Sickle cell anemia

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Sickle cell anemia

Post by georgina_009 on Sun May 18, 2008 1:14 pm

I've done some answers to this so could you check them and help with the last one?

Sickle cell anemia is an example of how fitness depends on environmental cond. In malaria prone places Hb^A Hb^S have advantage over normal. But Hb^S Hb^S usually die.
The relative fitness (w) of them in Nigeria:
wild type - 0.87 Hb^A Hb^S - 1.0 Hb^S Hb^S 0.05

a) What is the relationship between w and selection co efficient?
b) What is the selection coefficient (s) for each genotype?
c) Calculate the frequency of the three genotypes aftr one generation of selection if the genotype frequencies before selection are 0.4 wt, 0.46 for heterozygotes, Hb^S Hb^S - 0.14

My answers-
a) relative fitness - capability to give genes to next generation, coefficient - intensity of nat. selection against genotype. So would you say that relative fitness is dependant on selction co efficient?

b) s= 1-w,
wt- 0.13, hetero- 0, homozy- 0.95

c)
At reproductive age - wt-0.4, hetro - 0.46, sickle cell affected - 0.00
because these dont add to 1 - recalculate by dividing by .86 -
wt - 0.465 hetero- 0.535 affected - 0

I'm not sure what to do now (or if what i did was right)

Thanks

georgina_009
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Re: Sickle cell anemia

Post by Alla on Tue May 20, 2008 6:43 am

Most of it is correct.

A. Just add that as w increases, s decreases.
B. Correct.
C. I am not sure how you decided that "At reproductive age - wt-0.4, hetro - 0.46, sickle cell affected - 0.00".
I think that you have to take into account (w). So, if hetero is 100%, then 0.46x1.0=0.46 (hetero); 0.46x0.87=0.40 (wt); 0.46x0.05=0.02 (affected).
Now you divide by 0.88 as you did before:

wt - 0.454 hetero- 0.523 affected - 0.023

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Re: Sickle cell anemia

Post by georgina_009 on Tue May 20, 2008 7:19 am

I was looking at an example inmy text book for c) so maybe i interpruted it wrong. I'll have to look at it again.
Thanks for your help

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Re: Sickle cell anemia

Post by georgina_009 on Tue May 20, 2008 4:02 pm

ok - my text book says that homozygotes will have already died at reproductive age, thuse they are 0.00 - The example they give isent for sickle cell but seems to have the same principal. But then again it is likely that the qn requires w aswell.
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Re: Sickle cell anemia

Post by Alla on Wed May 21, 2008 6:59 am

I cannot say for sure, so I would suggest you ask your professor for clarification.

Good luck.

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