BSCI 410 |
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Questions about Homework 1.
The following are questions and answers from students regarding Homework 1 (2008). Each question is offset with a horizontal line.
There was a typo in the homework as distributed. The corrected question 3 should read, in part,
In each case, there are two GHI1 spores and two GHI2 spores. The pdf is now corrected, but it's probably not worth downloading and printing again for that small change.In other words, per 1,000 tetrads, how many of each of the following types do you expect?
- tetrads with two GHI1 GHI2 spores and two ghi1 ghi2 spores;
- tetrads with two GHI1 ghi2 spores and two ghi1 GHI2 spores; and
- tetrads with one GHI1 GHI2 spore, one ghi1 ghi2 spore, one GHI1 ghi2 spore and one ghi1 GHI2 spore
From a student:
3. "The number of tetratypes relative to the number of ditypes is determined by the rate of recombination between the markers being studied and the centromere."—class notes
Does this mean between the two genes? I wanted to work backwards from the equation
No, if the two unlinked markers are both at the centromere, you will get entirely ditypes. That is the pattern of segregation of the centromeres themselves. At the other extreme, if two unlinked markers are very far from the centromere you will get 2/3 tetratypes (we discussed this in class). The equation below applies to linked markers that are very far apart.
60 cM = [3NPD + (T/2)]/1000You also have, for problem 3, an equation giving the recombination rate in terms of the number of tetrads of different types (this is just a calculation of the fraction of spores that are recombinant) and an equation relating the map distance to the expected recombination fraction.
I have 2 unknowns and cannot decide if I can eliminate one. Am I thinking about this wrong?
I was also thinking:The 60 cM. is a map distance, not an an expected recombination rate. I gave you an equation relating these two.
Expect a crossover to occur between 2 genes 60% of the time Tetratype
Expect a crossover to occur after the two genes 40% of the time PD
5. In the study population, at least 1/3 are known to be affected. There is still a (1/4) chance that each of the other two children in each family could be affected. I am not sure how to combine these two concepts. Should I add to 1/3 or multiply using 1, because the first child is certain to be affected?Some families will have more than one child. You should definitely do problem 4 first and then do something similar for this problem. Also, since the question asks for a conditional probability you may want to review the equation for that and think about how to apply it.
6. I am having trouble applying the binomial distribution in part a.The most important thing is to be consistent. The number I gave you is the number of wild-type spores per 1,000 spores. The number of recombinant spores per 1,000 spores and the number of recombinant spores per 1,000 tetrads will be different.
A single crossover (or other tetratype-yielding crossover) will yield only 1 wild-type recombinant spore.
right.
Given the binomial distribution, (nCx)p^xq^y, it seems like 4 WT haploid cells would require 4 separate crossover "successes", as one single crossover, for example, yields 1 WT and 1 double mutant (2 recombinants). I am confused on the number of crossover "events," or rather n, because I know this does not necessarily equate the number of spores. I think there are 500 "rounds of meiosis" but is that n? p I think is .001 and then q and y can be calculated accordingly. I suppose I don't understand "n" and "x"
For part b, is this same formula used to accumulate "four or more?"Iif you use the right formula, and apply it correctly, you can use if for both parts, yes.