first crossover. Eventually, at large distance, only half the events will be odd-numbered, and thus “visible” in the progeny.
o What happens if ALL the events are even-numbered? Then the parental chromosomes will always result (A and B, and a and b). This would appear to be a 0% recombination frequency.
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
o This question required you to Analyze the information given, using logic, to dissect the problem and determine the answer.
o Did you recognize that double crossover events connect the genes like on the original chromosome?
o Did you try drawing a diagram to help answer the question?
o What if two genes always had exactly one crossover between them? How would they segregate?
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