Please post all your questions and comments on translocations under this section. Have fun solving this practice question!
Translocations make for challenging exam questions in 334. However, translocations are also responsible for a number of nasty leukemias and lymphomas. Have a look here, or just google in "chromosomal translocation and cancer".
8 comments:
I was just wondering if any of the class had got the "biology 334 Midterm exam package". Its the one they were handing out before the midterm with 2 old midterms and a bunch of other problems. The problems at the back of the package do not have answers! does anyone want to compare answers... otherwise i dont see a point in doing them. Thanks.
in Ch 6 ques 44,
when translocation homozygote selfed, they will be fertile because they have no problem pairing in M1?
However, I don't really understand why frequency ppss is 0.25.
in fact, is this question worth doing?thank you very much.
For question 44:
- we have 2 different S/s P/p plants here: the original one, and then the one we get from crossing the original one to a tester.
- the first dihybrid plant does not show semisterility upon selfing, but it shown pseudolinkage, that is, p and s segregate together more often than they should. You are correct that there's no semisterility because if it's homozygous for the translocation, then pairing occurs normally at MI (no cruciform or anything).
Let's look at that 1/4 ppss. If it was a dihybrid self with the 2 genes on separate chromosomes, we should get a 9:3:3:1 (with 1/16 being the ppss).
Instead, we get something/16 :something/16 : something/16 : 4/16.
The ppss are way more frequent than expected. We can even calculate the RF between p and s in this strain:
since ppss is higher than expected (and we are doing a self), they must be the result of 2 parental gametes (remember chapter 4...). The frequency of the p,s gamete should be the square root of 0.25, that is, 0.5. The other parental gamete, P,S, should be the same frequency, 0.5. Therefore, we have 0 recombiinants: the translocation has brought p and s (and P and S) genetically so close together that their RF is 0!
- so, when we cross the dihybrid to a tester, there's no semisterility, but there are no recombinants between p and s.
- however, the F1 that we obtain from the testcross is itself semisterile (as point #3 says), because it is now comprised of the translocated chromosomes of the dihybrid in combination with the normal chromosomes of the tester.
Sorry about the long answer....I hope it helps
thanks a lot...the explaination is very clear...but how can you tell if psudeolinkage due to physically linkage of gene or due to translocatied chromosome tend to inherit together?
You can't...unless you have additional info from the question!
has anyone solved the translocation problem here? the one with the Flatina with fat, teeth and ears?
When doing everything out i got all of the genes independently assorting...but now im not sure whether they are, i think the fat gene is since the F1 are all fat...but maybe the teeth and ears gene have been separated by a homozygous translocation? and thus appear unlinked even though they are? I havent got a clue. Help!
ok scratch that. so i looked at the answers online...but..how do you know which gene to look at first and decide if it is independently assorting or not?? when i looked at the fat gene, if it was independently assorting wouldnt we expect a 1:1 ratio of fat and non fat in the progeny? and thats what we get. the same with the teeth gene...I dont understand?
ok i get it now :)
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