Modifier Genes

Instead of masking the effects of another gene, a gene can modify the expression of a second gene. In mice, coat color is controlled by the B gene. The B allele conditions black coat color and is dominant to the b allele that produces a brown coat. The intensity of the color, either black or brown is controlled by another gene, the D gene. At this gene, the dominant D allele controls full color whereas the recessive d allele conditions a dilute or faded expression of the color expression at the B gene. Therefore, if a cross is made among mice that are BdDd, the following phenotypic distribution will be seen:

    9 B_D_ (black)
    3 B_dd (dilute black)
    3 bbD_ (brown)
    1 bbdd (dilute brown)

The D gene does not mask the effect of the B gene, rather it modifies its expression.

Modifier genes – genes that have small quantitative effects on the level of expression of another gene.

Marko:
Most breeders know the basic principles of inheriting certain traits from Gouldian finches.
It is difficult to understand how modifications of basic phenotypes arise.
These modifications of the basic phenotypes may be caused by the action of regulatory genes (eg A, B, C)
I have made diagrams that show some of the ways in which regulatory genes work.
In fact, it is even more complicated, because there may be more regulatory genes, e.g. 8 or 10.
Explanations.
Gene P – gene coding for some pigment (eg eumelanate, pheomelanin or lutein). In fact, this gene encodes an enzymatic protein that catalyzes the reactions of pigment production.
S – the substrate from which the pigment is made.
P – pigment (final product)
A +, B +, C + – regulatory genes for increasing pigment production (darkening genes).
A-, B-, C- – regulatory gene alleles, inhibiting pigment production. (brightening genes).
There is no domination here. The intensity of the color depends on the amount of genes that dim the color and the number of brightening genes that are found in the genotype. It does not matter which, A, B or C.
The subject (A+A+, B+B+, C+C+) will be very dark.
The subject (A-A-, B-B-, C-C-) will be very bright.
The subject (A+A-, B+B-, C+C-) will have an intermediate color intensity. Same as individuals eg (A+A +, B-B-,C+C-) or (A-A-, B+B-, C+C+)

Pol:
the genes A, B,C it is separate genes?? or it is part of Nucleotides of gene P??? They must be connected with gene P and be in the same locus? Or not necessary …and modifier genes can be placed in “other side of the DNA chain” ?

Marko:
Genes A, B, C are separate genes. They can be located even in other chromosomes.
All P, A, B, C genes inherit independently.
Genes A+, B+, C+ act as a gear lever in the car. They accelerate the activity of the P gene and in this way increase the production of the dye.
Allels A-, B-, C- work the other way around.
In another embodiment, the regulatory genes may affect the efficiency of the enzyme. There are many possibilities.
In fact, the relationships between genes are even more complicated because there is eg the phenomenon of epistasis.
I use only capital letters intentionally because there is no domination phenomenon.
Because it is not known how many regulatory genes are and which chromosomes are located on, it is difficult to describe the inheritance of the modification of basic genotypes.

Pol:
why genes modifiers can’t be recessive or dominant? They can’t have alleles?

Marko:
Yes, this is epistasis.
They have alleles. (A +) and (A-) are alleles.
In the regulatory gene alleles, domination and recession may occur.
The operation of regulatory genes in Gouldian finches is not investigated. So I made an analogy to the operation of regulatory genes that regulate the color of a person’s skin. The inheritance of human skin color is well understood.

Pol:
For example If these 3 lutino is genetically Purple breast. Then they have different modifier genes. And if I will make selection for darker mask probably I will make selection for specific modifier gene (or group of modifier genes). Right? And probably Blue Head gouldian or Light colored Pastel Green birds of Cuba it is same effect, of other not found yet and hidden genes modifiers.

Marko:
Yes. The selection of dark individuals allows you to accumulate regulatory genes that darken colors.
There is a small probability of obtaining a homozygote (A + A +, B + B + C + C +). In fact there are much more regulatory genes than 3. Dark individuals are heterozygous with a predominance of genes (+). Therefore, in the offspring of dark parents there will be different intensity of color.

COOPERATION OF GENES, CD. (Picture 3)
Modification of the basic phenotype takes place at the level of genes and biochemical reactions.
The P gene codes for the pigment. The amount of this dye is determined by the regulatory genes A, B, C. They accelerate or slow the work of the gene. In this way, they increase or decrease the amount of pigment in the feathers.
It’s not everything. Other, independent genes affect the location of the dye in feathers. It is not known how many such genes are and where they are located.
In my example, there are 3 such genes and I named them D, E, F.
As you can see, phenotype formation is very complicated. That is why it is difficult to describe it and put it in a genetic calculator.