Chromosome

  • Genetics of the zebra finch : Instructions for use

    1. Introduction

    The raising and presentation of zebra finch has grown considerably over the past fifteen years. To improve the size of the new mutations, breeders have also resorted to the classic "carrier" birds.
    Some manage to combine several mutations. All this made it essential to know a minimum of applied genetics. This is the minimum that I would like to introduce to new breeders.
    It is not a complete course of genetics, but the simple presentation of the method I use preceded by some basic concepts.

    2. The zebra finch and its mutations

    A zebra finch has a number of visible characters (size, shape, pattern, color, sex) that make up its phenotype. It may have, in addition to other characters not expressed (it is said that it is a carrier). The whole of the characters, expressed or not, is called the genotype.

    A young zebra finch comes from an egg cell, the result of the fusion of the nucleus of a spermatozoon of the father and the nucleus of the ovum of the female. The genetic program of the bird is already there: A sequence of cell divisions and coded information will trigger (or not) the appearance of the characters. The coded information is carried by genes located on long filaments contained in the nucleus: Chromosomes.
    All chromosomes go in pairs: each chromosome has its counterpart.

    There are two categories of chromosomes :

    - The sex chromosomes:
    • XX in the male
    • XY in the female

    - The chromosomes autosomes.

    The gray zebra finch living in Australia is the origin of all our zebra-reared finch. It has a whole set of genes distributed in its chromosomes.
    Whenever a new mutation has appeared, it is because there has been a modification of a gene of origin (and that it has proved to be hereditary). The gene of origin and the mutated gene are located at the same location called locus on each of the homologous chromosomes.
    Both genes are alleles.

    Genetique du diamant mandarin 1

    A bird is pure (homozygous) when all its alleles carry identical information.
    A bird is heterozygous when at least one pair of alleles carries different information about the same trait.

    There are currently about twenty different mutations of gray zebra finch.

    We distinguish :

    a) The dominant mutations

    Pastel, crested, cheeks (gray, brown), black face (black-face), cheeks clear.
    A mutation is dominant when it is expressed while the mutated gene exists in only one copy. This gene is located on an autosome chromosome.
    There are therefore no zebra finch carrying these mutations.
    Note: When the two homologous chromosomes each carry a dominant gene, the young is not viable. This is called a lethal factor.

    b) Recessive mutations

    White, variegated, saddled, white breast, black breast, orange breast, black cheeks, isabelle, agate, yellow beak, eumo.
    A mutation is recessive when it is expressed only if the two autosome chromosomes each have the mutated gene.
    If there is only one mutated gene, the character is not expressed. The bird is simply "carrier" of the mutation.

    c) Gender mutations

    Brown, pale back, masked old type, masked new type.
    A mutation is linked to sex when the genes responsible for this mutation are located on the X chromosome (s) of the bird (the Y chromosome of the female being empty of genes).

    The mutation is expressed in females since they receive from their father the mutated X chromosome. For it to be expressed in males, the mutated gene must be carried by each of the two X chromosomes. Otherwise, the male is only the carrier of the mutation; however, he can pass it on to half of his daughters.
    Notes: The "Light Back" and "Masked" genes are alleles of the same non-mutated gene. A Gray male may carry Pale Back and Masked.

    Genetique du diamant mandarin 2

    A pale-backed male can be a masked bearer, but not the other way around. In this case, even in a single copy, it is he who expresses himself.
    The same factor (Pale Back) can be recessive compared to Gray, but dominant over Masked.
    The gene "Brown" also located on an X chromosome does not have the same locus as the previous genes.

    Genetique du diamant mandarin 3

    Anterior to the other two, it is on a different chromosome.
    For these genes to be linked (Pale Brown Back, Brown Masked), it took the appearance of a phenomenon that is the subject of another article: Crossing-over.

    d) Combined mutations

    Many mutations as well as gray can be combined with each other. One can theoretically associate a lot but in practice, it is better to remain cautious: In addition to the many necessary crossings, it is necessary that the bird obtained remains typed and corresponds to the criteria of the standards.

    The most famous are :

    • Brown pastel
    • Gray or Brown cheeks
    • Isabelle Black Chest

    Black Brown Black Breast or Brown White Brown Pastel combine, for example, a sex-linked mutation, a dominant free mutation and a free recessive mutation.
    It is therefore necessary to know how to choose the best crossings to achieve this.

    3. Crossing technique

     a) Assign each mutation a symbol

    We begin by assigning each mutation a symbol: By analogy with the atomic symbols, we can choose one or two letters of the name of the mutation.
    The dominant mutations are in upper case, the others in lower case.

    Personally, I use the following symbols (From French abbreviations of mutations) :

      Fn: Black face
      bj: Yellow beak
      bl: White
      po: Orange Breast
      br: Brown
      Pl: Pastel
      pa: variegated
      pb: white chest
      pn: black breast
      J: Cheeks (Gray or Brown)
      jn: Black cheeks
      ag: Agate
      dp: Pale Back
      ma: Masked Old type
      mn: Hidden new type
      eu: Eumo
      se: Sellé

    Scientists have a + sign followed by the symbol of the unmutated gene.
    Example: H (Huppe); H + (not Crested); pb (White breast); pb + (no white breast).

    Personally, I find it more logical to write: H + (Huppé); H- (not Crested); pb + (White breast); pb- (no white breast).
    In the end, the results will be the same.

    b) Write the genetic formula of each bird

    On either side of a fraction bar, symbols of the genes carried by each homologous chromosome are transferred, starting with the sex chromosomes.
    Examples:

    c) Sex chromosomes

    Gray male: XN / XN; Gray female XN / Y
    In this case, N means Normal

    d) Gender mutation

    Brown male: Xbr + / Xbr +; Brown female: Xbr + / Y
    Same formulas with dp +, my +, mn +.

    e) dominant free mutation

    Male pastel gray: XN / XN PL + / pl-; Gray pastel female: XN / Y pl- / PL +
    Same formulas with H +, BF +, J +.
    Non-mutated recessive factors are written in lower case.

    f) Free recessive mutation

    Black-chest male XN / XN pn + / pn +; Black breasted gray female XN / Y pn + / pn +
    Same formulas with pb +, po +, jn +, pa +, se +, is +, and so on.

    g) Combined mutations

    Brown male black face black cheeks: XN br + / XN br + Fn + / fn- jn + / jn +
    Male pastel pale yellow pastel: XN dp + / XN dp + Pl + / pl- bj + / bj +

    h) Carriers

    Gray male / (/ means carrier) Pale back: XN dp + / XN dp-
    Female Brown / Black cheeks: XN br + / Y jn + / jn-
    Gray female black face / black breast: XN / Y Fn + / fn- pn + / pn-
    Gray male black face / black breast: XN / Y Fn + / fn- pn + / pn-
    Male pale back gray / Masked NT (new type): Xdp + / Xmn +. In this case, one could write DP +, since the Pale Back dominates its allele, the NT Mask.

    4. Place these formulas in a cross table

    We must first remember:

    • That each parent transmits to his or her young only one of the two chromosomes of each pair.
    • That the grouping in each gamete (spermatozoon or ovum) of these chromosomes is by chance: it is the genetic mixing.

    The more the parent has mutated genes on different chromosomes, the more combinations will be possible. This is the only difficulty in this method, but it is inevitable.

    Let's start with a simple crossover :

    a) Brown male: (XN br + / XN br +) X Female gray XN br- / Y

      XN br+ XN br+
    XN br- XN br+/XN br- XN br+/XN br-
    Y XN br+/Y XN br+/Y


    Each chromosome of the male (in this case, the sex chromosomes) finds its homologous chromosome provided by the female. It only remains to translate each formula.

    Results : XN br + / XN br- Male gray / brown (50%); XN br + / Y Brown female (50%)

    b) Male gray / brown: (XN br + / XN br-) X Brown female: XNbr + / Y

      XN br+ XN br-
    XN br+ XN br+/XN br+ XN br-/XN br+
    Y XN br+/Y XN br+/Y


    Results : XN br + / XN br + Brown male (25%); XN br- / XN br + (25%); XN br + / Y Brown female (25%); XN br- / Y Gray female (25%).

    Once the method is acquired, it is possible to find the result of any cross. It takes time, logic and patience (or a computer).

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  • 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)


    L enjambement image 1

    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.


    L enjambement image 2

    CHROMATID

    A : Normal
    B : Contracted spiral
    C : Schematized


    L enjambement image 3

    L enjambement image 4

    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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