This question was asken in an exam,


The answer they are saying is "Epistasis". But I think "Dominance" fits better, because it is not mentioned whether genes of same allele are to be considered or different allele. Also it is not told whether genes in same locus are being talked of or different.

Please give some reference if you can. I know only school level biology. I do not know even in detail what allele and locus are. As far as I could understand from wikipedea and other sources I think "Dominance" is correct.

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    $\begingroup$ Even I thought it to be dominance, but the above explanation makes everything crystal clear. Thanks @mdperry $\endgroup$ – user18661 Sep 4 '15 at 9:50

The definition of "gene", according to The American Heritage Science Dictionary is:

"A segment of DNA, occupying a specific place on a chromosome, that is the basic unit of heredity. Genes act by directing the production of RNA, which determines the synthesis of proteins that make up living matter and are the catalysts of all cellular processes. The proteins that are determined by genetic DNA result in specific physical traits, such as the shape of a plant leaf, the coloration of an animal's coat, or the texture of a person's hair. Different forms of genes, called alleles, determine how these traits are expressed in a given individual. Humans are thought to have about 35,000 genes, while bacteria have between 500 and 6,000. See also dominant, recessive. See Note at Mendel."

So, when the question refers to genes, it must be referring to two different genes on two different loci. If the question was referring to alleles of the same gene on the same locus, the correct answer would be dominance, but since it is referring to two different genes, the answer is epistasis.

  • $\begingroup$ "So, when the question refers to genes, it must be referring to two different genes on two different loci". Why is that? It is not mentioned whether same loci are being discussed or different. $\endgroup$ – user31782 Sep 2 '15 at 19:09
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    $\begingroup$ When you talk about two separate genes, you are always talking about two separate loci ("specific places on a chromosome"). Two different genes cannot occupy the same locus. Different alleles can occupy the same locus, but different alleles are just different forms of the same gene. $\endgroup$ – C_Z_ Sep 2 '15 at 19:17

Consider two different genes (call them two different loci, if you like). Gene A where + is a wild-type, functional copy of the gene, and a is a mutant, loss-of-function copy of the gene. Gene B where + is a wild-type functional copy of the gene and b is a mutant, loss-of-function copy of the gene.

An individual with this genotype:

+/+; +/+

Will be wild-type. And if the a allele of Gene A is recessive, and the b allele of Gene B is recessive, then an individual with this genotype:

+/a; +/b

Will also have a wild-type phenotype. In a diploid organism, dominance is when +/a individuals have a mutant phenotype. The same holds true for +/b individuals, if one copy of the mutant allele causes a mutant phenotype then the b allele is dominant to the + allele of the B gene.

If both mutant alleles are dominant then this individual would have two mutant phenotypes:

+/a; +/b

Most mutations, however, are recessive to the wild-type allele, so you would not see a mutant phenotype until there are two mutant copies and no wild-type alleles. Either:

a/a; +/+


+/+; b/b

The double mutant would have this genotype:

a/a; b/b

You cannot interpret, or make logical sense, of an epistasis experiment unless the alleles are recessive.

If this individual does not have a double mutant phenotype, but instead only shows the 'A' gene loss-of-function phenotype:

a/a; b/b

Then we would say that the A gene is epistatic to the B gene, because homozygous A gene mutants "hide" the B gene mutant phenotype.

If, instead the double mutant only has the 'B' gene mutant phenotype then we would say that the B gene is epistatic to the A gene.

If the double mutant has both phenotypes then there is no epistasis between these two genes, or loci. Conversely, if the double mutant has a wild-type phenotype then the mutations are suppressing each other.


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