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# TBR Genetics :( Hard problem!

Discussion in 'MCAT Study Question Q&A' started by greenseeking, Jul 2, 2012.

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1. ### greenseekingNew Member

Joined:
Jun 5, 2010
Messages:
124

Based on the information in Experiment II, estimate the minimum genetic map distance between the two genetic loci. (Note: One genetic map unit (m.u.)
gives a recombinant frequency (RF) of 1 percent.)

I scanned the whole passage in the attachment.

This is what I did:
B+/ba x ba/ba (parental strands)

If there is no crossover, B+/ba (black), ba/ba (white)
If there is one crossover (between the sister chromatids), BA/ba (albino), b+/ba (brown)

So in the crossover, since crossover was done in between the two alleles, 100+34/200= 67amu. (wrong)

The back of the book started doing crossover between non sister chromatids. What about the crossover between sister chromatids? In the genetics chapter, they have an example where they find out the distance between the genes through crossing over between sister chromatids.

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2. ### ReneeisnowhereNew Member

Joined:
Jan 14, 2012
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4
Status:
Pre-Medical
Let me see if I can help.

F1 gen is B/b; +/a
B/b; +/a(Black) x b/b; a/a (white)
What we have is :
Black offspring (must no be cross over)
Brown offspring ( must be cross over)
white offspring (mixture cross over/ not crossed)

Now if we add up the possibles of crossover, don't forget the whites that exist that are crossover.
One will assume here that if 34 brown crossover's happened, then about 34 white crossover's happened. That would mean that 2x 34/2= 34x 100%= 34 units. Don't be too quick in assuming just because they all look the same, they have the same genes. Epistasis means watch out and be careful.

hth,

Renee
3. ### greenseekingNew Member

Joined:
Jun 5, 2010
Messages:
124
hi renee,

that helped a lot thanks! but can I ask why you can assume that the white crossover mice have frequency of 34?? Also in crossover, it's the crossing between two sister chromatids of the same homolog right?
4. ### ReneeisnowhereNew Member

Joined:
Jan 14, 2012
Messages:
4
Status:
Pre-Medical
Remember that to figure out what the map units are, one effectively is figuring out the average distance between the two. It has to be under 50% to be on the same homolog, otherwise it is called something else, (can't remember right now) I do remember that the less total amount of the recombinants that occur, the closer the two genes are together. This makes sense if you think about playing with a pipe cleaner~ the closer the two points that you want to twist, the harder it is to twist. (sorry, my kids have been playing with pipe cleaners lately)

Gee, this link might help explain a little bit more