• Crossing-over

    It is in 1960 that appeared in Belgium the first Dos Pâles Bruns.
    Let's try to understand how such a combination of colors could be born.

    Brown and pale-back sex-related factors are known to be located on different X chromosomes and at different locations (loci): (1) and (2).
    They are therefore not normally linked (otherwise all the browns would be pale as well: which is not the case).
    How could they be linked on the same chromosome? (3)

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    When a brown male is paired with a pale gray - backed female (or vice versa), gray males with brown and pale backs are obtained each time. Each male therefore has two different X chromosomes: one carries the genes "brown" "not pale back", the other carries the genes "not brown" and "pale back". Being recessive, none of these genes can express themselves since it is in a single copy; being non-alleles, none can dominate the other; it is therefore a natural gray color that expresses itself.

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

    Chromosomes are very long molecules (2 millionths of a millimeter thick, 5 cm of average length in humans) entangled, in normal times, 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 exactly identical chromatids attached to each other by a centromere.
    Each chromatid then spirals. It is only then that the chromosome becomes visible under an optical microscope. The chromosomes cluster together and pair with each other in pairs.

    During this phase, two chromatids of the two contiguous chromosomes can cross, break and then be joined together by exchanging more or less important segments. This is the so-called crossing-over phenomenon.
    The "brown" gene could thus be linked to the "pale back" gene on the same X chromosome. A gray male with brown backs and pale backs can (but only in this way) produce pale gray, gray, brown backs and pale brown backs. (12.5% of each).

    With this spanning, this same male could have also :

    • Crossed with a pale-backed female: 12.5% pale gray-backed male with brown.
    • Crossed with a brown female: 12.5% brown male with pale back.

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

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    A : Normal
    B : Contracted spiral
    C : Schematized

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    We do not know if this is how the mutation actually appeared but the hypothesis seems likely. The brown masked that appeared at the same time in Great Britain may have had the same origin (a crossing between the chromosomes of a gray-backed gray-backed male with brown and brown can give females masked brown).

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