Morgan's student Sturtevant demonstrated that the recombination frequencies between a series of linked genes is additive. Examine the following recombination data from Sturtevant, and determine the proper order of the genes on the Drosophila X chromosome. Assume y is in the 0.0 position.

Question

A.  y m v w

B.  y w v m

C.  y m w v

D.  y w m v



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?

Answer 1

B.  y w v m

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses recombination frequency.
· What type of thinking is required?
        o This answer requires you to weigh and judge evidence – to Evaluate – and choose the best of the possible answers.
· What key words does the question contain?
        o Linked genes are genes that lie near each other on a chromosome – near enough that they do not segregate independently. They will segregate together unless there is a recombination event between them, which takes place at some frequency, depending on distance. This is the recombination frequency.
 
Gather Content
· What do you already know about recombination frequency?
        o Recombination frequency reflects the distance between two genes. If the genes are close, the frequency will be low; if far apart, it will be higher, up to 0.5.
        o The frequency between two genes will be the sum of the frequencies between those flanking genes and a middle gene (up to 0.5).
 
Consider Possibilities
· What other information is related to the question? Which information is most useful?
        o For this problem, drawing a diagram can be helpful.

  Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o We are told that yellow is at the 0.0 position – in other words, at the end of the chromosome, so start by drawing y on the left.
        o What recombination data do we have for yellow and other genes? 0.322 between y and v, and 0.010 between y and w. So white is closer to yellow and vermilion is much further.
        o Now you can draw y-w----------v.
        o What other data do we have, to help place miniature? 0.030 between v and m, and 0.337 between w and m. So miniature is close to vermilion, and far from white.
        o But is miniature between white and vermilion, or on the far side of vermilion? The recombination frequency is greater for white and miniature (0.337), than for white and vermilion (0.297), so miniature must be the furthest away.
        o Thus: y-w----------v ---m, or y w v m.
 
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 answer requires you to weigh and judge evidence – to Evaluate – and choose the best of the possible answers.
        o Did you recognize that recombination frequency reflects gene distance?
        o Did you realize that recombination frequencies are additive?
        o Did you draw a diagram to help you interpret the data?