Allele

  • 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 gray back female (or vice versa), each time you get gray 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 gray color 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 gray male with brown back and pale back can (but only this way) produce pale gray, gray, 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 gray 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

    CHROMATIDE

    A : Normal
    B : Spirally contracted
    C : Schematized

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