Allele

  • Forms of dominance

    A little history

    As we know today, Gregor Mendel, best known for his experiments with peas, was the basis of genetics. He demonstrated through his experiments that if he crossed two peas (F1) with different characteristics such as flower color, leaf size ... Then the offspring (F2) kept the characteristics of a single parent. All the flowers of these young peas had the same color and size of leaves.
    It’s as if they “lost” one of the properties. When he crossed these young F2 peas with each other, the F1 characteristics reappeared among the offspring of the F3 generation. Mendel called these characteristics shown by F2: Dominant. And the characteristics hidden in the F2 have been called recessive.

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

    Some concepts

    We know that genes carry characteristics and that these genes are located on chromosomes. There are genes that deal with the color of the eyes, the color of the legs, the size of the beak… Chromosomes are found 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 legs, the size of the beak ... However, the functioning of the “color of the eyes” genes is manifested only in the eyes. In the paws, the "eye color" genes do not show up. Each cell therefore “knows” where it is located in the body and which genes it must activate.
    Chromosomes go in pairs, all genes are found in pairs. So for the eye color gene, we have two genes. This also applies to the color of the legs, the size of the beak ... These two genes for the color of the eyes can cause a color of the eyes blue. It is also possible that one gene is responsible for the color blue and the other for the color brown. (which does not mean that the being will have one blue eye and the other brown, the brown of the eyes is dominant over the blue, so both eyes will be brown).

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

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  • Pale back, masked and masked old type, three allelic versions

    Why are the mutations in zebrafinch of pale back, masked (new type) and masked old type combined with each other so difficult to predict !?
    Quite simply because we cannot speak at the genetic level of different mutations but rather of allelic versions of a single gene. The pale back, the masked and the old type mask are due to the same gene but which has three allelic versions.
    To understand well let's make the parallel with man, the color of the eyes for example, whatever our eye color, our iris color and coded by the same gene, but this gene has many different versions (alleles) which allow us to have the color panel that we know.

    Now that we know a little more about what complicates these crosses, let's take a look at how each allele behaves in relation to each other.
    Everything is a story of dominance and co-dominance or recessivity.

    A small table to illustrate all this :

    Allele / allele Pale back Masqued Masqued OT
    Pale back x Pale back Pale back
    Masqued Pale back x Masqued
    Masqued OT Pale back Masqued x

    *OT = Old type

    In this double entry table you can see that it allele dominates the other, the bird will therefore have the phenotype of the allele which dominates, be careful, it is not because the allele is dominated that it does not not influence. See pale back / OT mask, the back is more diluted because of the masked OT allele.

    From this result we can draw the first conclusions :

    • The pale back can be masked or OT masked.
    • The masked can be a masked OT split but cannot be a pale back wearer (pb dominates masked = pb / masked). *pb = pale back
    • The masked OT cannot carry a pale back, nor a masked person because the latter two dominate him.

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  • The crossing-over

    It was in 1960 that the first zebrafinch pale Brown back appeared in Belgium.
    Let us try to understand how such a combination of colours could have been born.

    It is known that the brown and pale back factors, related to sex, are located on X chromosomes, but different and at different locations (loci): (1) and (2).
    They are therefore not normally linked (otherwise all the browns would also be pale backs: which is not the case).
    How could they get linked on the same chromosome ? (3)

    Crossing over1 1

    When you mate a brown male with a pale grey back female (or vice versa), each time you get grey males, carriers of brown and pale backs. Each male therefore has two different X chromosomes: One carries the "brown" "no light back" genes, the other carries the "no brown" and "light back" genes. Being recessive, none of these genes can be expressed since they are in a single copy; being non-alleles, neither can dominate the other; It is therefore a natural grey colour that is expressed.

    How will these genes be transmitted by the male to his offspring ? To understand it, some explanations are necessary.

    Chromosomes are very long molecules (2 millionths of a mm thick, average 5 cm long in humans) that are normally entangled with each other in the nucleus of the cell. At the time of meiosis (cell division allowing, in males, the formation of spermatozoa from the mother cells of the testes), these chromosomes split into two rigorously identical chromatids linked together by a centromere.
    Each chromatid then spirals. Only then does the chromosome become visible under the optical microscope. The chromosomes group together and join two to two homologous pairs.

    During this phase, two chromatids of the two joined chromosomes can cross, break and then join together by exchanging more or less important segments. This is the phenomenon called spanning.
    The “brown” gene could thus be found linked to the “pale back” gene on the same X chromosome. A grey male with brown back and pale back can (but only this way) produce pale grey, grey, brown back and pale brown back females. (12.5% of each).

    With this crossing over, this same male could also have:

    • Crossed with a pale back female: 12.5% of pale grey back males split brown.
    • Crossed with a brown female: 12.5% of brown males split pale backs.

    By mating one or the other of these with their "pale brown back" sister, it is possible to obtain (in 3rd generation): 25% pale brown back males and 25% pale brown back females.

    L enjambement image 2

    CHROMATID

    A : Normal
    B : Spirally contracted
    C : Schematized

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