The forms of dominance

A little history

As we know today, Gregor Mendel, best known for his pea experiments, has been at the root of genetics. He has shown by his experiments that if he crossed two peas (F1) with different characteristics such as the color of the flower, the size of the leaves ... Then the resulting (F2) kept the characteristics of only one parent. All the flowers of these young peas had the same color and size of leaves.
It's as if they had "lost" one of the properties. When he crossed these F2 peas between them, the F1 characteristics reappeared among the F3 generation. Mendel called these features shown by the F2 : Dominant. And the hidden features in F2 have been called recessive.

Currently we still call it dominant and recessive. However, we know that Mendel discovered "complete dominance". There are indeed other forms of dominance. We already know these forms so we will especially bring some provisions to remember.

Some concepts

We know that genes carry characteristics and that these genes are located on chromosomes. There are genes dealing with the color of the eyes, the color of the legs, the size of the bill ... The chromosomes are located in the cells of the body: They are stored in the nucleus of each cell. In each nucleus of each cell are the genes for the color of the eyes, the
color of the paws, the size of the beak ... However, the functioning of the genes "eye color" is manifested only in the eyes. In the legs, the genes "eye color" do not appear. Each cell "knows" where it is in the body and what genes it needs to activate.
The chromosomes go by pair, all the genes are found in pairs. For the eye color gene, we have two genes. This also applies to the color of the legs, the size of the bill ... These two genes for the color of the eyes can cause a color of the blue eyes. It is also possible for one gene to take care of the blue color and the other for a brown color. (This does not mean that the being will have one blue eye and the other brown, the brown eye is dominant on the blue, so both eyes will be brown).

Les formes de la dominance 1 Les formes de la dominance 2
 

This blue and brown are called genetics "alleles" for the color of the eyes. Alleles are the different values ​​that a gene can take. For eye color, the brown allele dominates the blue allele. Blue is called "recessive".

Note that we are still talking about a single gene. The allele, dominant, recessive concepts only make sense in the context of a single gene. An allele can also have several values: The eyes can be brown, green, blue. From a pair of 2 genes with the same value allele (eg 2 x brown) we will say that they are: Homozygote. In the presence of 2 different alleles (eg 1 x brown and 1 x blue) we will say "heterozygous" for the gene of eye color.
Genes take care of the color of the eyes, the color of the legs, the size of the beak ... These eyes, legs, beak are the effects of these genes. The effect of all these genes of all cells is called phenotype : Either the appearance of the creature. The values ​​of these gene alleles of this being is called genotype.

Full Dominance

In a heterozygous being in case of complete dominance, the effect of the dominant allele will be visible in the phenotype. A being with a brown allele and a blue allele will have brown eyes. in a homozygous being (for example 2 blue alleles) this allele value is apparent in its phenotype. In a being with blue eyes it is therefore certain that both alleles have the value "blue" In a being with brown eyes, the 2 alleles can have the value "brown" or 1 allele the value "brown" and the other the value "blue"; or 1 "brown" allele and 1 "green" allele. In these 2 last cases, only one thing is certain : At least 1 allele has the value "brown".

Incomplete dominance

In this form of dominance, the phenotype of a heterozygote is intermediate between the two different alleles. Many call it a mixture of alleles. This phenomenon happens more frequently in the world of plants. In plants such as the snapdragon where the red and white variants exist, we also find the pink variant. The homozygous flowers are either red or white. But the red allele is dominant on the white allele, and is shown by a pink flower in the variant heterozygote. In this case, we speak of "incomplete dominance".

Les formes de la dominance 3

Codominance

This form is similar to the form "incomplete dominance". But the effect on the phenotype is entirely different. Both alleles express themselves in codominance. In human blood, there are three variants: A, B and O. The A and B alleles are dominant. The O allele is recessive with respect to A and B. These alleles A and B provide a different antigen on red blood cells. A person with the AB group (heterozygous A and B) has red blood cells with a type A antigen and a type B antigen in the same cell. Note here that A and B are present in the same cell (see illustration).

Note : antigens are molecules that are able to induce a response to the immune system. This is the reason why the blood group of the recipient must be known. If he receives another group than his own, then the blood received is attacked by his own immune system.

Les formes de dominance 4

Pseudo-dominance

In complete dominance, we gave the example of the color of the eyes: The brown color supplants the other possibilities blue or green in the phenotype. Only a subsequent growth (and its results) could give a definitive answer as to whether these brown eyes hid blue or green.
With pseudo-dominance, we see the same effect in the phenotype. The result is the same : One allele represses the effect of the other. So where is the difference ? If one of the alleles (usually the dominant) is coupled to a lethal factor then pseudo-dominance appears. This phenomenon occurs in Drosophila.

Les formes de la dominance 5

Drosophila is the favorite subject of researchers in heredity. In these Drosophila there is a mutant with curly wings. The opposite form to smooth wings. The "curly" allele is dominant on the "smooth" allele. When crossing two subjects with the looped allele, in generation F2 we obtain : 2 young with curly wings and 1 with smooth wings. In case of complete dominance we would have obtained 3 with curly wings and 1 with smooth wings. In Drosophila a lethal factor causes one of the young to die in the bud. There are never homozygous drosophila with curly wings. The distinction between complete dominance and pseudo-dominance can only be established by very serious precision in rearing results, not only for hatchlings but also for egg-dead youngsters.

Zebrafinch and dominance

Let's see what all this gives with our zebrafinch :

1. Gray and brown : Gray is totally dominant on brown.
2. Clear back and masked: Clear back is completely dominant on masked.
3. Pastel : It is a factor (allele) that clears the base color (gray or brown). Pastel is dominant over the base color. But pastel creates a mortality effect in the bud. The double pastel factor does not exist. Pastel is pseudo-dominant.
4. Blackface : Some white feather fields are changed to black / brown. The Blackface is dominant.
5. Gray and brown cheeks : Acts on the cheeks and also on the rest of the plumage. The Joue factor is completely dominant.
6. Other factors : Recessive such as white breast, black breast, isabelle, yellow beak, black cheek ...

In young heterozygous couples we can often see split characteristics. It is therefore an incomplete dominance.
So far we have not mentioned the white and the variegated. The factor (allele) that plays a role in this case prevents another color from manifesting itself in the phenotype. This is called an "inhibitor". Given the effect of an allele (factor) in each cell (see above) this can cause plumage partially or completely white (white or variegated). In a white or variegated, we do not speak of dominant but of prevention. White and variegated are recessive.

Note : With the general concept of "dominance" we are still talking about two alleles of the same gene. This in the whole of genetics. The black breast can not be dominant on the orange chest, the clear back not be dominant on the black cheeks ... We also know that several alleles can work together to achieve a global result. Know also that a scale of variation is related to the development of a gene in the phenotype and that in this development; the environment (food ...) plays a role.


Luk Lievrouw
Belgische Zebravinken Club

genotype phenotype Allele Dominant Recessive homozygote heterozygote dominance codominance inhibitor