TheNacho63
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I seem to have trouble understanding Epistasis in terms of dominant and recessive. I know that it is interaction of different gene loci but I don't understand the examples in the OCR A2 Biology text book and my teacher hasn't explained it so well.

If someone can help explain or knows of clearer learning resources for it, it would be much appreciated
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ParticularParrot
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Right, this is tricky to explain in a message, but I'll have a go.

Epistasis is when a particular allele or combination of alleles at one locus *masks* the expression of a gene at another locus. i.e. one gene prevents another one from being expressed.

The difference between dominant and recessive epistasis is in the combination of alleles required for this to happen.

Recessive epistasis: if both alleles at locus 1 are recessive i.e. aa, gene 2 will be masked
Dominant epistasis: if either allele at locus 1 is dominant, i.e. AA or Aa, gene 2 will be masked

So, the combinations possible look like this:

Recessive epistasis:
If locus 1 is:
AA - Gene 2 expressed
Aa - Gene 2 expressed
aa - Gene 2 not expressed

Dominant epistasis:
If locus 1 is:
AA - Gene 2 not expressed
Aa - Gene 2 not expressed
aa - Gene 2 expressed

So dominant and recessive epistasis both involve masking - but what causes gene 2 to be masked is the opposite.
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Dynamo123
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(Original post by TheNacho63)
I seem to have trouble understanding Epistasis in terms of dominant and recessive. I know that it is interaction of different gene loci but I don't understand the examples in the OCR A2 Biology text book and my teacher hasn't explained it so well.

If someone can help explain or knows of clearer learning resources for it, it would be much appreciated
One popular example of Epistasis, that I once studied somewhere (I dunno where) is the Bombay phenotype (I don't know why it's called that). In this example, we take the ABO blood group system. You know that a particular antigen on the surface of an RBC shows the blood group--whether it is A or B. If both are present, it is AB, and if none of them is present, the blood group is designated as O. Now there is this wonderful phenomena that a gene present at an entirely different loci on a different chromosome controls the binding of the antigen to the surface of the RBC. This has something to do with glyocsylated compounds, but I won't go into the details now--they can be digressive. So you see, what happens is that this other gene, controls the binding of any antigen to the RBC. If your original gene says hey presto! your phenotypic blood group must be AB because both A and B antigens are being coded for, this other guy gets in the way and says, lemme have a go at them first.
Now suppose this other guy has two allelic forms, a dominant H and a recessive h. If by chance your original gene codes for an AB blood group system, but this H gene is present genotypically as hh (recessive) then the A and B antigens will not be able to bind on the surface of the RBCs. And the result? As stated above, you will be genotypically AB blood group type, but phenotypically you will be O type blood group due to the lack of A and B antigens on the RBC surface.

One more point: epistasis is not dominance. Dominance is when the genes are at the same locus-level. Epistasis is the effect of a gene on a totally different locus on another gene which controls the actual phenotype.

I hope you understand it better now
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Chloeheart
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Okay,

So an episatic gene masks the expression of another gene.
Many different genes can control the same characteristic - they are called polygenic, as they interact to form the
phenotype.
This is possible because the allele of one gene, will mask (or block) the expression of the alleles of other genes. = Epistasis.

A popular example of epistasis is flower pigment colour in plants, which are controlled by TWO genes.
Gene 1 codes for a yellow pigment and this is the dominant allele (Y) and gene 2 codes for an enzyme that TURNS the yellow pigment orange. (R is dominant Orange allele)
If you don't have the Y allele, it won't matter if you have the R allele or not as the flower will be colourless, (as without the Y, there is nothing to turn orange)
Gene 1 is epistatic to gene 2 as it masks the expression of gene 2.

A colourless pigment needs at least 1 dominant Y allele, (YY, Yy) to turn the flower yellow.
And to produce the enzyme to turn the flower orange you also need a dominant, (RR or R)

In terms of phenotypic ratios:

Dihybrid cross with a recessive epistatic allele : 9:3:4 ratio of Orange/Yellow/Colourless flower colour, if you crossed two homologous pairs, (YYRR X yyrr) - dominant and recessive.

And for a dihybrid cross with dominant alleles : 12:3:1 ratio.
Having at least 1 copy of the dominat epistatic allele masks the expresion of the other gene. If you cross a homologous recessive parent and a homologous dominant parent.
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