Lou FCD
Posts: 5455
Joined: Jan. 2006
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| Quote (bfish @ Nov. 10 2008,20:05) | | Quote (Lou FCD @ Nov. 10 2008,11:43) | So I got the results of my Biology exam back this morning.
anyway, I got a 97. |
Hey, congratulations!
Are you still doing your drosophila lab, or is that over with? I haven't kept track if that was one generation ago or two. When it's finished, I'd be curious to hear what the experiment was. |
Thanks to you both.
That lab we finished last Monday.
Here's what it was:
We were given a bunk of knocked out fruit flies and told how to tell the males from the females. We picked out five males and five females, and put some maggot food (blue sticky stuff) in the bottom of a tube that was similar to a really fat test tube. We kept the tubes on their side, so the unconscious flies didn't get stuck in the maggot food and starve to death.
Now, we were given this information: Our flies were F1 generation flies, and the P generation was kept in a lab out in Durham or Greensboro or somewhere. In the P generation, all the males had red eyes, and all the females had white eyes.
In our F1 generation, I checked them out under a microscope and all the males had white eyes and all the females had red eyes, exactly the opposite of the P generation.
So, the question was, "What will the F2 generation have in the way of eye color?"
If eye color were an autosomal Dominant/recessive allele, then we would expect that barring any hanky-panky skewing of the F1 generation by picking out flies with particular eye colors, we would expect that our flies would have a 3:1 ratio of the eye color coded for by the Dominant allele to the eye color coded for by the recessive allele, and that it would be the same ratio regardless of the sex of the flies.*
Since we had such a definite correlation to sex, it was pretty obvious that the gene was sex linked. Since females and male were demonstrably capable of having either eye color, it had to be linked to the X chromosome (since girl flies don't have a Y chromosome).
Ok, so we know then that the gene for eye color in fruit flies is coded for in the X chromosome. So which color is Dominant, and which recessive?
If white were the Dominant trait, then all the P generation males would have to have the recessive, w, and the females either heterozygous Ww, or homozygous Dominant, WW.
Remembering that females inherit an X chromosome from their mother and an X chromosome from their father, we would expect that the F1 generation females would then have either half white and half red eyes if their mothers were heterozygous, or all white eyes if their mothers were homozygous Dominant. (Each of the males would inherit their X chomosome from their mother, so we would expect them to all have white eyes if their mothers were homozygous Dominant, or half red and half white if their mothers were heterozygous. - remember they only get one X, and Dad donates the Y)
IF: Pfemales = WW and Pmales = w, then F1females should = Ww and have white eyes.
IF: Pfemales = Ww and Pmales = w, then F1females should = 50% Ww and have white eyes and 50% ww and have red eyes.
ALL our females had red eyes, so white could not possibly be the Dominant trait.
BUT IF: Red is the Dominant allele, then we know that the P generation females were homozygous recessive:
SO : Pfemales = rr and Pmales = R, then all F1females should = Rr and have red eyes. The males would all be r, and have white eyes, which is exactly what we saw.
That then was my hypothesis, because it matched the available evidence.
A week after we put the flies in, we knocked them out again with fly nap, removed the F1 adult flies, and set them free (or euthanized them, our choice), leaving a squirming mass of maggots in the blue stuff at the bottom of the tube.
Given that, our task was now to form a prediction as to the eye color of the F2 generation flies.
Because the F1 flies broke down this way according to my hypothesis:
Females: Rr - Males: r
and the females would get one X from each parent, the males one X from Mamma fly, I predicted that:
Females: 50% Rr with Red eyes, 50% rr with white eyes
Males: 50% R with Red eyes, 50% r with white eyes
Last Monday, we checked out our tubes full of F2 generation flies.
Our count:
Females 22 Red eyes, 13 White eyes
Males 17 Red eyes, 13 White eyes
...which was a bit disconcerting.
At first glance, I thought class totals were more in line with my hypothesis. However, excluding our count, the totals were:
Females 82 Red eyes, 68 White eyes
Males 71 Red eyes, 65 White eyes
Still not perfect, but closer. We still seemed to be a bit heavy on Red eyed females.
Looking at the x2 goodness of fit, we see that our group:
((22 - 17.5)2 / 17.5) + ((13 - 17.5)2 / 17.5 = 2.314, which yields a p-value of between 0.1 and 0.2 (with one degree of freedom) for our females.
Kinda weak, but above passable support for the hypothesis.
For the males, we have
((17 - 15)2 / 15) + ((13 - 15)2 / 15) = 0.533, which yields a p-value of between 0.3 and 0.5 (with one degree of freedom) for our males.
I'm much happier with the male prediction.
The class as a whole, including our results,
((104 - 92.5)2 / 92.5) + ((81 - 92.5)2 / 92.5) = 2.85 and a p-value of between 0.10 and .05 for our females.
((88 - 83)2 / 83) + ((78 - 83)2 / 83) = 0.602 and a p-value of between 0.30 and 0.50 for our males.**
I haven't yet run the numbers on the mixed sex results, as that will require some thought regarding degrees of freedom, I think. My brain hasn't caught up to me on that yet.
There were a couple groups (ours was one) who were way off on females, and are the bulk of the cause of the odd female numbers. The other groups were pretty close to 50/50 on the females, and most of the groups (but not all) were pretty close with the males.
That leaves some questions to be answered, but with the lab done, all that's left is the report. I'd like to run the experiment several more times, just to find out what's up for my own personal edification.***
*Fruit flies have an XX / XY sex chromosome set-up like humans
**Raw numbers for those of you interested:
F(red) F(white) M(red) M(white)
Group 1 (us) 22 13 17 13
Group 2 18 11 21 16
Group 3 11 13 6 8
Group 4 12 14 8 12
Group 5 16 12 11 9
Group 6 25 18 25 20
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Totals 104 81 88 78****
***I'll get the chance next semester, but that's a (VERY happy, cool) story for a different post.
**** Feel free to check my math, as well as the rest of the post. It's late, and I'm tired.*****
*****Editated to try and straighten out the asterisks.
Edited by Lou FCD on Nov. 10 2008,21:59
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“Why do creationists have such a hard time with commas?
Linky“. ~ Steve Story, Legend
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