Dominant vs. Recessive

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Dominant vs. Recessive

Post by Alla on Thu Mar 20, 2008 2:23 am

Paragonidae wrote:so that brings me back to the beginning, how is it that one allele is expressed over the other while if it the recessive one on its own (with its homologous) they are expressed, is it a question of quantitative effect where the dominant is expressed stronger and the recessive expression is insignificant in comparison and when there are two of them they make enough product to be expressed phenotipicaly. sorry for repeating this but it is really hard for me to understand. maybe you can recommend some reading material on this subject.

Dominant allele is said to be the allele responsible for phenotype of the heterozygous individual (simplified version).

Taking plants:

Different alleles for the same that occur in wild type plants (different varieties of the same plant) usually differ in the regulatory sequences. Dominance of one allele over the other depends on what the gene in question does. Usually, alleles that produce more of the product are dominant over those that produce less of it (but that is not always the case). I try my best to explain with the hypothetical examples.

Example 1:

Let’s say Gene A is responsible for plant height and product of gene A promotes growth. Two alleles (A1 and A2) are different in the promoter region. A1 has a strong promoter and makes 75% of gene product while A2 has a weak promoter and produces only 25%. Suppose that 75% or more of the gene product results in normal size plants and less than 75% in dwarf. Then plants with A1A1 (150%A) and A1A2 (100%) genotypes will be normal size while those with A2A2 (50%) will be dwarf. So in this case, A1 is dominant and A2 is recessive.

Example 2:

Let’s say Gene B is a repressor of Gene A. When 75% or more of B product is produced, Gene A is sufficiently repressed to have phenotypic consequences (considering only plants with A1A1 and A1A2 genotypes). Each of two alleles of Gene B (B1 and B2) produces 50% of gene product, but B1 is controlled developmentally and expressed only during flowering while B2 is constitutively expressed (all the time). Then, plants with B1B1 and B1B2 genotypes will be normal height since repression of gene A will occur only during flowering while B2B2 will be dwarf because gene A will be repressed all the time. So, in this case, B1 is dominant while B2 is recessive.

Now consider Gene A and Gene B combinations.

A1A1B1B1, A1A1B1B2, A1A2B1B1, and A1A2B1B2–should make plenty of A; B1B1 and B1B2 can repress Gene A, but only during flowering. So the plant will grow normally and stop growing only while it’s flowering.

A1A1B2B2, A1A2B2B2, and A2A2B2B2 – Produce plenty of Gene B product that will suppress most of Gene A and result in dwarf phenotype regardless of Gene A allele combination.

A2A2B1B1, A2A2B1B2, A2A2B2B2 – will be all dwarf regardless of Gene B allele combination because of A2A2 allele combination.

This is an example of allele of one gene displaying dominant epistasis with regard to allele of the other gene. In this case, both B1 and B2 alleles display dominant epistasis over A1 allele (whenever phenotype directed by A1 allele, B1 or B2 can override/modify it); while A2 allele has dominant epistasis with respect to both B1 and B2 alleles (whenever A2 dwarf phenotype is present Gene B does not really matter in terms of phenotype).

Example 3:

Let’s suppose what happens in both cases if we introduce new alleles – A3 and B3 – that have null mutations and do not produce any gene products.

A3A3 plants will be dwarf.

B3B3 plants will be normal size since gene A will not be repressed (considering only plants with A1A1 and A1A2 genotypes).


A1A3 will produce 75% of A and will be normal size, so A1 is dominant over A3.
B1B3 will produce 50% of B during flowering, not sufficient to repress gene A and, therefore, plant will not stop growing while flowering – giving a new dominant phenotype due to B3.

B2B3 will produce 50% of B all the time, again not sufficient to repress gene A and, therefore, will not stop growing while flowering or any other time– making B3 dominant over B2 as well.

B3B3 is a good example of dominant phenotype due to a non-functional allele.

If you need more help, let me know.

I would appreciate it, if you registered to this site and used it directly.
Good luck!


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