I desperately need some help with thisquestion!!...I really thank you for your t

Question

I desperately need some help with thisquestion!!...I really thank you for your time and knowledge!

1. Two genes, a and d, are closely linked on the same chromosome.After completing test crosses with these two genes, the rate ofrecombination or recombination frequency is found to be 0. What isthe best explanation for this?
2. In a series of test crosses, the rate of recombination betweena and c is 0.3; between b and c is 0.47; and between a and b is0.2. Generate the linkage map for these three genes.

    c. Gene d (on the samechromosome) has a rate of recombination of 0.05 with gene b and arate of   0.25 with gene a. Where would the gene beplaced on the previous map? I desperately need some help with thisquestion!!...I really thank you for your time and knowledge!

1. Two genes, a and d, are closely linked on the same chromosome.After completing test crosses with these two genes, the rate ofrecombination or recombination frequency is found to be 0. What isthe best explanation for this?
2. In a series of test crosses, the rate of recombination betweena and c is 0.3; between b and c is 0.47; and between a and b is0.2. Generate the linkage map for these three genes.

    c. Gene d (on the samechromosome) has a rate of recombination of 0.05 with gene b and arate of   0.25 with gene a. Where would the gene beplaced on the previous map?

1. Two genes, a and d, are closely linked on the same chromosome.After completing test crosses with these two genes, the rate ofrecombination or recombination frequency is found to be 0. What isthe best explanation for this?
2. In a series of test crosses, the rate of recombination betweena and c is 0.3; between b and c is 0.47; and between a and b is0.2. Generate the linkage map for these three genes.

    c. Gene d (on the samechromosome) has a rate of recombination of 0.05 with gene b and arate of   0.25 with gene a. Where would the gene beplaced on the previous map?

Explanation / Answer

(A) If the recombination frequency is 0, this would indicate that thegenes are pretty much adjacent to each other on the chromosome -they are always sorted together and recombination does notoccur because there is not enough space in between the genes forcrossing over to happen. Either that, or the 'two genes' areactually 1 gene. (B) 'Rate of recombination' is the same as 'recombination frequency'.Also remember that 1 map unit is equal to 0.01 (1%) recombinationfrequency. So we know that A and C have a recombination frequencyof 0.3, or 30%, which means they are 30 map units apart: A                            C    |---------30---------| We also know that B is 47 map units from c, so there are a numberof possibilities:    A        B?              C    |--------30---------|        Not possible, since B->C must be 47 map units, and A-->C isonly 30 units. A                            C                      B    |---------30---------|--------47------| B          A                            C   |----17----|---------30---------|     (17 + 30= 47 map units)    |-------------47----------------| However, we also know that A and B are 20 map units apart, so wecan rule out the A-C-B arrangement, since this has A and B 30+47 =77 units apart. Although BAC has them 17 units apart - this isclose enough to 20! (C) Now gene D is 5 map units from B and 25 map units from A - it isfurther away from A than it is from B. This means it must occurbefore B:    D        B              A                       C     |---5--|-----20-----|-------30-------|     |--------25---------|              |-------------~47------------|

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