Since Mendel 1855-56; Genetics teaches that genes can dominate one another. This was based on purely qualitative genetic analysis. This article just says that only quantitative genetic analysis can show the reality behind gene interaction. Enzymes that carry on gene actions do not act qualitatively. Instead, Enzymes act quantitatively and the results show that there is no such thing as a dominant or recessive gene.
Genes control the manifestation of biological traits that appear in individuals as phenotypes via the activity of enzymes, proteins having catalytic proprieties. Genes can be transcribed into various types of RNA (transfer RNA or tRNA, messenger RNA or mRNA, ribosomal RNA or rRNA, and less studied regulator RNA). Genes that govern phenotypes directly are structure genes that are transcribed only into mRNA carrying the plans of proteins (some being just structural proteins, many being enzymes and some others hormones like well-known insulin) that get synthesized within ribosomes.
In the simplest cases, each enzyme thus made through protein synthesis catalyzes a chemical reaction by transforming a certain amount of a source molecule (the substrate) into a certain amount of a new molecule called Product of the reaction. So an enzyme action has both a qualitative aspect (the type of substrate it acts on and/or the type of product it produces) and a quantitative aspect (the amount of substrate it is able to transform and/or the amount of product it can produce). For some reasons, Geneticists often only consider the qualitative aspect of enzyme action and at the same time gene action, oblivious of the quantitative aspect of enzyme or gene action that is always present. The principle of a gene dominating another gene came from this partial interpretation of gene action deriving from enzyme action. When the quantity of the product of an enzyme action is taken into account, which automatically includes the qualitative aspect, then it becomes obvious that no gene can dominate another which is supposed to be recessive.
Let us look at the real gene effect behind the seed color of Peas studied byMendel (1822-1884), the Father of Genetics. There are two alleles, one Y1 producing let say a unit of yellow pigment, and the second Y0 producing no unit of yellow pigment. There are two homozygotes Y1Y1 and Y0Y0, the former with yellow seeds because of the pigment synthesized and the later with green seeds because when in some plants no pigment (other than green) is produced the organ (here the seed of the Pea) will be just green like the leaves. In cereals the non colored seed will be white because of the white starch it contains. In Pea, the default color is green. If a plant organ wants to be colored otherwise, it has to do some extra work : the synthesis of the desired pigment, the yellow pigment in this case.
The homozygote 1 has the genotype Y1Y1 (made of 2 alleles) produces two units (1+1, each from each allele) of yellow pigment and has a phenotype that is qualitatively yellow seeds and quantitatively intense yellow seeds.
The homozygote 2 has the genotype Y0Y0 produces zero unit of yellow pigment and has a phenotype that is both qualitatively and quantitatively green seeds.
The heterozygote has the genotype Y1Y0 produces one unit (1+0) of yellow pigment and has a phenotype that is qualitatively yellow seeds and quantitatively yellow (non intensive yellow) seeds, so quantitatively different from that of the homozygote1 and thus no dominance is any more involved here.
It means that when you cross both homozygotes (Y1Y1 and Y0Y0) you get a heterozygote Y1Y0 with yellow seeds indeed because of the action of one Y1 allele action. If you look only at the color yellow you will say that both the homozygote Y1Y1 and the heterozygote Y1Y0 have the same phenotype (yellow seeds) and thus the allele Y1 appears to be dominant over the allele Y0. But this is only a partial interpretation of the reality. How about the amount of yellow pigment synthesized in each case, does it count for nothing ? Why ? The whole interpretation clearly shows that the heterozygote fully resembles none of its parents. Its phenotype is in between those of its parents.
From the above data it can be seen that 1+1 = 2 is different from 1+0 =1 and thus the heterozygote does not have the same phenotype as the yellow seed homozygote. Also 1 + 0 = 1 does not mean that 1 dominates 0. It just does not make any sense to say that. So there is no dominance in the expression of the phenotype of the hybrid Y1Y0.
Around 1910 the Sweeden Nilson Ehle used this quantitative effect of gene action to explain the inheritance of Wheat seed color depending on two to three genes (loci) and considered as a quantitative trait instead of a qualitative trait as color has been considered until then. He supposed for the very first time that two loci both with an active allele and an inactive one controlling the same trait (the seed color) could explain why when crossing two strains of Wheat, one with red seeds and the other with white seeds, leads to a hybrid with pink seeds and to an F2 generation with 5 different phenotypes. Both active alleles (each from one locus) have the same action which is the production of a unit of red pigment. In F2, the phenotypes rank progressively from 4 units (the red strain) of red pigments to 3, 2 (pink seeds), 1 and 0 units (the white strain).
But how exactly genes act to express the phenotypes of qualitative traits, there seems to be no explanation of that, until now. Enzymes are the doers, the workers of the biological activities. And their work is both qualitative and quantitative. At the same loci different alleles are supposed to have the same qualitative action but must differ in their quantitative effects. When they all act on the same substrate, they may be more or less able to transform it into the expected amount of the product. In most cases, when there are only two alleles at a given locus, one has a positive effect on the substrate and is able to transform it into at least a unit of the product, and that is the active allele. The second allele which is inactive has no effect on the substrate and therefore produces no amount of the product.
When a locus has more than two alleles one may have a nil effect and others different positive effects. No two alleles at the same locus can have exactly the same effect and still be identified as different alleles by their specific effects.
Since genes act through enzymes that always work by transforming substrates into products so that a certain amount of product (nil or positive) is produced as a result of an enzyme or gene action, each phenotype can be expressed quantitatively, no matter the type of trait (qualitative or quantitative) it pertains to. There is no such thing as a purely qualitative phenotype. When a biological trait is controlled by just one locus it can be called a qualitative trait but that does not mean that its phenotypes are purely qualitative. Enzyme/gene action is a quantitative work ! It always deals with transforming certain amounts of source molecules or substrates into certain amounts of new molecules or products. Just like in computer software action a locus with two alleles follows the 1 or 0 principle. The action of an allele with positive effect is 1 and for the non active allele it is 0.
Since there seems to be no dominance between genes, contrary to what has been said and thought so far, what changes in Genetics from now on ?
No thing much except to replace a supposed dominant allele with an allele with a positive effect and a supposed recessive one with an allele with zero or nil effect. In that case, the heterozygote derived from crossing two different homozygotes will have a phenotype of its own, not one of its parents. This is just closer to what happens really at a given locus. Dominance was just a misinterpretation of the reality. Science is supposed to be clear and precise, isn’t it ? Then we need quantitative tests in assessing more accurately the effects of genes in individuals, populations and species.