F215 - Revision thread 13th June 2011

4th time?! Oh man im still on my first time round and havent even finished yet! Still got most of ecosystems and animal behaviours!

dont worry... 113 hours left to go.... you can make it..

This isn't reassuring

112 now

:P

I wish I didn't need sleep!

Don't we all, but tbh just try not to worry to much - the more you stress, the more you'll panic and forget.
I keep thinking of impossible questions that are going to come up in the exam, and since i've dreamed impossible questions, i can't answer them LOL!

Thanks.
Past papers dating back to 01- (of the old spec from freeexampaper site)

dont worry... 113 hours left to go.... you can make it..

Yeah just 113 hours left to cover the last 2 modules.. fun. Also seeing as you've gone through it four times, could you explain recessive and dominant epistasis to me please

Im gonnna fail this unit and its cos of a few genes... I dont know nothing about genetics, module 1. Its going in one ear and coming out the other. What am i gona do.. 4 days left ((((

Epistasis is really simple, you have two separate genes at two different loci (different parts on the chromosome) and the presence of a certain allele of one gene stops the expression of the second gene.

It'll be easier to understand with an example; so if it was fur colour in mice and you have an allele for Brown (B) and white (b). So the presence of the B allele will give a brown coat as it is dominant.
Now a completely different gene has two alleles A & a. Now for the Brown fur colour you need to also have the dominant A allele present so;
BBAA = brown
BbAA = brown
BBAa = brown
BbAa = brown

BBaa = white
Bbaa = white
bbaa = white
bbAa = white
bbAA = white

if the dominant A is not present the coat colour is white. This is recessive epistasis because if there are only the reccessive alleles it supresses the epression of the other gene. So, regardless of whether or not there is the dominant B for brown fur, if there is no dominant A the brown will not be expressed and the fur will be white.

A way to remember it so it makes sense is that the dominant A gene codes for an intermediate compound e.g a colourless pigment, and the B allele changes this intermediate into the final brown colour. If the intermediate is not present the B allele cannot have its effect.

Then dominant epistasis is just the reverse, so If here is the presence of a dominant allele at one loci, the second gene wont be expressed, so you need to have BOTH the recessive alleles for it to be expressed.

Yeah just 113 hours left to cover the last 2 modules.. fun. Also seeing as you've gone through it four times, could you explain recessive and dominant epistasis to me please

Epistasis is really simple, you have two separate genes at two different loci (different parts on the chromosome) and the presence of a certain allele of one gene stops the expression of the second gene.

It'll be easier to understand with an example; so if it was fur colour in mice and you have an allele for Brown (B) and white (b). So the presence of the B allele will give a brown coat as it is dominant.
Now a completely different gene has two alleles A & a. Now for the Brown fur colour you need to also have the dominant A allele present so;
BBAA = brown
BbAA = brown
BBAa = brown
BbAa = brown

BBaa = white
Bbaa = white
bbaa = white
bbAa = white
bbAA = white

if the dominant A is not present the coat colour is white. This is recessive epistasis because if there are only the reccessive alleles it supresses the epression of the other gene. So, regardless of whether or not there is the dominant B for brown fur, if there is no dominant A the brown will not be expressed and the fur will be white.

A way to remember it so it makes sense is that the dominant A gene codes for an intermediate compound e.g a colourless pigment, and the B allele changes this intermediate into the final brown colour. If the intermediate is not present the B allele cannot have its effect.

Then dominant epistasis is just the reverse, so If here is the presence of a dominant allele at one loci, the second gene wont be expressed, so you need to have BOTH the recessive alleles for it to be expressed.

BULLT has explained it really well with an example....

but in simple words...



eg. Allele C will result in white coat, not matter how many other allele's are present

Recessive epistasis is when a recessive allele masks the expression for a dominat allele

eg. allele A will mask the expression for allele B..

sorry not very good at explaining...

Thanks!! I think I understand it now but need to do some past questions on it!