You can find easily in textbooks that definitions for dominant and recessive alleles as follows:

Dominant allele: produces working protein and shows its effects in hybrid state.

Recessive allele: produces defective protein and remains suppressed in hybrid state.

But I have these confusion:

  1. Defectiveness or benefit is relative thing.

  2. There are many genetic diseases which are caused by dominant genes, for example short toe fingers. The protein related to it is also defective.

Another definition includes dominant as wild type most circulated while recessive as mutant allele. But even this seems to be unsatisfactory as every mutation cannot be recessive or defective.

Then, finally, what determines dominance of an allele over others?

  • 1
    $\begingroup$ These definitions are wrong. Can you please directly quote a definition from a specific textbook(s) (so that we can boo the author if the textbook is indeed wrong). While the recessive allele is probably more often associated with the production of a non-working protein, it is definitely not how the term recessive is defined. $\endgroup$
    – Remi.b
    Aug 22, 2017 at 2:26
  • $\begingroup$ Yeah, these are bad definitions. A better one would be "Two alleles are in a classic dominant/recessive relationship if the heterozygous phenotype is the same as the homozygous dominant phenotype, and different from the homozyogus recessive phenotype" $\endgroup$
    – swbarnes2
    Aug 23, 2017 at 17:03
  • $\begingroup$ Possible duplicate of Is protein production doubled if you have homozygous dominant genes as opposed to heterozygous genes? $\endgroup$
    – Remi.b
    Aug 24, 2017 at 14:22

1 Answer 1


Phenotype determines dominance. The "rules" you state are just general possibilities that are most likely; they are not correct definitions. "dominant as wild type most circulated" is just completely totally false.

It's common to have mutations that cause loss of function. These often end up as recessive alleles, because the second copy still makes a functional protein. It's not about the allele being suppressed necessarily, and doesn't have anything to do with benefit or not, it's just that the other copy is able to do the job, so you don't see much effect. Because there are often feedback mechanisms for gene expression, having 1 less copy in the genome often means much less than a 50% reduction in protein.

It's also common to have mutations that cause negative gain-of-function: these are often dominant, because you only need one copy for that negative effect to be observed.

However, as you point out, these aren't always the case. You could have an allele where loss of a single copy causes a deleterious phenotype. Sometimes these might more appropriately be considered "incomplete dominance."

I think it's important to note that these terms are really labels based on phenotypes, they are not biological realities. An allele could be thought to be completely dominant, but under further study actually demonstrates a form of incomplete dominance or co-dominance.

The Wikipedia page on dominance has some better definitions for you.


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