A2 Biology OCR June 2015 Revision Thread

Does anyone have any predications?
A big question on meiosis hasn't come up in a while :/

I hope there isn't as well! but it's hard to predict what could up come tbh after f214

Epistasis is the interaction of gene loci where one gene locus masks/suppresses the expression of another gene locus (so it is not expressed in the phenotype) via an enzyme pathway

There are three forms of epistasis:
1. Recessive epistasis:
*A homozygous recessive condition at either gene locus suppresses the expression of the alleles at the other gene locus regardless of what they are
9: 4: 3 is the ratio this manifests itself within
2. Dominant epistasis:
*A dominant allele at one gene locus masks the expression of the alleles at the second gene locus regardless of whether they are dominant/recessive
13: 3 or 12: 3: 1 ratio
3. Complementary epistasis:
*A dominant allele needs to be present at both gene locus' for the phenotype to be expressed
9: 7 ratio
*Usually involves the product of one reaction being the substrate for the next reaction (is a multi enzyme pathway)

I'm sure the OCR book says something along similar lines. Is there anything specific you're confused about?

So it depends on the gene locus to determine its epsitasis?

Hi guys can anyone check if this answer is correct for a question that asks

"outline how to sequence a genome"

1.mechanically shear samples of the genome and place into BACs and transfer to E.Coli

2. Ecoli will replicate and form clone libaries , then extract DNA from a clone library and cur into smaller fragments using R.enzymes

3. Seperate DNA fragments by electrophoresis where smaller fragements travel furher up agrose gel than longer fragments

4. Now use chain termination method to produce many lengths of DNA fragments . Mixture should have primers , DNA polymerase , DNA nucleotides + nucleotides with fluorescent marker

5. When fluorescent marker is added DNA polymerase is "thrown off " leading to many lengths of DNA fragements

6. Put DNA fragmnets though laser to read colour sequence = Base sequence

Any help greatly appreciated as this is my most difficult topic !

I hope this is correct bec. this is just about what i would write in the exam! iA.

Outline the steps involved in sequencing the genome of an organism.
If a question asked this what would you write because there are two ways mentioned in the textbook
1. Page 166-167 it talks about BACs
2. Page 172-173 it talks about automated DNA sequencing

I'm hoping chi squared and hardy Weinberg both come up haha


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I'm hoping chi squared and hardy Weinberg both come up haha


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loool i hope so too iA.

It depends on what alleles are present at each gene locus- whether they're homozygous recessive, dominant, etc that determines what type of epistasis it is